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K1 9 . h‘5. ‘ .r‘ .nufl ' ' l..."'>.- - " ..‘ .- . l ‘ ‘ As ‘ v Masai» 7“ - ' I V ,.I’ ‘__\.'J \Lx *6 : \ -‘ "Ga-"h a; " "2' .u 0* a . _ " "5’3 _ “1““ ‘.~'._'~ u ' 21‘"; WT, ~- '- - Mi-‘f/Q, ‘ J "‘ ‘. ‘ " .1 J o .- ‘- . ‘ I . - ~ . ‘ J $331 ;'v I U“Q”‘" .h -l _ "‘ . . ‘ "‘5 ‘U " :o' [:‘e-“ _ ‘ \ . " f ‘ a ‘ "‘ 9| “ r ‘ . “'l t _' - L Q) @"1‘ A STUDY OF THE MECHfimISM BY WHICH HOMOGHVIZATION INFLUENCES FLAVOR DEVELOPMVT IN MILK A STUDY OF THE MECHANISM BY WHICH mmocmlzm'lou INILUENCES FLAVOR DEVELOPMENT IN MILK by PAUL BYBEE LARSFN A THESIS Submitted to the Graduate School of Michigan State College of Agriculture and Applied Science in partial fulfilment of the requirements for the degree of MASTER OF SCIENCE Department of Dairy Husbandry 1940 THE-518' ACKNOWLEDGMENTS The writer wishes to express his sincere appreciation to Dr. Earl Weaver, Head of the Dairy Department, for the assistantship which made this study possible; to Dr. G. M. Trout for his guidance throughout the study, and for his suggestions and careful criticism in the writing of this thesis; and to Dr. I. A. Gould for his suggestions in the planning of this study and for his valued assistance in the Judging of the milk. TABLE OF CONTENTS page INTRODUCTION ................................................... 1 REVIEW OF LITERATURE ........................................... 2 I. OXidi-zed Flavor 0fM1lk OOOOOOOOOOOOOOOQOOOQ.00....000... 2 E‘requency Of OX1dized flavor ooooooooooooooooooooooooo 2 The effect of season on oxidized flavor .............. 3 The cow as a source of oxidized flavor ............... 4 Relationship of feed to oxidized flavor .............. 5 The effect of apparent acidity on oxidized flavor .... 7 Effect of bacterial action on oxidized flavor ........ 7 Relation of ascorbic acid to oxidized flavor ......... 8 a. Disappearance of ascorbic acid and oxidized fla- vor development ............................... 8 b. Effect of light, oxygen and metals on ascorbic acid .......................................... 9 0. Effect of feed on the ascorbic acid content of milk .......................................... 10 d. Effect of adding ascorbic acid to milk ........ 12 Effect of carotene on oxidized flavor ................ 12 The relation of enzymes to oxidized flavor ........... 14 Relation of various constituents of milk to oxidized flavor ............................................... 15 a. Effect of fatty constituents .................. 15 b. Effect of non-fatty constituents .............. 18 11 page II. Relation of Processing to Oxidized Flavor DevelOpment ... 19 Effect of metals on oxidized flavor development ...... 19 The relation of oxidation-reduction potentials to oxi- dized flavor ......................................... 20 Effect of heat on oxidized flavor .................... 22 Effect of light and radiation on oxidized flavor ..... 24 Effect of gases, aeration and vacuum on oxidized flavor 26 Effect of agitation, freezing and thawing on oxidized flavor ............................................... 27 Effect of methods of sterilizing equipment on oxidized flavor ............................................... 27 Effect of antioxidants on oxidized flavor ............ 28 III. The Effect of Homogenization upon the Preperties of Milk. 29 a. Physical properties .............................. 29 b. Chemical preperties .............................. 31 The effect of homogenization on oxidized flavor ...... 33 Theory of protective action of homogenization ........ 35 Effect of light on homogenized milk .................. 35 SCOPE OF INVESTIGATION .......................................... 37 EXPERIMENTAL PROCEDURE .......................................... 38 RESULTS ......................................................... 41 The relation of oxidation-reduction potentials to the develop- ment of oxidized flavor in unhomogenized and homogenized milk when stored over long periods at a low temperature ........... 41 111 page The effect of cOpper upon the oxidation-reduction potential and upon the oxidized flavor in unhomogenized and homogenized milk over long storage periods .............................. 49 The effect of removing the fat globule membrane by churning and washing upon the develOpment of oxidized flavor in the re- made homegenized milk ....................................... 59 The deveIOpment of oxidized flavor in milk made from unhomo- genized cream and skim milk and homogenized cream and skim milk after storage for various periods at a low temperature . 66 The ascorbic acid content and the development of oxidized flavor in raw, unhomogenized pasteurized and homogenized pas- teurized capper-free and cepper-treated milk after various storage periods at a low temperature ....................... 70 The effect of sunlight upon the oxidation-reduction potential and the develOpment of off-flavor in unhomogenized and homo- genized milk after various storage periods .................. 77 The development of rancid flavors and increases in titratable acidity due to lipolysis in milk made by mixing raw milk with homogenized pasteurized, homogenized raw milk with homegenized pasteurized milk and raw milk with homogenized raw milk after various storage periods ..................................... 86 DISCUSSION ..................................................... 104 SUMMARY ........................................................ llO LITM'IIIRE CITED 0.00.00.00.000000...........OOOOOOCOOOOOOO0.... 112 INTRODUCTION Flavor defects due to chemical reactions are often encountered in high quality market milk as it reaches the consumer. One of the most prevalent and serious flavor defects in the market milk industry today is the one known as "oxidized”. This flavor is also known as tallowy, metallic, card- board, cappy, papery, oily and emery, dependent largely upon its intensity. There are numerous causes and factors involved in the development of oxi- dized flavors in milk, which have resulted in many complications in regards to its control. Contamination of milk with cepper and iron has long been known to cata- lyze oxidized flavor develoyment. Within recent years, extensive investi- gations have been conducted showing the effects of feed, carotene, vitamin C (ascorbic acid), lecithin, enzymes, bacterial action and apparent acidity on oxidized flavor develOpment. IMore recently, the effect of heat treat- ment upon the develOpment of oxidized flavor has received considerable attention. Recent research has shown that the homogenization of milk at sufficient- ly high pressures prevents or retards the deve10pment of oxidized flavor, even when the milk is highly contaminated with such catalytic metals as cep- per and iron. However, no one has shown by what means homogenization pro- tects milk against such flavor deveIOpment. Whether the mechanism by which the milk is stabilized, is chemical, physical, or a combination of both re- mains to be shown. Likewise, the exact cause of hydrolytic rancidity in homogenized raw milk is undetermined and needs further study. The object of this study is to determine, if possible, how the homogenized milk is stabilized against oxidized flavor development and what relation it has, if any, to hydrolytic rancidity in homogenized raw milk. REVIEW OF LITERATURE I. Oxidized Flavor of Milk Introduction. Since Golding and Feilmann (1905) first called attention to a developed "alkaline mealy flavor” which was "driving away the custom of the retailer", a flavor resulting from capper contamination, many data have been collected on the oxidized flavor of milk. Considerable of these data are without the scOpe of this paper, but the factors responsible for the develoyment of oxidized flavor may be so closely related to the retard- ing effect of homogenization on oxidized flavor development that inclusion herewith of the salient facts seems desirable. ‘gzgggengy of oxidized flavor. Roland and associates (1937) studied the frequency of oxidized flavor defects in commercial market milk from 19 dairies and classified 20.9 per cent of the samples as being oxidized to some degree, with the defect being the most troublesome in the highsfat, premium.quality milk. Honing and Dahlberg (1939) found the occurrence of oxidized flavor in pasteurized milk at 24 hours after pasteurization, reaching a high in March, being absent from May to September inclusively, and recurring in October. The milk scored on the third day after pasteur- ization showed somewhat the same general trends as that scored the first day after pasteurization, excepting that the frequency of oxidized flavor was much greater. Roadhouse and Henderson (1935) reported that 24.2 per cent of 349 samples showed oxidized flavor. Among the pasteurized samples 35.3 per cent were oxidized; among the raw samples only 18.2 per cent showed the defect. Roland and associates (1937) found the percentage occurrence of oxi- dized flavor in raw’ milk started down in.April, went up in lay, was zero - 5 - in August and September and increased again in October and Nevember. Brown and associates (1937a) stated that mixed raw milk never developed oxidized flavor when kept free from iron and cepper contamination. Dahle and Palmer (1937) reported that a temperature of 145° F. for 30 minutes greatly accentuated the off-flavor. Greenbank (1936) pointed out that oxidized flavor developed more rapidly when stored at 5° c. than at 15° c. Trout (1937) noted that 5.5 per cent of the samples of pasteurized milk studied over a six weeks period in late summer showed oxidized flavor de- velOpment the first day after pasteurization with an increase up to 20.7 per cent on the third day. The effect of season on oxidized flavor. The greater occurrence of oxi- dized flavor in winter milk has resulted in.much investigation along this line. Guthrie and Brueckner (1933) found that the oxidized flavors were more pronounced and more widespread in winter than in summer. Anderson and associates (1937) stated that a sudden hot humid weather caused cows which had been producing off-f1avor:milk to produce good milk temporarily, while sudden cold spells appeared to aggravate off-flavors making them more intense. Mattick (1927) believed the contributing factors were prob- ably limited by a rise in temperature which corresponded to summer con- ditions. Webb and Hileman (1937) have shown that summer milk is able to re- sist the development of oxidized flavor even in the presence of high oxi- dation-reduction potentials. Honing and Dahlberg (1939) found the frequency of oxidized flavor in pasteurized milk to be high in March, to be absent from May to September and to recur in October. Garrett and Bender (1940) observed that milk produced during the sums mer by simulated winter feeding conditions was more susceptible to the de- velopment of oxidized flavor than was milk produced by typical pasture feeding, and concluded that season seemed only incidental and the type of roughage was the important factor. The cow as a source of ggigized flavor; Many investigators have studied the cow as a source of oxidized flavor. Guthrie and Brueckner (1933) found that 21 per cent of 155 cows studied gave milk consistently in which distinct oxidized flavors developed after pasteurization on three days storage, while another 10 per cent produced milk in which.oxidized flavors deve10ped only slightly. Dahle and Palmer (1937) found that approximately 37 per cent of the cows yielded.milk during March which develOped an oxi- dized flavor. This dropped to 25 per cent and 5 per cent respectively in .May and October. Both Guthrie and Brueckner (1933) and Dahle and Palmer (1937) stated that apparently no relation existed between the breed or stage of lacta- tion and the development of the oxidized flavor in the milk. Honing and Dahlberg (1939) found that milk from first calf heifers showed a higher incidence of oxidized flavors than.milk from older cows, while Guthrie and Brueckner (1933) reported there was no relation between the age of the cow and the develOpment of the oxidized flavor. Beck, Whitnah and Martin (1939) reported that oxidized flavor occurred in 6.1 and 7.8 per cent respectively of 480 samples of Jersey and Guernsey :milk and in 15.8 and 19.4 per cent respectively of 448 samples of Holstein and.Ayrshire milk. Greenbank (1936) found that one cow which had just freshened produced milk which throughout her lactation showed oxidized flavor on storage and concluded that this defect may not necessarily always develOp late in the lactation period. - 5 - Brown and associates (1937a), Fox (1937) and Tracy and associates (1933) reported that there was considerable variation among individual cows with respect to the tendency for oxidized flavor to develOp in the milk. The latter investigators thought this difference to be due to cells or other anti-oxidizing substances contained in milk. Stebnitz and Sommer (1937b) stated there were considerable variations in the stability of the butterfat toward oxidation from different cows and from an individual cow at different times. Guthrie and Brueckner (1933) observed that oxidized flavors were found in foremilks, middle milks and last milks; being a little more noticeable in the foremilks, which.may have been due to a smaller amount of butterfat.in the milk. There was a variation in the intensity of the oxidized flavor which deve10ped in the milk from different quarters of the udder, indicating that dry feeds were not the sole cause of the development of the flavor. The oxidized flavor appeared in the milk of some cows for several weeks at a tune; the milk from others was irratic in this reapect. Relationship of feed to oxidized flavor. Brown and associates (1937a) found that changing cows from dry feeding to dry feeding plus pasture caused the milk to become non—susceptible to oxidized flavor development. Appar- ently pasture grasses contain one or more substances which pass into the milk where they act in such a manner as to retard or prevent oxidized flavor development even though capper or iron is added. Stebnitz and Sommer (1937b) found that fat from.cows receiving grass in their ration was less saturated and hence more susceptible to oxidation, but the presence of increased amounts of protecting substances in pasture produced milk prevented the development of the defect. Thurston (1935), Dahle and Palmer (1937) and Greenbank (1936) have reported the ability of - 5 - grass pasture to stabilize milk flavor. Greenbank (1938b) noted that the inhibition of oxidized flavor when cows were fed green feed was paralleled by a decrease in oxidation-reduction potential and an increase in poising action. . Garrett and Bender (1940) found that milk produced during the summer by simulated winter-feeding conditions was more susceptible to the develOp- ment of oxidized flavors than was milk produced by pasture feeding. Brown and associates (1937a) stated that so-called industrial feeds, especially dried beet pulp lacked reducing substances thus seeming to favor oxidized flavor develOpment. Honing and Dahlberg (1938b) found that the feeding of mangels appar- ently exerted no influence in preventing the deve10pment of oxidized flavor. Dried beet pulp neither prevented nor increased the susceptibility of the milk toward oxidized flavor defects. In an earlier work (1938a) they found that the level of feeding did not have any effect on the flavor of milk or the frequency of oxidized flavor. Prewitt and Parfitt (1935) showed that the ration containing soybean oil either direct or in the unprocessed beans had a tendency to produce mdlk which.upon holding develOped less degree of oxidized flavor than did milk from cows fed other rations. Fox (1937) found little relationship between various rations and oxidized flavor deveIOpment. However, some milks had a lesser tendency to become oxidized when the cows were placed on a low fat ration. Liebscher (1937) observed no effect on the taste or capper content of milk when cows were fed beet tOps treated with capper. The effect of apparent aciditygon oxidized flavogg_ Anderson, Dowd and Steuwer (1937) found that high acid milk of about 0.19 per cent acidity frequently develOped oxidized flavor after 48 hours storage. When these samples were neutralized until the acidity was 0.15 or below, none of them deve10ped an oxidized flavor in 48 hours. The degree of acidity at which the development of oxidized flavor was retarded or prevented varied with different milks from different sources. They explained that the retarding effect of neutralization on the development of oxidized flavor was due to a balancing of the sodiumrcalcium-phosphorus-casein complex in milk. However, Brown and Dustman (1939) could find no relation between the acidity of freshly drawn milk and the tendency to deve10p oxidized flavor. Both.normal and oxidized flavored milk were found in high and in low acid milks in about the same preportions. The standardization of the acidity of milk to 0.13 per cent did not affect the tendency for the milk to de- ve10p oxidized flavor when contaminated with cepper. Greenbank (1936) increased the pH of milk 0.1 and found it suffi- cient to prevent the development of oxidized flavor after 24 hours stor- age and decreased the intensity after 48 hours storage. Effect of bacterial action on oxidized flavor. Since some bacteria are known to have reducing powers it would seem probable that they would lower the tendency for~milk to become oxidized. ‘Hattick (1927) suggested that bacteria, although not directly involved in the reaction, act in a retard- ing capacity by their own utilization of oxygen, or by the production of acidity which carried the system outside the limiting pH. Tracy et a1 (1933) working with living yeast cells found that they retarded the develOpment of tallowiness in.milk stored at 400 F. Dead 11!)!1 cells or the filtrate of a yeast suspension had no such effect. The bac- teria and yeast caused a change in potential towards the reduction phase, suggesting that a removal of oxygen occurred through the metabolism process of the organisms. Roland and associates (1937) found there was a marked tendency for samples falling in the oxidized flavor group to show considerable lower bacterial counts than those free from oxidized flavor. Dahle and Palmer (1937) found there was no significance between par- ticular bacterial counts and oxidized flavor, although high counts retarded the develOpment of the defect. Incubation also helped retard the develoP- ment of oxidized flavors. Both incubated and unincubated pasteurized samples developed a very strong flavor. They suggested that the heat treatment in addition to the destruction of bacteria affected some reduc- ing substance which incubation could not restore. Dahle (1938) found that starter bacteria would delay the develOpment of the off-flavor. Tracy and associates (1933) found that incubation retarded the development of tallowy flavors. Relation of ascorbic acid to oxidized flavgpp a. Disappearance of ascorbic acid and oxidized flavor developpent, Chilson (1935) reported that all the ascorbic acid in.mdlk was found to have been oxidized by the time oxidized flavor was detectable by taste. He pointed out that the oxidizing enzyme in the skimmilk was the major factor involved in causing the destruction of ascorbic acid when the milk was pasteurized at 143° F. without the presence of metals. Dahle and Palmer (1937), Sharp and associates (1936), Trout and GJessing (1939) and Guthrie and associates (1939) have confirmed the work of Chilson's and reported a close correlation between the disappearance of ascorbic acid and the develOpment of oxidized flavor. Sharp et a1 (1936) stated that the accelerating effect of soluble capper on the development of the oxidized flavor and the oxidation of ascorbic acid was more pronounced in the presence of the active enzyme. Trout and Gjessing (1939) found the rate of disappearance of ascor- bic acid was greater in winter milk than in summer milk, being relatively greater in the irradiated and in the grade A than in the pasteurized milk, and when a distinct oxidized flavor was noted, little or no ascorbic acid was present. Guthrie and associates (1939) stated that the relationship between the destruction of ascorbic acid and the development of oxidized flavor was often so definite that conditions affecting the develOpment of the oxidized flavor could be followed indirectly by studying the ascorbic acid and factors influencing its disappearance. Henderson (1939) found no direct connection between the complete destruction of ascorbic acid and the development of oxidized flavor. b. Effect of ligptp_oxygen and metals on ascorbic agpgp_ The destruc- tive action of metals, especially capper, on ascorbic acid has been ob- served by Chilson (1935), Guthrie et a1 (1939)and Josephson and Dean (1939). The presence of oxygen is thought to accelerate the oxidation of as- corbic acid. Dahle and Palmer (1937) found the ascorbic acid content of Inilk to diminish in the absence of oxygen but not so rapidly as in samples containing free oxygen. However, Guthrie and associates (1939) obtained results showing that even in the presence of sunlight the oxidation of as- corbic acid did not occur in oxygen-free milk. Hand et a1 (1938) heated ldlk at 63° C. for three hours after the addition of 0.1 milligram per liter of dissolved capper and found no appreciable oxidation of reduced ascorbic acid. ‘When cepper treated milk was vacuum cooled no destruction of ascorbic acid was noted. - 10 - ,Light has been found to have a very destructive action on the ascor- bic acid in.milk. Hand and associates (1938) believed lactoflavin to be the sole agent in milk reaponsible for the sensitivity of ascorbic acid to light. Guthrie et al (1939) reported that the ascorbic acid might disap- pear within five minutes to one hour when milk was exposed to sunlight, depending on the intensity of the light and the mixing of the milk. IKon and Watson (1936) considered the reaction was due mainly to visible radi- ation of short wave length (white and blue) although ultraviolet radiation was also probably active. They reported that milk exposed to sunlight on the doorstep for half an hour and then kept for one hour in the dark lost its original antiscorbutic preperties. Huruiana (1937) stated that one factor reaponsible for the reduction of methylene blue in milk exposed to sunlight was the oxidation by cata- lytic dehydrogenization of the ascorbic acid present in the milk. Henry and Eon (1938) found sterilized milk to behave normally on exposure to light, giving no titration with indol-phenol reagent. _c_.____ Effect of fifteed on the ascorbic acid content of mp1}; The effect of various feeds on the ascorbic acid content of milk has been studied by many investigators. Riddell and associates (1935) and Riddell et a1 (1936) could find no significant difference in the ascorbic acid content of milk due to feeds. Anderson (1936a) found that feeding cabbage, which was high in ascorbic acid, had no beneficial effects in regards to deveIOpment of rancid and oxidized flavors. He believed that ascorbic acid was not the factor responsible for good flavored milk, whereas,Brown and associates (1937a) feeding one quart either of lemon juice or tomato Juice or one gram of crystalline ascorbic acid per day, found the milk to resist oxi- dized flavor deve10pment. They concluded that ascorbic acid in rations - 11 - of dairy cows might reduce or entirely eliminate the susceptibility of milk to oxidized flavor develOpment. Brown and associates (1939) found the addition of ascorbic acid to the ration did not increase materially the ascorbic acid content of the milk. They believed the ascorbic acid supplement acted in some manner other than by directly increasing the ascorbic acid content of the milk. Garrett et a1 (1940) found that feed had no influence on the amount of ascorbic acid secreted in the milk. Beck and associates (1939) found that the mean ascorbic acid content of fresh.milk was practically the same in milk susceptible to oxidized flavor as in milk that was non-susceptible. No relation was found between the amount of ascorbic acid in the original milk or between the amount lost during storage and the develOpment of oxidized flavor. Trout and Gjessing (1939) found the ascorbic acid content of com- mercial bottled winter milk to be lower than that of Spring, summer or fall milk. Brown et a1 (1939) reported that when ascorbic acid was fed to cows on a ration low in carotene there was a slight increase of carotene in the milk produced. Guthrie and associates (1939) found the ascorbic acid con- tent of milk from different cows varied greatly, but remained relatively constant from.milking to milking in individual cows. Garrett et al (1938) found a correlation between ascorbic acid and flavor after three days storage and believed ascorbic acid to help sta- bilize flavor. WOessner and associates (1939) found the milk from Brown Swiss cows to contain more ascorbic acid than the milk from Holsteins, Jerseys or Guernseys. His work also indicated that homogenization tended to destroy the ascorbic acid in milk. - 12 - d. Effect of addinggascorbic to the milk. Chilson (1935), Dahle and Palmer (1937), Dahle (1938), Greenbank (1936) and Guthrie et a1 (1939) have all shown that the addition of pure crystalline ascorbic acid to milk at the rate of 20 to 60 milligrams per liter retarded or entirely pre- vented the deveIOpment of oxidized flavor. Effect of carotene on oxidized flavor. Since carotene has reducing prop- erties and reducing substances in milk tend to prevent an oxidizing reac- tion, it would be expected that carotene would exert a protective action on the flavor of milk. Anderson (1936a) (1936b) found the feeding of carrots to be effective in improving the flavor of milk toward both oxidized and rancid flavors. The feeding of pure vitamin A was not as effective as was carotene in the form.of carrots. However, Dahle and Palmer (1937) found that the addition of carotene to milk did not prevent the occurrence of the off-flavor. Stebnitz and Sommer (1937b) stated there was no relationship between the carotene content, as evidenced by the color of the fat, and the stability of fat toward oxidation. Dahle (1938) found that beta carotene mixed with fat and then emulsified in milk did not prevent the occurrence of off- flavor. Anderson et a1 (1937) found that enriching the ration of cows pro- ducing either rancid or oxidized.mdlk with plant materials of high caro- tene content, enabled those animals to produce milk again of very good flavor after a period of ten to fifteen days. They believed that rancid, oxidized and insipid flavors had their origin in carotene deficient ra- tions. Brown and associates (1937a) observed that green feeds as well as fresh hay produced from it, which are high in carotene, contained con- - 13 - siderable amounts of reducing substances which tended to prevent or de- crease the intensity of oxidized flavor. Anderson (1936b) found machine cured alfalfa was more effective in reducing oxidized flavor than was field cured alfalfa. Dahle (1938) found very little oxidized flavor oc- curred after molasses alfalfa silage was fed. Garrett and associates (1938) observed a positive correlation of 0.9339 between color and first day flavor and a correlation coefficient of +0.6039 between first day color and third day flavor, showing that carotene did help stabilize flavor. Garrett et a1 (1939) observed that milk of high yellow color tended to lose less of its flavor on storage than did.milk of lower color. Beck and associates (1939) observed a relationship between color in- tensity of milk fat, as produced by different breeds, and the development of oxidized flavor, with the defect being more prevalent in milk which was below breed average in fat color intensity. The deveIOpment of oxidized flavor in raw’ milk was effectively prevented by feeding as little as 206 milligrams of carotene per cow daily. Brown and associates (1939) added a carotene supplement to the ration at the rate of one-half pound (contain- ing 350 milligrams of carotene in oil) per cow which rendered the milk non- susceptible to metal induced oxidized flavor. Supplementing the ration with carotene increased the carotene content of the milk. However, when cows were placed on a low carotene ration the milk did not deve10p an oxi- dized flavor spontaneously, indicating that some other factor besides a low carotene content was responsible for naturally occurring oxidized flavors They believed that the carotene must be in solution in the fat in order to affect the susceptibility of the milk. Garrett, Hartman and Arnold (1939) and Garrett and Bender (1940) com- pared the flavor of milk produced on molasses grass silage, which was high in carotene, with the flavor of milk produced on beat pulp and corn silage, ‘which.were low in carotene and found the milk produced on the grass silage to be of superior initial flavor, held their good flavor longer in storage and withstood the destructive effects of soluble capper'more than milk produced on either corn silage or beet pulp. The relation of enzymes to oxidized flavor. ‘Kende (1931) postulated that oiliness in whole milk was due to the enzyme ”oleinase” activated by an exogen or endogen contamination with heavy metals. Chilson (1935) showed that the enzyme was in the skim milk phase by developing an oxidized flavor in a milk remade from raw skim.milk and cream.heated to 1700 F. for 10 minutes. The addition of capper was neces- sary in some cases to cause the off-flavor, showing the enzyme alone was not a sufficient catalyst. Dahle and Palmer (1937) found that when susceptible cream was mixed with.normal skimmilk the resulting milk was generally susceptible to oxi- dative changes. They found the enzyme to be inactivated at temperatures of 165° and 1689 F. and concluded that the causative factor was destroyed, rather than that reducing substances were formed. Webb and Hileman (1937) presented evidence that the mechanism of oxi- dation by oleinase was entirely different from the mechanism of the catal- ysis by capper, since the former did not involve high oxidation-reduction potentials, while the latter did. Chilson (1935) believed that the enzyme in the skimmilk was the major factor involved in causing the destruction of ascorbic acid in pasteurized milk. Sharp and associates (1936) found the accelerating effect of solu- ble copper on the deveIOpment of the oxidized flavor and the oxidation of ascorbic acid to be much.more pronounced in the presence of the active enzyme. Anderson (1937) (1939) found that the addition of small amounts of pancreatic enzyme to milk prevented the develOpment of oxidized flavor. The enzyme was used in concentrations varying from 1 part of enzyme powder to 40,000 to 80,000 parts of milk. Relation of various constituents of milgpto oxidized flavor: a. Effect of fatty constituents. Early investigators were of the Opinion that the oxidation of fat was the cause of oxidized flavor in milk. Whitehead (1930) treated skim milk with the sodium salts of palmitic and oleic acid and found the sample containing the sodium oleate to cause the reduction of methylene blue within a short time, whereas sodium palmitate had no effect. He believed that the reduction of methylene blue, the de- velopnent of off—flavors by sunlight, and the oxidation of unsaturated fats were closely related. Stebnitz and Sommer (1937a) found the deveIOpment of peroxides in butterfat ordinarily preceded the appearance of a tallowy flavor. Hender- son and Roadhouse (1934) found that direct sunlight markedly increased the susceptibility of milk fat to oxidation and that an increase in iodine number was accompanied by a shorter induction period. Stebnitz and Some mer (1937b) found considerable variation in the stability toward oxidation of the butterfat from different cows. The amount of linoleic acid rather than the oleic acid was found to govern the stability of butterfat. Dahle and Palmer (1937) found a decrease in iodine number of the fat in the samples which deve10ped an oxidized flavor. - 16 - Brown and associates (1937) stated that it was possible that in high- ly oxidized milk the butterfat might become oxidized, whereas,in mildly oxidized milk only the substances of the adsorbed film of the fat globule were oxidized. There was no measurable change in iodine number of the fat as a result of the deveIOpment of oxidized flavor. Tracy and Ruehe (1931) found skimmilk to obtain a metallic flavor on addition of capper, but when fat was present a tallowy flavor resulted, the intensity of flavor increasing with the fat content. Roland et a1 (1937) noted that the highest percentage of oxidized flavor occurred in highefat, premium-quality milk. Fifty per cent of the samples having a fat content between 4.0 and 5.1 per cent showed an oxidized flavor, where- as,only 4.8 per cent of the samples having a fat content from 3.6 to 3.9 per cent showed the defect. Roland and Trebler (1937) found that a vari- ation of one per cent fat in the range of whole milk could be detected by a significant change in flavor score. Hammer and Cordes (1920) found sunlight to have a greater effect on low fat milk than high fat milk. Evidence has been presented by Thurston et al (1955) indicating that the phOSpholipid portion of the milk, rather than the butterfat, was the substance which became oxidized when an oxidized flavor develoPed. They found that when the adsorbed layer was removed by washing and the butter- fat redispersed in fresh skimmilk, no oxidized flavor could be detected in the remade milk. By dispersing tallowy butterfat in fresh skimmilk a flavor different from the typical oxidized flavor resulted. The intensity of the flavor was greater in cream, buttermilk and butter than in skimmilk or whole milk. Chilson (1938) also believed the typical oxidized flavor of whole milk to be due to an oxidation of lecithin or similar substances adsorbed - 17 - on the fat globule, rather than to oxidation of the true fat, while a tal- lowy flavor was due to the oxidation of oleic acid of the true fat. Thurston and associates (1936) suggested that the protective effect of vigorous agitation, freezing and thawing, and homOgenization against oxidized flavor was due to some realignment of the material adsorbed on the fat globule, with lecithin being undoubtedly concerned in this realignment. If the lecithin were transferred from the adsorbed layer on the fat globule to the plasma by these treatments, it would indicate that lecithin, while in the adsorbed layer, was readily oxidized to give rise to oxidized flavor, whereas lecithin dispersed in the plasma was not oxidized in this manner. Roland and Trebler (1937) found mechanical separation of milk produced a marked decrease in its sensitivity to cOpper-induced oxidized flavor as evidenced by tests made by recombining cream and skimmilk. They attributed this to the removal of lecithin or related substances by the separator or a change in their distribution between the fat and aqueous phase. Thurston (1938) found that the dispersion of purified lecithin, which became oxidized during purification, in skimmilk yielded a mixture having a typical oxidized flavor. Dahle and Palmer (1937) working with.mixtures of lecithin free fat and susceptible skimmilk produced an oxidized flavor upon the addition of copper, whereas,mixtures of butter oil and normal skimmilk showed little or no flavor develOpment. ~Their results indicated that it was possible 'for oxidized flavor to occur in the absence of lecithin in the fat globule membrane providing the enzyme oleinase was present. Josephson and Dean (1939) gave supporting evidence that phospholipids were responsible for tallowy flavor. Brown and associates (1937b) while studying the effect of metal- induced oxidized flavor on the iodine number of the milk fat concluded - 18 - that the lecithin of the adsorbed film was the constituent oxidized. Cephalin, however, might also be responsible. They did not find any changes in the iodine number of the lecithin or find any measurable change in the iodine number of the milk fat when milk became oxidized. Swanson and Sommer (1940) found reductions of 44.41 and 30.89 per cent in the 10- dine number of the phospholipid fraction upon the development of oxidized flavor. Fox (1937) found only a slight relationship between the per cent lecithin in the fat of the milk and the intensity of the oxidized flavor which deve10ped in that milk. The oxidized flavor deve10ped over the en- tire range of lecithin percentages in all degrees of intensity. Beck and associates (1939) obtained similar results. b. Effect of non-fatty constituents. The deveIOpment of off-flavors in milk due to sunlight have recently been attributed to changes in the non-fatty constituents of milk. Doan and Meyers (1936) believed that the burnt flavor caused by sunlight apparently had its source in the casein- free milk and albumin-free serum of the milk. Weckel and Jackson (1936) believed that the activated flavor originated in or was closely associated with the protein fraction of milk. Albumin obtained from.milk unduly ex- posed to radiation possessed a more intense activated flavor than did the casein. The flavor was also produced in the filtrates after removal of casein and albumin and was believed to be due to the soluble minor proteins and to adsorption from.casein and albumin. Flake and associates (1939) found a one per cent solution of gelatin to develOp an activated flavor on exposure to radiation. They believed that the activated flavor and the burnt flavor produced by sunlight to be identical or practically so. -19- ;;;__Relation of Processing to Oxidized Flavor Developmggg Effect of metals on oxidized flavor developmggtél The catalytic effect of cOpper on the develoyment of oxidized flavor in milk is well known. Thurs- ton (1935) and Roadhouse and Henderson (1935) showed that copper and iron must be in solution in order for it to catalyze the development of oxidized flavor. Brown et a1 (1936) and Josephson and Dean (1939) found that cOpper was more effective in causing an oxidized flavor when added after pasteuriz- ation. Thurston (1935) and Brown et al (1937a) and Greenbank (1936) found that considerably more ferrous iron than capper was required to produce oxidized flavor. Greenbank (1936) found that ferric iron, which was an oxidizing agent, was an inhibitor of oxidized flavor, sapecially in the higher concentrations. Tracy et a1 (1933), Thurston (1935), Webb and Bileman (1937) and Gould and Sommer (1939) have shown that the addition of copper and iron to milk caused a rise in the oxidation-reduction potential. Webb and Hilsman (1937) believed that capper increased the oxidation-reduction potential of the milk to a point sufficiently high to bring about a change in some milk constituent. Thurston (1935) found tin and aluminum salts to lower the oxidationereduction potential of milk. Roadhouse and Henderson (1935) stated that metallic salts gave a taste preperly designated as metallic, some of them, however, acted as catalysts, and hastened the oxidation of the milk fat. Ennsiker et a1 (1929) found that Allegheny metal, tin and heavily tinned copper had no effect on the flavor of.milk. Monel metal, Enduro, Ascoloy and nickel had only a slight effect on flavor, while tinned iron, cOpper, galvanized iron, iron and zinc affected milk flavor the most. - 20 - Guthrie and associates (1931) found cepper and capper alloys pro- duced oxidized flavor in.milk. Chromium alloys caused slight oxidized flavor at times,‘whereas,ehram1unrnickel alloys, pure aluminium, glass enamel and carefully tin-plated metals produced no oxidized flavor in milk. Gould and Sommer (1939) showed that cOpper raises the point to which milk must be heated in order to prevent an oxidized flavor, and also raised the temperature at which sulphide liberation began. Ferrous iron was found by Gould (1939) to have little effect on sulphide liberation or the temperature at which milk must be heated to prevent oxidized flavor. Josephson and Dean (1939) found c0pper to retard sufhydryl formation during heating and oxidized those which were formed after heating. The relation of oxidation-reduction potentials to oxidiged flavor, Sommer (1938) stated that the intensity of oxidizing conditions in milk or other liquids could be measured electrically and expressed in terms of oxidation- reduction potential. Tracy, Ramsey and Ruehe (1933) showed that freshly drawn milk had a normal tendency toward reduction. The addition of copper moved the potential toward the oxidation side, whereas,incubation of milk usually caused a rapid drop in potential. They believed that oxidation- reduction potentials were related to fat oxidation in dairy products. Thurston (1935) found that the oxidation-reduction potentials of milk treated with copper were always higher than the controls, and the poten- tials of tin and aluminum treated samples were always lower than the con- trols. The oxidation-reduction potentials were of no practical value in predicting samples which had a tendency to develOp oxidized flavor. Green- bank (1936) found that milk with the lowest potential was best, but the -21- difference in potentials were not great enough to indicate that this was a property which controlled the deve10pment of the flavor. Webb and Hileman (1937) determined the oxidation-reduction potentials of milk from individual cows and concluded that the absolute value of the oxidation-reduction potential of unmixed milk pasteurized in glass had no relation to the degree of oxidized flavor which develoPed. Oxidized flavor deve10ped in some samples when the oxidation-reduction potential was very low. Evidence indicated that the flavor of mixed milk in the winter was directly related to the oxidation-reduction potential. The de- velOpment of oxidized flavor in milk by the addition of capper was due to or accompanied by an increase of the potential to a point sufficiently high to bring about a change in some mdlk constituent. Summer milk was able to resist the deve10pment of oxidized flavors even in the presence of a high oxidation-reduction potential. Fox (1937) found little or no relationship to exist between the poten- tial of individual milk samples and the development of oxidized flavor. Greenbank (1938a) suggested a method of determining milk which would become oxidized. A.amall amount of cOpper was added to milk and the in- crease in potential was determined. Unusual increases in potential indi- cated samples which might become oxidized. In another work (1938b) he stated that thermal inhibition of the flavor was shown to act through a lowering in oxidation-reduction potential, and the inhibition of the flavor by green feed was paralleled by a decrease in potential and an in- crease in Poising action. Gould and Sommer (1939) showed that the oxidation-reduction potential of’milk heated momentarily remained unaltered until temperatures of 800 C. or above were used. At these temperatures the potential showed a definite _ 22 - trend toward a more negative level, the higher the temperature the lower became the potential. Changes in the oxidation-reduction potential were feund to occur coincidental with the appearance of the cooked flavor. When a 30 minute holding period was used the first appreciable lowering of the potential occurred at a temperature of 72° C. They attributed this decrease to the liberation of sulphides within the milk. Josephson and Dean (1939) confirmed the work of Gould and Sommer and further reported that raw milk and milk heated under 1800 F. exhibited a rise in potential upon storage, whereas,samples heated above 180° F. showed no rise in potential. Whitehead (1931) reported some interesting work on the effect of sunlight on the oxidation-reduction potential of milk. He found that when milk was exposed to sunlight, considerable changes in Eh.were soon observed coincident with the decoloration of methylene blue. The thme re- quired to bring about the change and also the extent of the change varied considerably with different milks, and with the intensity of sunlight. The form of the curve obtained with.whole milk exposed to sunlight was quite different from that obtained by plotting oxidation-reduction poten- tials with.time in milk of high bacterial counts. The lower limit of the Eh.value of whole milk was found to be about -0.20 volts. ‘When skimmilk was treated in the same manner little change in Eh occurred in some samples, whereas,in others it fell to about zero and then showed a tendency to rise again to an Eh of about 0.20 volts. Effect of heat on oxidized flavor. Frazier (1928) noted that pasteurized samples usually develOped a tallowy flavor and odor before raw samples. This has been confirmed by Sharp et al (1936), Dahle and Palmer (1937), and many other investigators. Frazier (1928) found that when milk was sterilized the defect ap- peared more slowly. Guthrie and Brueckner (1933) showed that pasteuriza- tion temperatures of 160° F. and higher for a period of 30 minutes de- creased and even prevented the tendency for the deve10pment of oxidized flavor. Dahle and Palmer (1937) stated that it would appear reasonable to conclude that the causative agent was destroyed rather than that re- ducing substances were formed. Thurston (1938) believed that heating from 165° to 168° F. probably resulted in the destruction of the causative en- zyme. Greenbank (1936) and Dahle (1938) also noted the inhibiting effect of high temperatures on the develOpment of oxidized flavor in.milk. Greenbank (1938b) reported that the thermal inhibition of the flavor was shown to act through a lowering of the oxidation-reduction potential. Sharp et a1 (1936), Greenbank (1936), Guthrie and associates (1939) and Gould and Sommer (1939) have noted that the undesirable effect of capper was much less evident in the milk heated to the higher temperatures. Chilson (1935) found that when skimmilk which had been held at 170° F. for 10 minutes was mixed with raw cream to form a 4.0 per cent milk no oxidized flavor occurred, but when the cream was heated to 170° F. for 10 minutes an oxidized flavor deve10ped. Gould and Sommer (1939) found a relation between oxidized and cooked flavors. They pointed out that the apparent coincidence between the tem- perature necessary to prevent the oxidized flavor in milk and the temper? ature required to cause a cooked flavor indicate the possibility that the failure of highly heated milk to show an oxidized flavor might be partially due to the covering up effect of cooked flavor, and secondly to the forma- tion of reducing substances which were closely allied with the cooked flavor. - 24 - The addition of c0pper necessitated a rise in temperature in order to pre- vent oxidized flavor and to cause sulfhydryl liberation. Josephson and Dean (1939) confirmed these results. Effegt of light and radiation on oxidized flavor. The accelerating ef- fect of light on oxidized flavor deve10pment in milk has been studied by Hammer and Cordes (1920), Frazier (1928), Thurston (1935), Trout (1937), Dean and Meyers (1936), Whitehead (1930) and many others. Hammer and Cordes (1920) found an exposure of ten minutes to sunlight was sufficient to cause an off-flavor while an exposure of 45 minutes was sufficient to cause a definite tallowy flavor. These flavors did not deve10p when the milk was placed in brown glass bottles during exposure to sunlight. Frazier (1928) exposed milk to diffused light and found a character- istic cardboard flavor and odor to develop. Skimmilk did not develop the cardboard flavor. He pointed out that there was a difference in the flavor caused by exposure to light and the flavor resulting from.mixing tallowy fat in fresh skimmilk. Tracy and Ruehe (1931) showed that a tallowy flavor deve10ped when milk was exposed to diffused light and a burnt flavor when the milk was exposed to sunlight. They found the defect was more notice- able in skimmilk than in whole milk and .believed that the defect was due to an action upon the milk serum rather than the fat. Davies (1931) stated that the rays which affected milk in bottles were those which were chemical- ly active and capable of passing through clear glass. Homogenized milk was found to be more susceptible to off-flavor develOpment than untreated milk when exposed to light. . Whitehead (1930) gave evidence supporting the theory that the reduc- tion of’methylene blue in milk by sunlight was an oxidation-reduction phenomenon in which unsaturated fats were oxidized and methylene blue acted as a hydrOgen acceptor. Henderson and Roadhouse (1934) found direct sunlight markedly in- creased the susceptibility of milk fat to oxidation. Stebnitz and Sommer (1937a) observed that the end of the induction period during the oxidation of butterfat was marked by a rapid bleaching of the color, and that light, especially ultra-violet, exerted a marked catalytic effect on the oxida- tion of butterfat. Aluminum.foil wrappers were found to exclude the light entirely and prevented the oxidation of the butterfat. Dark green and dark red transparent wrappers were more effective than other colors in pre- venting the reaction. Doan and Meyers (1936) found blue and green colored paper bottles or blue and green ce110phane wrappers on paper bottles re- tarded the development of tallowiness and burnt flavors. They further re- ported that when whole milk and skimmilk were exposed in clear glass they acquired a burnt flavor, whereas,when eXposed in paper bottles they com- monly acquired a milk tallowy flavor. Milk in the paper bottles with— stood 10 minutes more exposure than did milk in the clear glass bottles without exhibiting an off-flavor. Weckel and Jackson (1936) distinguished between oxidized flavor and the flavor due to light. According to them the activated flavor originated in or was closely associated with the protein fraction of milk. When al- bumin was separated from milk unduly exposed to radiation it possessed a more intense activated flavor than the casein. The presence of the flavor in the filtrate after the removal of casein and albumin was believed to be due to the effect of radiation on the minor soluble proteins and to ad- sorption from casein and albumin. Radiation might possibly exert an ef- fect upon a reactive substance in whole milk not represented by any one of the constituents fractionated and studied. -26 ... Thurston (1938) also noted a difference in the flavor produced by sunlight and the so-called oxidized flavor found in spontaneous milk or in capper treated milk. Dahle and Palmer (1937) found sunlight caused the off-flavor to oc- cur in samples in which the free oxygen was replaced by nitrogen. Flake, Wackel and Jackson (1939) found preheating to temperatures of 150° - 200° F. resulted in a slightly greater intensity of the acti- voted flavor in the milk subsequently exposed to radiation. The addition of various amounts of salts commonly found in.milk had no effect on the intensity of the flavor noted when the mixtures were irradiated. The addi- tion of a small amount of hydrogen or calcium peroxide either before or after irradiation caused a marked decrease in intensity of flavor. The activated flavor was removed by the addition of copper followed by bubbling air through the milk. The flavor was inhibited by replacing the oxygen of milk with.nitrogen. Colored glass filters which eliminated all wave lengths less than 4600 A° were effective in preventing deveIOpment of activated flavor. Effect of gases,_aeration and vacuum on oxidized f1avggg Hammer and Cordes (1920) found that exposure to air apparently had some influence on the de- velOpment of tallowiness in mdlk. Hattick (1927) expressed the idea that the reaction was, to a large extent, dependent on free access to oxygen in the molecular state. Dahle and Palmer (1937) replaced the dissolved oxygen in the milk with nitrogen and found it prevented or reduced the deveIOpment of the off-flavor. However, sunlight caused the offoflavor in these samples. When oxygen was passed through samples heated to 180° F. from which the oxygen had been removed previously, the flavor failed to deve10p. Dahle (1938) showed, however, that oxidized flavor would deve10p in milk deprived of its free oxygen by nitrOgen when copper was added. Flake and associates (1939) found that the production of activated flavor either by sunlight or artificial ultra-violet radiation was inhibited when nitrogen replaced the natural gas balance exisiting in the milk. Greenbank (1936) found aeration increased cOpper tolerance of milk by four times. Aeration plus pasteurization gave greater protection than aeration alone. When oxidized flavored milk was aerated some of the flavor was removed. They believed that in this case the flavor was probably ad- sorbed by the fat or protein which made it more difficult to oxidize. Ken and Watson (1936) and Guthrie et a1 (1939) found that oxidation of ascorbic acid did not occur in oxygen free milk even in the presence of sunlight. Effect of agitatiog! freezfpg_and thawggg_9n oxidized flavor, Thurston and associates (1936) showed that vigorous, prolonged agitation of milk at low temperature had a marked effect in reducing or eliminating the de- ve10pment of oxidized flavor even when copper was added in sufficient quantities to cause the off-flavor in untreated milk. Such agitation was found to cause some movement of the lecithin from the adsorbed layer on the fat globule to the plasma. Freezing and thawing was found to have the same effect. Thurston (1938) suggested that this reduction in the development of oxidized flavor in milk agitated or frozen might possibly be explained by a reduced fat surface. The effect of_methods of sterilizing equipment op;oxidized flavor,~ Dahl- berg and Carpenter (1936) studied the influence of methods of sterilizing equipment on oxidized flavor development and found that when hot water - 28 - sterilization was employed the raw milk and hot milk which passed through the equipment, after the first 50 pounds had been passed through, kept well (for three days without the development of oxidized flavor, whereas the first 25 pounds through the equipment deve10ped a trace of oxidized flavor 'in one day and was intense in two days. When chlorine sterilization was used the first 25 pounds of milk passed through the equipment was almost unfit to drink within one hour due to a coal tar flavor. There was a tendency for the milk processed in equipment sterilized with chlorine to be poorer in flavor than other milk after a storage of three days. Thereffect of antioxidants on oxidized flavor. The use of oat flour as an antioxidant in milk has been studied by Dahle and Palmer (1957), Dahle (1838), Mueller and Mack (1939) and Garrett (1940). Dahle (1938) observed that the addition of 0.1 to 0.2 per cent of oat flour reduced the deve10p- ment of oxidized flavor and 0.3 per cent was sufficient to prevent its de- velopment. Mueller and Mack (1939) using copper contaminated milk noted the anti-oxidative prOperties of cereal flour at the beginning of the sec- and day of storage. Garrett (1940) found oat oil to be more effective as an anti-oxidant than oat flour. The antioxidative pr0perties of ascorbic acid, when added to milk in crystalline form, has been observed by Chilson (1955), Dahle and Palmer (1937), Dahle (1938) and Guthrie et a1 (1939). Dahle and Palmer (1937) and Dahle (1938) found hydroquinone to be an effective antioxidant when added at the rate of two p. p. m. Greenbank (1936) found that formaldehyde did not have much effect upon the develop- ment of oxidized flavor, whereas,hydr0gen peroxide prevented the deve10p- ment of the flavor. Hammer and Cordes (1920) showed that small amounts of commercial lactic acid did not have any important influence on the deve10p- ment of tallowiness. Dahle (1938) found malic acid to intensify the off- flavor. Anderson (1937) (1939) showed that the addition of small amounts of pancreatic enzyme to milk prevented the development of oxidized flavor when added at the rate of one part of enzyme to 40,000 to 80,000 parts of milk. l;_. The Effect of Homoggnization upon the Properties ofggilk a. Physical_properties. The fact that the homogenization of milk reduced the size of the fat globules and prevented creaming has been shown by'Weigner (1914), Baldwin (1916), Doan and Swaps (1927), Smallfield (1929) and Halloran and Trout (1932). Weigner (1914) found the diameter of the fat globules in normal milk to average 2.9 microns, while those in home- genized milk averaged 0.27 microns, whereas Baldwin (1916) reported values of 5.0 to 6.0 microns for normal milk and 1.0 to 2.0 microns for homogene ized milk. Similar results have been obtained by Halloran and Trout (1932) and by Doan (1958). Halloran and Trout (1932) showed that pressures of 1,500 pounds per square inch at 90° F. or at 145° F. was usually sufficient to prevent the formation of a cream layer. Trout and associates (1935) found that at any given pressure the reduction in the size of the fat globule was greater when the milk was homogenized at 145° F. than when processed at 90° F. ‘Whitaker and Hilker (1937) confirmed these results and further showed that milk with hardened fat when.homogenized at 50, 60 and 70° F. failed to show reduction in the size of the globules. However, some reduction in size was noticed at 80° F. Halloran and Trout (1932) and Doan and Minster (1983) found that the protein stability toward alcohol was decreased by homogenization. The latter investigators attributed this to the greater amount of adsorbed casein; such casein being fixed and was in the first stages of coagula- tion. Doan and Minster (1930) stated that the surface tension was increased slightly in homogenized milk, while Balloran and Trout (1932) found homo- genization increased the surface tension of pasteurized milk but lowered the surface tension of raw milk. Doan and Minster (1933) confirmed these results and attributed the decrease to the presence of free soluble fatty acids. The fact that homegenization lowers the curd tension of whole milk has been shown by Smallfield (1929), Halloran and Trout (1932), The0philus and associates (1934) and Doan (1938). Whitaker and Hilker (1937) have shown that this reduction in curd tension occurs whenever the homogeniza- tion temperature was 90° F. or above, with.more effective reduction re- sulting by cooling down to 90° F. than by heating up to this temperature. The0philus and associates (1934) found that homogenization at pressures of 500, 1,000 or 2,000 reduced the curd tension of milk approximately 25, 46 and 53 per cent respectively, with the greatest reduction occurring in milk with high original curd tension. - Bollingsworth (1931), Trout (1933), Babcock (1934b), Hood and White (1934) and Doan (1938) found that homogenization presented a problem in securing a satisfactory Babcock fat test; a lower test was usually ob- tained when the milk was homogenized. A.plug at the base of the fat col- umn has been noticed by Trout (1933) and Hood and White (1934), the re- moval of which necessitated the use of sulphuric acid at a lower specific gravity and at more carefully controlled temperature than those commonly employed. - 31 - No important changes in specific gravity could be found in milk due to homogenization either by Weigner (1914), Trout (1933) or Babcock (1934b). A brown to yellow sediment in homogenized milk has been noticed by Trout and Halloran (1932), Babcock (1934a) and Charles and Sommer (1934). Babcock (1934a) found the sediment to consist largely of leucocytes and epithelial cells, and later (1939) reported there was no relation between temperature and pressure of homogenization and the formation of sediment. Trout and Halloran (1933) analyzed the sediment and found its composition to be approximately 72 per cent water, 11 per cent fat and 17 per cent solids not fat. Clarification is used as a means of eliminating the sedi- ment. Weigner (1914) and Doan and Minster (1933) noticed an increase. in viscosity when milk was homogenized which they explained was due to in- creased adsorption of substances on the fat. Trout and associates (1935) reported decreases in the viscosity and foaming ability when milk was homogenized raw but these were increased when the milk was pasteurized be- fore homogenization. Several workers have shown that homogenized milk and cream are diffi- cult to churn and whip. Clayton (1935) attributed these phenomena to the increased adsorption of milk protein, notably casein, on the fat globule. By using viscosity measurements, Weigner (1914) calculated that of the casein in milk, 2.27 per cent was adsorbed in ordinary milk and 25.2 per cent in the homOgenized milk, based on the assumption that only casein is adsorbed and that the mean thickness of the adsorbed layer is 6.8 milli- microns. b. Chemical prgperties. The most noticeable chemical change in milk caused by homogenization is the change in the acid degree of the fat re- - 32 - suiting in.a rancid flavor. Dorner and Widmer (1932) and Halloran and Trout (1932) working independently discovered almost simultaneously that a rancid flavor and an increase in acidity occurred in raw milk within one-half to four hours after homogenization, with the acidity and degree of rancidity increasing with the pressure. Sharp and de Tomasi (1932) showed the increase in acidity to range from 0.04 to 0.08 per cent. The develOpment of rancidity and the increase in acidity is attrib- uted to the action of lipase on the increased fat surface by Dorner and Widmer (1932), Halloran and Trout (1932) and Doan and Minster (1933). Sharp and de Tomasi (1932) working with homogenized cream substrate con- cluded that the increased activity of lipase cannot all be explained on the basis of increased surface otherwise they would have obtained greater increases in acidity. Pfeffer, Jackson and Weckel (1938) also believed the increased activity of lipase was not due entirely to decrease in fat globule size. Doan (1938) stated that homogenization probably caused the destruction of the adsorbed layer on the normal fat globule and allowed the lipase to come in contact with the fat. The causative agent of rancidity was inactivated by heating to a temperature of 131° F. as shown by Dorner and Widmer (1932). Doan (1933) found a flash heating of 147° F., a temperature of 134° F. for 15 minutes or a temperature of 132° F. for 30 minutes would prevent rancidity in milk homogenized at 2,000 pounds. Trout and associates (1935) found subse- quent pasteurization did not decrease the acidity resulting from home- genization. The acidity remained constant regardless of the pressure when the milk was pasteurized prior to homogenization. Homogenized raw skimmilk showed no increases in acidity. Babcock (1934b) found the Optimum temperature of homogenization for the development of rancidity in raw milk to be from 30 to 40° C. Dorner and Widmer (1932) and Whitaker and Hilker (1937) showed that when the milk was homogenized while the fat was in a hardened condition rancidity did not deve10p. Dorner and Widmer (1932) found that distinctly alkaline milk became rancid rapidly after homogenization, but acidified mdlk became rancid only slowly. He further concluded that the agent of rancidity was not contained in the fat or in the whey. Doan (1933) and Pfeffer, Jackson and Weckel (1938) believed that the lipase was to be found primarily in the milk plasma rather than being associated with the fat. Gould and Trout (1936) found no appreciable difference in the ReichertéMeissel number, the Polenski number, the refactive index, or in the acid degree of the fat when pasteurized milk was homogenized. They found the acid degree of the fat to increase four to six times within a few'mdnutes'when raw milk was homogenized, with the greatest daily change occurring during the first 24 hours when an increase of 1,652 per cent was noted. weessner and associates (1939) observed that homogenization tended to destroy the ascorbic acid in milk. The effect of homogenization on oxidized flavor. Tracy, Ramsey and Ruehe (1933) and Doan and Minster (1933) were perhaps the first to show that homogenization retarded the deve10pment of oxidized flavor in milk which had been contaminated with capper. Their results have been confirmed by Thurston and associates (1936), Dahle and Palmer (1937), Ross (1937), Trout and Gould (1938) and others. The ability of homogenized milk to withstand oxidative changes depends upon the pressure of homogenization as shown by Ross (1937), who found that pressures of 500 or 1,000 pounds could not be depended upon to prevent the development of oxidized flavor, while at pressures of 1,500 pounds no samples deve10ped the defect. Samples containing capper added before homogenization did not deve10p the defect within 96 hours when pressures of 1,500 pounds were employed. Almost iden- tical results were obtained when the c0pper was added after homogenization. Trout and Gould (1938) found that pressures of 1,500 pounds had a marked effect in stabilizing the flavor. .Pressures of 2,500 and 3,000 were sufficient to inhibit the develOpment of the oxidized flavor when copper was added at the rate of 5.0 p. p. m. prior to homogenization. A pressure of 3,000 pounds inhibited oxidized flavor deve10pment in milk contaminated with 2.5 p. p. m. of capper added after processing, but was ineffective when 5.0 p. p. m. of capper were added. Ross (1937) treated cream in the same manner as he did milk and obtained almost identical re- sults. Trout and Gould (1938) supplemented these results by showing that homogenization stabilized the flavor of milk containing from.2.0 to 10.0 per cent fat. However, the process was ineffective when fat was present in inappreciable quantities as in skimmed milk. They also found the effectiveness of homogenization was not lessened by the addition of 1.0 to 4.0 per cent of serum solids to the milk before homogenization. Ross (1937) observed that homogenization did not destroy the oxidized flavor when the flavor had deveIOped before the milk was homogenized. Helm, Greenbank and Deysher (1925) found that homogenization improved the keeping quality of milk powder by increasing the ability of the fat to withstand oxygen adsorption. Bell (1939) showed that homogenization caused condensed milk to be more resistant to the changes which result in oxidized flavor. -35- Theory of protective action of homogenization, The mechanism by which homogenization retards oxidized flavor develOpment is not known. Ross (1937) gave a hypothesis based on the assumption that oxidized flavor was caused by enzymic action on the fat globules. under this assumption the finely divided fat globules, resulting from.adequate homogenization, are surrounded by a film which protects them from enzymic action, thus pre- venting the development of oxidized flavor. Dahle (1938) stated that it was thought that the process added a fairly heavy film on the surface of the globule which protected it from oxidation. Tracy, Ramsey and Ruehe (1933) thought the retardation of oxidation to have been due to certain physical changes in the milk which.might have made the oxidized flavor less detectable. Thurston (1938) stated that the most likely theory to explain the non-development of oxidized flavor in homogenized milk was the increased adsorption of protective protein on the surface of the fat globules. Effegt of light on homegenized milk. That homogenized milk is more sensi- tive to sunlight than unhomogenized milk has been demonstrated by Davies (1931), flood and White (1934), Doan and Meyers (1936), Dahle (1938) and Flake, Weckel and Jackson (1939). Davies (1931) pointed out that homogenized milk exposed more fat surface than the same sample of natural milk, and, coupled with this was the fact that during atomization the milk had become completely saturated with atmospheric oxygen. Such treatment of milk was likely to lower the protective effect of the hulls or coatings of the fat globules thus render- ing the fat more susceptible to deterioration. This flavor defect due to light has been called sunshine oxidized, tallowy, burnt and activated. The term burnt or activated as suggested by Flake and associates (1939) described the flavor more adequately and seemed more desirable. Dahle (1938) used the term.oxidized to describe the flavor defect in homogenized milk after exposure to the sun. Doan and Meyers (1936) found homogenization of whole milk to accelerate the development of tallowy flavor in both glass and paper bottles, but the burnt flavor was not evident. At high pressures of homogenization the degree of tallowiness was greater in the paper bottles than in the clear glass bottles. They also found that homogenization had no noticeable effect on the deve10pment of the burnt flavor in skimmilk exposed to light. Flake and associates (1939) found that a slightly'more intense flavor resulted when homogenized milk was irradiated than when milk was irradiated only, or irradiated then homogenized. They suggested that per— haps the parent substance or substances which gave rise to the flavor might be-made more available to, or~more susceptible to the action of ultra-violet rays and to the sunlight by the homogenization process. - 37 - SCOPE OF INVESTIGATION This study was conducted in an effort to determine if possible the mechanism by which homogenization retards the develOpment of oxidized fla- vor in pasteurized milk and accelerates the development of rancidity in raw'milk. 1. 2. 4. 5. 6. 7. The experiment included a study of the following points: To compare the effect of long storage periods on the flavor and oxidation-reduction potentials of unhomogenized and home- genized, pasteurized milk. To compare the effect of long storage periods on the flavor and the oxidation-reduction potentials of milk when capper was added. To determine the effect of removing the fat globule membrane by churning and washing on the development of oxidized flavor in the milk and the lecithin content of the buttermilk. To study the develOpment of oxidized flavor in milk remade from unhomogenized skim.milk and cream, and homogenized skim milk and cream. To compare the vitamin C content of raw, pasteurized and homo- genized milk as related to the develOpment of oxidized flavor. To determine the effect of sunlight on the oxidation-reduction potential and flavor in unhomogenized and homogenized milk. To study the devlepment of rancidity in homogenized raw milk. EXPERIMENTAL PROCEDURE The mdlkzused in this experiment was obtained at the collegehcream- cry, and usually consisted of mixed milk from two or three producers. The quality was similar to that received at the average milk plant, with the butterfat content varying from.3.5 to 4.0 per cent. Pasteurization was accomplished at a temperature of 143° F. for 30 minutes in an cpen, stainless steel, 20 gallon cheese vat. Homogenization was accomplished at pasteurizing temperatures, except for the raw milk which was homogenized at 90° F, with a 200 gallon per hour Cherry-Burrell viscolizer at a pressure of 2,500 pounds, unless other- wise stated. The milk was cooled immediately, bottled and stored at 35° to 40° F. Oxidation-reduction potentials were determined at various periods throughout the year by means of a Beckman pH meter using a platinum elec- trode against a saturated calomel cell. The readings were taken when the E. M. F. had become constant and were converted to Eh by adding the E. M. F. to the voltage resistance of the saturated calomel cell which was de- termined against a normal hydrOgen electrode. In studying the effect of washing the fat on the oxidized flavor, a normal centrifugal cream testing about 35 per cent fat was obtained from pasteurized milk. The fat globule membrane was removed by churning the cream in a glass daisy churn. The butter was washed once with cold water and then redispersed in the original skim.milk by homogenizing at 500 pounds pressure. A.portion rehomogenized at 2,500 pounds pressure, constituted the sample washed once. The remaining milk was reseparated; the cream re- churned; the butter rewashed and redispersed in its skim.milk as before. - 39 - A.portion rehomogenized at 2,500 pounds pressure constituted the sample washed twice. The remaining milk was treated in the same manner for a third time and constituted the sample washed three times. Care was taken to see that the remade samples had the same fat content as the original milk. The phospholipid content of the buttermilk from each of the three churnings was determined according to the method suggested by Herrall (1935) except for a few variations in the procedure. The samples of buttermilk were weighed instead of being measured. The dissolved ash.was transferred to a 200 ml. volumetric flask and made up to volume. A.five ml. portion of the ash solution was transferred to a 100 ml. volumetric flask and made up to volume. The phosphorus content of this solution was determined. The molybdate and stannous chloride solutions were made as suggested by Bodansky (1932). The recombined milk studied were prepared as follows: cream and skim milk, separated from pasteurized milk, were each divided into two lots, one lot of cream.and one lot of skim milk being homogenized at 2,500 pounds pressure and the other lot kept unhomogenized. The following mixtures were then made: 1, homogenized cream plus homogenized skim.milk; 2, home- genized cream plus unhomogenized skim milk; 5, unhomogenized cream plus unhomogenized skim.milk; and 4, unhomogenized cream plus homogenized skim milk. The remade samples were standardized so as to have the same fat con- tent as the original milk. Ascorbic acid determinations were made on raw, pasteurized and homo- genized pasteurized milk by means of the rapid titration method recommended by Sharp (1939). The effect of sunlight on the flavor and oxidation-reduction poten- tial of unhomogenized and homegenized milk were studied by exposing the milk in eight-ounce glass jars to direct sunlight. Duplicate samples were exposed in oxidation-reduction cells. These cells were eight-ounce Jars fitted with four hole steppers. Two platinum wire electrodes were inserted into the milk through the holes, with an agar bridge extending from the .milk through one hole into a side tube filled with a saturated potassium chloride solution, into which the saturated calomel cell was placed when making connections. The fourth hole in the stepper served only as an air vent so the stepper could be firmly fitted into place. All milk was studied organoleptically for oxidized flavor by two or three experienced Judges. The samples were numbered and scored without the identity of the samples being known. The presence and intensity of the oxidized flavor was indicated as follows: -, no oxidized flavor; ?, questionable oxidized; +, slightly oxidized; + +, distinctly oxidized; and + 4 t, strongly oxidized. In summarizing flavor scores, each intensity was given a numerical value of O, l, 2, 3, and 4 respectively. Rancidity develoyment was studied by making mixtures of raw milk plus homogenized pasteurized milk, raw milk plus homogenized raw milk, and homogenized raw milk plus homogenized pasteurized milk. These samples were studied organoleptically. The presence and intensity of the rancid flavor was indicated in the same manner as were the oxidized flavors. The increase in titratable acidity was determined by direct titration, using a ten.ml. sample of milk and 0.05 normal sodium hydroxide, the increase in acidity being calculated as lactic acid. - 41 - RESULTS Eggprelgtign of oxidation-reduogiondpotentials to the deve10p- 222::9f oxidized flavor in unhomOgenized and homogenized milk ghgn stored over long periods at a low temperature. In an effort to determine the relationship between oxidation-reduction potentials and the development of oxidized flavor in unhomogenized and ho- megenized milk, flavor determinations and oxidation-reduction potential measurements were made immediately after processing and after 1, 3, 7, 10, l4, 17, 21, 28 and 35 days of storage at 35° to 40°F. In this series of experiments the milk was pasteurized at 143° F. for 30 minutes and homo- genized at a pressure of 2,500 pounds. The data obtained are presented in tables 1, 2, 3 and 4; with the average results of all trials presented in table 5. An examination of the data shows there are considerable variations in the oxidation-reduction potentials of different milk immediately after processing. An increase in oxidation-reduction potential occurred when the milk was stored, with the rate of increase being slightly greater in the homogenized than in the unhomogenized milk. The day at which the max- imum.Eh occurred varied considerably, depending upon the milk. Winter milk did not show very large changes in oxidation-reduction potential dur- ing the storage period as compared to the milk produced in the spring or' fall. An examination of the average results on all samples presented in table 5 shows the highest potential occurred in both the unhomogenized and homogenized milk after 14 days storage at which time the potentials were 0.4094 and 0.4175 volts respectively. After reaching the maximum the po- tential decreased until at the end of storage period it was approximately the same as at the beginning of the storage period. The rate of decline -42- J potentials and the development of oxidized flavor in unhomogenized and homogenized pasteurized r he oxidation-reduction Table l o milk after various sto page periods. Fall milkI 1938. O. O. Apotentials and flavor when Fall milk was Oxidation-reduction O. O. 0. Storage ized :Fla- Homegen O O 0. Fla- : Unhompgenized Homogpnized Fla- 1 Fla-z Unhomogenized O O O 0 (MW) Eh VOI‘ Eh 00 vor .0 Eh vor .0 Eh Trial II Trial I r 3 SOUI‘ + : 0.3052 : 0.2879 _— : Insufficient sample) Trial IV 0. 0. Trial III I 0. O. 00 O. .0 O. z 3 sour O 0 0.3584 + : 0.3373 : : : z + - 8 002919 - 3 003222 c o 0000000 03707 5 ilms is rials Average (5 t Trial V .0 0. O. O. O. O. O. Q. 0. O. 4.0.Avauozavmw 0960171 33432 m 0.0... 0000008 see. see. so est n .*.9 ...”...3 ...... .91 .7 C 1 00:90.. ee .00.... m 4 626 onmglq. n $5 9231 3 335IL o . 000000 a 37 785 0 m123 *Average of three trials ent of Homegenized ,J¥l_____. oxidized flavor in unhomogenized and homogenized pasteurized milk after various storage periods. 0 Winter milk 1939. I : Unhomogenized ntials and flavor when Winter milk was ppotential and the devel - 43 - pots Homo enized _-_ The oxidation-reduction : Oxidation-reduction 'Unhomo enized Table 2. Storage r . ......8 ...-.6 .2. t t mmwwmmm 10 t t if v .1 .1 AunununununuAu .0 .b s 00 .0 O. .0 O. O. O. O. O. O. O. O. .0 O. O. O. 0, fl .0 O. O. O. .0 O. 9. O. .o.ons.4.o ,nvnuo. 92 4. aunv9~0.4.2~ 84602 02% 8 \1 I 101281.. h 512151.. 81 1% e ...... gems-.13 w. Ti enquququququ w“ eunuouqu a; .92uu .auauqJ I .0000. so... 5 see... nuxu.ununu.u .U.U.U.u.u m In nonununununu O. .0 d O. O. .0 O. .0 O. I. O. u 0. O. O. O. O. O. .0 8 e O. O. O. O. O. O. O. O. . an .9.9 +++ an .++?¢”t E 00 3 ar T Oflfi+++ m... 9+ .O n m ozwwmfivm 10 .v ... “We 6 oeoeeoo FV 1.1 V 0332253 .0 c .A .0 o. o. O. O. O. .0 0. O. O. .0 O. O. O. .0 O. O. .1 O. 0.. O. O. Q. I. O. O. w 6624.22 04.397 m 10%716 .n v.ov1.nc no 7mg nuns suave. E 3mg 5 ”Wm. 19 n 119115 3 3 3.3 .../“32 T. 532333 0 0 O O O O 0 O O O 0 O O nununvnunvnu nununuAuAu AUAUAUAUAUhV* O. .0 O. O. O. .0 0. I. O. 0. O. O 0. O. O. 0. I. .0 0. O. O. O. O. O. O. .0 O. O. 0. 9. O. 00 . w r ar ..?. . .uw ...... .....W 10 .v _ FV .1 .o “m .0 .0 00 O. O. O. O. O. O. 0’ 00 00 O. O. O. O. 00 O. O. O. O. O. .0 O. 8 6929 91 20 232 .nL a. «success 1.1.uupuau my m~m~1.o.au Aw E 951274. 12%95 1 $9819 I enaumJauesau some .euqu ezecaues see... I 00.00 m .00.. m 000000 I 00000 00000 T. 7i 8 V. s ...: 1..................... 1......+.....+..+....t 1.............+..mt er a N”... “a. a +N++ M a +Hw+fin IO .1. ... .9 ... .1 o. 6 FY r mm“ .1 r .11 T. 0 mi .0 c .0 O. ’0 00 00 .0 O. O. O. .0 00 O. .0 O. 06 O. 00 1 O. 0. 00 O. .0 .0 O. 1 . W“ “m 33 32 26 8960 m mmemm meme“ m memes a «sausage queue“ as .1. mueaeaaueu T. O O O O O O O O C O... O O 0 O O nunU.U.U.U.U nvnvnunvnv nUnunUAUnu 0.. O. O. .0 O. O. I. O. O. .0 00 O. O. 00 0. O. 0. O. O. O. CO 00 O. O. C. .0 O. .0 .0 O. O. O. 8 v. mun. 1. «on. Ru nvmuucnuax mu m 0 0 m; 0 mama.» 12 3 I‘ * Average two trials milk 93 f oxi- : Fla— Bomo enized SpringmilkJ 1939. Unhomo enized: Fla- potential and the development 0 enized and homogenized pasteurized .15 after various storage periods. vor : Fla- 0 o enized : Fla- Oxidation-reduction potentials and flavor when Spring_milk w The oxidation-reduction —:‘ dized flavor in unhom : Unhomo enized : (Days) : Table 3. Storage : ... r . . . . . . . + . . + . . e e r n.0.n.nvnv nvnu e. +01% 00620 06 t8 ‘IJ 00000.00 its 8 00000010 .08 1 O. O. O. O. O. O. O. O. .0 O. .0 O. .0 .0 0. O. .0 O. a O. .0 I. .0 O. O. Q. .0 .0 .1 66072752 4.827%624 r 25134726 68756516 5367 067 t 79085 14. 0a.. 1.06%.].20. 25.471673. 159%]. 29 mu .443 “Aeneas mu queuuuauxequaqu a. «cause .Aeuuaung I 00...... 0....00.IJ0.00000 I 00000000 1 00000000 6 00000000 “0. O. .0 O. CO .0 O. O. .0 a O. .0 O. O. 0. O. O. 00 .0 $0. I. .0 O. .0 .0 O. O. I. .1. .1..?9.?§ r .+.+++++ mo 00 r “H T ++H++++ e OMWWBMGW T + +4 ... ...... v o....... A 01232334 0. .0 O. .0 I. O. O. O. O. 00 .0 O. I. I. O. O. .0 .0 0. O. .0 O. O. O. O. .0 CI 05772777 26524777 788475 8 «counvsuxeau119. 1i1iqusvoamwevo. J.quuoaxenc .o 23:73.140 22849 97 2 .47 943 34.34444. 33233322 3333 333 .....00. ......0. 0......0 00000000 00000000 0000000.x.0 O. z 0. O. O. .0 0. .0 .0 .0 O. O. .0 O. 0. O. 0. 9. O. O. O. O. 0. O. O. .0 0. .0 O. .0 O. 0. .0 Ir r IT ......69 ...-.63.. .....ee tt .70 +0...» +0.70 t tt .11 .1 1.1 .Db .0 ..Db 0. O. .0 O. O. O. O. O. O. 9. 0. O. O. .0 O. O. O. .0 O. .0 O. .0 .0 .0 O. .0 .0 87590222 24.882827 9222824. 1% 38902 50892072 6974.161 R... m~c2412 92.589942 2167008R~t 33 ”4.4332 T. 23333333 33333228 00......I 00...... ......00 I 00000000 I 00000000 7 0000000L fl 0. .0 0. 0. 00 .0 .0 0. .. 1m 00 0. .0 00 0. 0. .0 0. 0. fl;.0 .. o. 0. .0 00 0. 0. .0 .1.9..9+?¢§+ .1 .++++¢++ .1 ......” ... r ++++§+ r 0......1”? r + ... T ...... ....o T ++ ... T .0 O. 0. O. .0 .0 O. O. .0 O. O. O. O. O. .0 O. .0 .0 O. 0. O. 0. O. O. O. O. 0. 86690472 7m820777 6228224. neemmele m1.ememm mm a en. 5 u 33w4lnwwuwmm 33333332 33.33338 00...... 00...... 00000.. 00000000 00000000 0000000.“ 0. O. 0. O. O. O. O. O. O. .0 O. .0 0. O. O. .0 O. .0 .0 O. O. O. O. .0 0. O. O. O. .0 0. .0 O. 0 037041 5 03704.1 5 03704185 112 3 112%3 11223 * Average of four trials. - 45 - reduction potential and the development of The oxidation- Table 4. oxidized flavor in unhomogenized and homogenized pasteurized milk after various stora Springgmilkj 1940. egg periods. 8d ‘ a . w an .x1 2“: n m .0 m p a m 1. o r .n a .0000. n e .d . .n e a W ”H w w v a. a o n m .n a a m 0.0... S 1. . m an .t znr n .....L a a: m a n m .4 "m 1 .t. 0...... u Au 3.. e e a ... m1 m. a: 1 m m m .m n nu no Storage (Days) VOI‘ .121; V0? VOI‘ Eh O. VOI‘ Eh Trial II 0.: Trial I O. O. l O. .0 O. O. .0 O. O. .0 .0 .. 0. .0 0. 002...... O. .0 .0 O. O. .0 .0 .. .. 0. 0. .o .0” O. C. .C O. .0 .0 lost .. O. O. z 0. O. O. O. C. .52 3% 3 3 +? : 0.3796 : ..0....... .....d.. 957 6 18...0 . SO” sssu‘umwo ... .1 0000.0 0. O. O. O. .0 z 0. O. 0. 01370418 1122 Trial IV C. 0. II I Trial Q. r r ....e.8r +0 t” t .t 1 1S .0 .0 O. O. O. 00 0. 00 00 00 22 4444.2 7.47t.9.4.o_®.4 0.02829” 3 2304.3 30 ..1..... 00 0.000% O. O. O. .0 O. O. O. .0 rr .. 9.1.5.66 o. tt _t.t 11 bb O. 0. O. .0 00 O. O. O. 78 5274.9 291060972 118M .950... 330 .33? .01....0 00 00000 O. O. .0 00 00 .0 9. O. ... .... ......OOOOOOzO. W49 33 5 o.o.+.nu mm 2258mm 3330 35 ...1.0.. 000 0000 00.00.000.000... ... 0...... ..9 . 00.00.000.000... 45 53 588 52 +0620 2&539497 3 034.33 . .1 . 0 . 1 00 .0 0000 000.000.0000.... 013704.18 1122 L—f Average 14 trialslp 0.00.00.00.000000 mmmmmmmm 0.9.0.0.0.0.0.0 00 I. .0 O. O. O. .0 O. .0 ans 4.805 3 122 225 926 335 “3433 00000000. 00000000 .. .. .. 0. 0. .. .0 .. 0. 0 666000 0 666000 0 O O O O O O .ununuaiguququgg 0. 0. O. O. 9. O. O. O. .0 . 44.1752 ovoqaonunu1l 9.9.qua. «uuuqvau 0.... nununununu * t t O. O. .0 O. .0 01370 1 38 14 :#0.42 . 7 3 0 .w 0. 1 2 * Average of three trials. # Average of two trials. Ebmggenized - 45 - :Oxidation-reduction potentials :and flavor when milk was of oxidized flavor in unhomogenized and homogenized pasteurized milk after various storage_periods. Average of all trials. The oxidation-reduction potential and the develogment Storage : UnhomOgenized : Table 5. ommmmmMM2. OOoOOoOoOOOOO O. .0 .0 O. O. I. I. O. .0 .0 0. u“ 1.0.Av aumu1‘v; Fla- vor 34.6 301 m2 M331WMMM OOOOOOAWOoOoO O. .0 O. O. .0 .0 O. C. O. O. .0 0064060971 0000.0.nU-82001 O O 009m2325333 O. .0 O. .0 O. O. .0 I. O. .0 O. 3 66 ”41602 ”96762 122 ewe—O52 333 3. 3: O. OOOoOOoOAWAUoOO O. .0 O. .0 O. .0 .0 .0 .0 .0 I. .0 .0 O. O. Eh O. O. : Fla- vor Average jall trials) Eh 013704—7185 111223 (Dave) t 0.44 0.40 '74 0.32 /,' / UNHOM0./ ‘\ O \ ' \ I HOMO. ,.O----------.-. ‘ .’.o.-..-.o...-Io..-..-'°'-O..000---.°O ' . - - 0. 28 ‘\t\.<»a 0 5 /0 /5 20 25 .30 35 DAYS rigure 1. Relationship between oxidation-reduction po- tentials and the development of oxidized flavor in unho- mogenized and homogeni7ed milk over long storage periods. FLAVOR IN TENS/TV - 4a - was slightly less rapid in the unhomogenized than in the homogenized milk. However, no very significant differences in oxidation-reduction potential occurred between the unhomOgenized and homOgenized milk. No oxidized flavor occurred in any of the samples after only one day of storage, but was usually quite pronounced by the third day of storage. 0n the third day of storage the unhomogenized milk merited an oxidized flavor ration of 2.06. After seven days of storage the rating increased to 2.94, remaining relatively constant until the seventeenth day of stor- age when it increased to 3.20 and stayed above 3.00 throughout the re- mainder of the holding period. The development of oxidized flavor in the unhomogenized milk was paralleled by an increase in the oxidation-reduc- tion potential. An oxidized flavor deve10ped in only a very few cases in the homo- genized milk even though the milk was stored as long as 35 days, showing the stabilizing effect of homogenization against oxidized flavor develoP- ment over long storage periods. A rise in oxidation-reduction potential in the homogenized milk very similar to that observed in the unhomogenized milk, was not accompanied by the deve10pment of oxidized flavor. Results similar to these were secured in milk obtained at various periods through- out the year as shown by the various tables. The summarized results of the oxidation-reduction potential and the deve10pment of oxidized flavor in unhomogenized and homogenized milk pre- sented in table 5 are shown graphically in figure 1. The effect of copper upon the oxidation-reduction_potential and upgn the oxidized flavor in unhomogenized and homogenized milk over long_storage_periods. In this study the pasteurized milk was divided into three lots. Lot I was kept as a control while lots II and III were homogenized at 1,500 and 2,500 pounds pressure respectively. Each lot was then divided into three portions to which cOpper was added at the rate of 0, l and 3 p.p.m. respectively. Oxidation-reduction potential measurements and flavor de- terminations were made immediately after processing and after 1, 3, 7, 10, 14, 21 and 28 days of storage at 35° to 40° F. The eXperimental data are tabulated in tables 6, 7 and 8 with the average results of all trials summarized in table 9 and plotted on figures 2, 3 and 4. An examination of the data shows that the addition of l p.p.m. of c0pper caused a rise in oxidation-reduction potential in both the un- homogenized and homogenized milk. The addition of 3 p.p.m. of cepper caused a further increase but the rate of increase per part of cOpper added was not so great as when only 1 p.p.m. of capper was added. After 1 day of storage the potential was practically the same in the milk con- taining l p.p.m. of copper as it was in the milk containing 3 p.p.m. of added copper. The maximum potential of the copper treated milk usually occurred after one day of storage. After reaching the maximum the po- tential decreased, with the milk containing 3 p.p.m. of capper decreasing more rapidly and to a lower final potential than the milk containing only 1 p.p¢m. of added capper. The unhomogenized milk treated with capper showed a slightly more rapid decrease in potential than the homogenized milk which.was treated with capper. - 50 - The unhomOgenized and homogenized milk to which no capper had been added showed a gradual rise in oxidation-reduction potential on storage. The maximum.potential occurred on the fourteenth day of storage, after which there was a tendency for the potential to decrease. The maximum potential attained by the milk not contaminated with cOpper was practically the same as that attained in the cOpper treated milk. However, the day of storage at which the maximum.potential was reached was greatly different, being reached the first day of storage in the latter but not until the fourteenth day of storage in the former. An examination of the develOpment of oxidized flavor shows that in the milk not treated with capper the oxidized flavor deve10ped after 10, 21 and 28 days of storage in the unhomogenized milk and the milk homo- genized at 1,500 and 2,500 pounds reapectively. The addition of 1 p.p.m. of capper to the unhomOgenized milk caused a slight oxidized flavor after one day of storage. The intensity of the flavor increased rapidly until the seventh day of storage, after which the intensity decreased a little, but gradually increased again until at the end of the storage period when a flavor rating of 4.0 was obtained. The addition of 3 p.p.m. of copper to the unhomogenized milk caused a strong oxidized flavor to deve10p quickly. After one week of storage the milk had an oxidized flavor rat- ing of 4.0 and remained very strongly oxidized throughout the storage period. Homogenization at 1,500 and 2,500 pounds pressure delayed the deve10p- ment of an oxidized flavor in milk to which capper had been added. The higher pressure was more effective in retarding the flavor than the lower pressure. The addition of l p.p.m. of cOpper to the homogenized milk did not cause as intense an oxidized flavor as did the addition of 3 p.p.m. - 51 - ai pper upon the oxidation-reduction potenti The effect of co Table 6. nized and upon the develOpment of oxidized flavor in_pnhomoge riods arious storagegpe steurized milk after v and homOgenized_pa at a low te (Trial 11. erature. 35L L :Oxidation-reduction potentials and flavor when I genized at various pressures milk was homo O. : 1500i 02? O. .0 :added Storage :Copper (Days) :(p.p.m.): Fla- 3 VOP Eh vor Eh vor 0. 0.3260 0. .0 O. O. O. .0 .0 .0 .0 O. O. .0 O. .0 0. O. - : 0.3186 : + : 0.3929 : 0.3183 0.3909 0.3912 0.3220 : 2 0.3819 : O O 0 l ? 0.3954 0 O O O : 0.3939 : 3 O. .0 .0 O. O. O. O. O. .0 0.. .0 O. O. .0 O. O. 00 00 ’ : 0.3832 3 - : 0.3982 + : 0.4002 : : : 0.3412 : 0.3334 0 1 3 O. O 0 003982 3 0.3992 + 3 : +++ : 0.3908 : 0.3892 0 C 0 .0 O. 4.9u9u EUR” . O nan co 0. ea 0.38 : 0.3984 : 003892 3 b? 0 O ' 3 003778 : ? +++ 0 O O 3 0.3372 0.3964 2 0.3612 2 0 1 3 O. Q. 0. 10 O. .0 O. Mwnu RV Oufiu Qun% e e nvnu .0 .0: O. O. O. O. .0 mw O. O. O. .0 O. .0 .3480 :bitter .3442 :sour 03577 : +§§ 0 0 0 0 O O O O O ? ++ +++ O. .0 O. .o n: .onw.i Aeneas «among 0 O O nonunu O. .0 O. 9. + + w + + + O. .0 .0 ounces ...... v m. z.u“9. O O O nonvnv O. .0 .0 n01iau .0 x O. .5 no J zed and Fla- vor 2500# V01‘ Trial 2 . Qgenized at variousppressures - 52 - pper upon the oxidation-reduction pptential and the development of oxidized flavor in unhomogeni milk was hom : Oxidation-reduction potentials and flavor when hpmogenized pasteurized milk after various storage periods The effect of co gt a low temperature. r Table 70 . . 9. . . . . . + . . . . . + . + + . + + + + + + + + + t 00 00 o. O. o. .0 0. O. O. 00 O. O. 00 00 00 00 .0 00 00 O. O. O. O. .0 .0 O. o. 00 .0 O. 00 00 877 688 266 4.72 277 227 4%8 74.6 ems was mam ”m2 was 80m wee awn mw mu mo.emw .bmw.4 svmumw «equmw mwmw . agave. «09392 O o o O o O O 0 o o O O 0 O 0 O 0 o o O O o O O 000 000 000 000 000 000 000 000 o. 90 O. 0. O. O. O. 0. O. O. O. .0 Q0 00 0’ 90 .0 0.. .0 00 00 .0 o. O. o. .0 O. O. O. O. 00 00 . . . . . . . 9. . . + . 9... . . . + e r + a . a . a . u . + + o + s o. O. O. O. o. 00 .0 o. O. 0. 00 O. o. o. O. O. o. O. O. O. o. O. 00 00 00 z 00 .0 90 0. 00 00 920.0. Avmwmm 92923. 4.9~9. n60.9u enouv. 4.Rvmw n:.l.o m a m was aw yea 36% we was memo Remum" EVMHA. «gauze «one mwquu eunumw au1l9~ 000 0.0 000 00. 000 000 000 000 000 000 000 000 000 000 000 OnwO O. 00 0. O. o. .0 00 O. O. 0. O. 00 o. O. 00 O. o. 90 O. O. o. 00 .0 90 O. 00 00 00 00 .0 00 00 . . . . . + . + e . + + . + + N + M + + f 9 f + e + e + e + M M f + + + + e + + e e + + + + 00 00 O. o. o. 00 O. .0 o. O. O. 00 o. O. 0. 0.. 00 O. I. 00 O. 90 O. O. 00 O. O. .0 o. 9. O. 00 9.7. 4.Avnv 92920. 9~4.7. 969292 mwnuv. o.4.m~ mwo.4. 0.92 1i1l.o .O.l.o .4.b.4 R61. 0.4. coco ages 1.Avo. genomw sumwai «30.nv none ”wage. sane Rogue. «vague "0.4 3. 4. «among ,quau «one «any «eaves 00. 00.0 00. 000 .0. 000 00. 000 000 000 000 000 000 000 000 000 o. 00 o. 00 o. 90 O. O. o. 00 00 O. 00 .0 O. O. O. 90 o. o. 00 00 O. O. O. 0. O. 00 o. 00 O. .0 013 013 013 013 01.3 013 013 013 I. 00 O. 00 00 00 00 00 0. .0 00 00 00 o. .0 .0 00 O. .0 O. 00 00 o. O. O. 00 Q. Q. 00 00 O. O. o 1 3 7 0 4 1 m . 1. l 2 - 53 - 13.1.. enized pper upon the oxidation-reduction pptent genized pasteurized milk after various storagp periods n the development of oxidized flavor in unhom £9, 2;; effect of co Table 80 Qg J and u r pLTrial 3 . perature. and homo at a low tem : Oxidation—reduction potentials and : flavor when milk was homogenized : at various pressures Storage :00pper : 0. 2500 added : (Days) :(p.p.m.): : Fla- Eh O. .0 O. : 0.3327 : : 0.3137 : O. O. .0 .0 0.3587 0.3602 0. .0 O. 0. 0.3807 0.3757 13 .0 : 0.3362 : 0 l O. O. 0. O. 1 0.4637 0. O. 61 9mg C.R 9» 3. ..4 000 00:00 . .9 ... ++* 00 .0 0. 1.1.1. mazes 34. 0.0.0 00:: 013 0. : 0.36:7 : Q. 77 PI. L 3 0 0 00 O. O. O. O. 0. O. 0. n0 7 +4“? 2 new A. O. 00 00.0 ... ... ... ++e :0. O. O. .0 O. .0 O. O. 0 1 O. nU.00 O. 0.... 013 0. .0 O. 4 1.. - 54 - ntial ppper upon the oxidation-reduction_pote pon the deve10pment of oxidized flavor in unhomog» The effect of co Table 9. zed eni and u 'Avergge of all trialspl rature. and homogenized pasteurized milk after various storagepperiods at a low tempe ‘ : Oxidation-reduction potentials and flavor when milk : was homogenized at various pressures 2500 .0 00 1500 Fla- : Fle- YO]? YO? Eh VOI‘ Eh .u.u.u .0 0. 0. 0. .d.l.o % 6 «eaumw 0 . 0 .ununu nUnunu .U.u.0 . 0 0 nun0nu 1. 9. no 4. 2 6 9 auuuau 0 . 0 nonunu .0 0. .0 .0 nunu o o m 0 . 0 .un0nu .0 .0 .0 .0 n.127 nvnomW 2 6 «wave. 0 0 0 AUAUAu 00 0. 0. .0 nv1.uu 0. 0. 00 0. nu mm“. ... 000 .0 0. 00 mfio Mia nmnwo. 00 .0 010 .0 00 90a 33 . z 0.00 : 0.3394 : 0 0 : 0.3365 : 0.67 : 3 : 0.4157 : 0.4158 00 0. 0. 3 0. 00 000 o . 002 .0 0. .0 859 mwm nU.0.nv. 0. 0. 00 mmm 002 0. 0. 0.. 82 4.0% 08 333 000 mwm .0 .0 00 0.3518 0. O. 00 0. 0. 007 006 ... 011 00 0. 00 0. 00 000 50 .305 0.. 0. 023 123 0000000000000. 794. 981 new 511 9 333 m: .0. 000 0.0.0. .0 0. .0 .0 .0 0. .0 000 00 055 00 .0. 0.. 223 234. 00 .0 .0 0. 0. .0 00 856 2 2 579 am. 242 .389 333 332 0.. 0. 000 000 ......0...00.0 00 000 50% .900 0.. 0.. 234. 24.4. .0 .0 00 .0 .0 .0 .0 899 188 “.21.. 1% (11 4. 333 332 000 .0 000 000 00.000.00.000. 013 013 000.000.00.000 1 8 2 2 figure 2. Effect of COpper upon the oxidation-reduction potential and upon the development of oxidized flrvor in unhomogenized milk. FLAVOR INTENSITY 0.42 0.30 f— ' ' ' —- ‘ "o ’ — /P. P. M/CO/v 7901. “9’ 0 4 8 /2 /5 20 24 28 DAYS (3 ~\. no (£1 £3 bigure 3. Effect of cepper upon the oxidation-reduction potential and upon the develOpment of oxidized flavor in milk homegenized at 1,500 pounds pressure. n. ,4 VOR IN TEA/SI TV 0.30 0 4 a /2 /5 20 24 28 DAYS figure 4. Effect of copper upon the oxidation-reduction potential and upon the development of oxidised flavor in ‘- milk homogenized at 4,000 pounds pressure. FLAVOR /N TENS/TV -58- of cepper. The rate at which the oxidized flavor increased in intensity was more gradual in the homogenized than in the unhomogenized. However, the oxidized flavor in the homogenized milk containing 3 p.p.m. of added capper was nearly as intense at the end of the storage period as it was in the unhomogenized milk to which c0pper had been added. An increase in oxidation-reduction potential in the unhomogenized, cepper-free milk was accompanied by a delayed deveIOpment of oxidized flavor, whereas,the increase in the potential of the capper treated milk was closely paralleled by the deve10pment of oxidized flavor. An increase in oxidation-reduction potential in homogenized milk not treated with capper was not paralleled by the develOpment of an oxi- dized flavor. There was some parallelism, however, between an increase in oxidation-reduction potential and the deve10pment of oxidized flavor in homogenized milk treated with capper. EggLefggct of removing the fat globule membrane by churning and washing upon the development of oxidized flavor in the remade homogenized milk. In an effort to show the effect of removing the fat globule mem- brane upon the develOpment of oxidized flavor in milk, flavor studies were made on unhomogenized milk, homogenized milk and remade homegenized milk in which the fat had been washed 1, 2 and 3 times as outlined in the procedure. Cepper was added to the samples at the rate of 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 and 1.5 p.p.m. and were studied organoleptically after 1, 3 and 7 days of storage at 35° to 40° F. The data are presented in table 10, with the average results of all trials presented in table 11. An examination of the data shows considerable variations in the susceptibility of different milks to the development of oxidized flavor. The milk in trials 1, 2 and 3, which were run during march and April,were quite susceptible to oxidized flavor development, whereas, trials 4 and 5,which were run during June when the cows were on pasture,was very re- sistant to the deve10pment of oxidized flavor. Oxidized flavor deve10ped more rapidly in the unhomogenized samples than it did in the unwashed homogenized or in the washed homogenized samples. The unwashed homogenized samples were more resistant against oxidized flavor develOpment than the control or the washed samples. The greater susceptibility toward oxidized flavor of the washed samples than the unwashed homogenized samples may have been due to the severe treat- ment, such as separation, churning, washing and homogenization to which the fat was subjected during processing. The samples which were washed once and twice were a little more susceptible to off flavor development than the samples which were washed three times. This seems to be in keeping with the theory that lecithin is the substance which becomes oxidized and causes the oxidized flavor, the removal of which from the milk results in greater stability against oxidized flavor. The addition of cepper increased the intensity of the oxidized fla- vor that deve10ped in the unhomOgenized and the washed homogenized samples. An astringent or puckery flavor was often encountered in the remade milk especially in those samples containing the higher concentrations of copper. The lecithin content of the buttermilk frmm each of the three churn- ings was determined in order to see what relationship existed between the amount of lecithin removed by churning and washing and the deve10pment of oxidized flavor. An examination of the data shows that the buttermilk from the first churning contained the largest percentage of lecithin. The buttermilk from the second churning contained less lecithin than the buttermilk from the first churning, but more than the buttermilk from the third churning, showing that each successive churning and washing lowered the amount of lecithin that remained in the milk. The lecithin contents of the butter- milk frmm the first, second and third churnings were'0.1663, 0.1160 and 0.0914 per cent reapectively. The percentage of lecithin in the fat of the buttermilk varied considerably because of large variation in the fat content of the buttermilk. The average percentages of lecithin in the fat of the buttermilk from the first, second and third churnings were 2.5308, 1.2195 and 0.7337 per cent respectively, again showing a decrease in the lecithin content in the milk due to churning and washing. obule membrane by churnin The effect of remo n the fat Table 10. of oxid zed flavor in the remade temperature. eriods at a low homogenized milk after storage for various 'U C) H 0 +3 :0 c: 0 .21 B d) :3 In «3 3 'd :1 a1 :1 O H .p (U N ...-1 c1 (1) 8° 5% £10 A o” ‘25“ +ab~ C) L0 '36 N as ‘i o '6 01:0 OF «'3 N as 0H 53" 53 H MO gnu. a“ E a E c: H p a i O H 0 > d) '6 g 0.000. I:- m H 94 'U a) 'U N o -H N :1 H m 'U I! 3 o O o '2 .d E! t3 00000...” Washed once J— Weshed three times Washed Twice Unwashed Control COpper added 3 days:7 days:l dayz3 days:7 days 3 1 da 3 days:7 days: 3 days:7 days:l day :7 dayszl day: 3 days m.):1 day Trial I (March 28, 1939) O. O. O. O. 0. ++ ++¢ O. O 0.000.. OOOOOHH r++ 9+ 6. ++ 00 +++ 6‘2 #90 ++§ O O O 0 09+ 64+ +++ eee : +++ e+o 94+ 0+ ++ 00 : +++ +¢+ 0. $100 4'? O. .0 +«b +++ O. .0 OK) O. O. O. O. O. O. O. O. .0 O. +s O. O. O. .0 0. ++ +? +? ++ +# veo :- Ob‘ #4"? It #9 ++ 00 0. 00 O. .0 00 Q. e++ +++ O. O. O. O. O. 0. ++ +++ O. O. .0 +++ ¢++ 3 +++ O O O O I O e+¢ +++ +++ 6 6 I a I O (Continued) Table 10. O. O. O. O. O. 0. ++ 00 O. .7.- . O. .0 .0 : 4» 004 +++ 0. 4+ 0. +++ ++ 00 *- J 19391; June 12 3:. TialIV( O. - 62 O. O. O. 0.0 0.2 O. O. O. .0 0. ff 0. 0. ++ 0.8 1.0 0. ++ 00 O. O. .0 O. 00 0. ID Trial V (June 29,_1939L O. O. O. O. O. O. O. 0.0 O. O. 0.2 0. O. 0.4 O. O. O. O. O. O. 0.6 O. O. 0.8 1.0 O. .0 O. O. O. .0 O. +++ ent ge for various periods at a low effect of removing_the fat globule membrane by churnin and washin u on the develo The Table 1.1 o .— lk after stora genized mi (Average of all trialgl J of oxidized flavor in the remade homo temperature. The develOpment of oxidized flavor in milk subjected to homogenization and washing when stored for l, 3 and 7 days. o O O O O. Homeggnized j2500#) Unhomggenized Copper 0. O. .0 8 added - 53 - m a :9 0 00000200 5 'O O . . 0 O Q .1 "" O OOOOOO +3 c~ 00 o. o. co ’0 00 co .0 .0 0 m E a o OOOOQHD 'U o 00000., P o OOOOOH 'U {a 00 on 90 9‘ 9° 00 00 00 g 2‘ o mfi‘VI‘CDGDtD ”1 'U e 000000 :5 0 000000 a .4 so 00 ., e. co co '9 00 00 00 o. “:2 a <2eaeeee o OOOOHN 0 c- O o. 00 00 co .0 00 00 o. 0. En £3. 9 s <2eeeece '6 r4 F1P4C>C>C3rfl 0 so “a o. O. 00 00 ’9 .0 .0 00 o. 3 5' <3 CD‘o'¢<3<> Pavia: 8 E: are. 00 co co co 0. co :1 ID 0 5* cc ¢>Glc>c>cc<3 'U '0 O ......1 O oOHHHr-{N ll: '52 00 co o. '0 00 00 o. a. ,«o a ,0 e s <2ee.tte QOHOOOr-ca on 00': 00 oo o. '0 00 co .0 co 3. Q) a eeet.et C3‘3m¢\ 7 figure 6. The ascorbic acid content and the development of oxidized flavor in cepper-tneeted raw, unhomogenized pasteurized and homogenized pasteurized milk after various storage periods. FLAVOR /NTEN$/TV -76- was paralleled by the develOpment of oxidized flavor in the capper-treated unhomogenized pasteurized milk, whereas, in the c0pper-treated homogenized pasteurized milk very little oxidized flavor deve10ped even though the ascorbic acid disappeared. Apparently ascorbic acid is not a factor concerned in the resistance of homogenized milk against oxidized flavor development. The data presented in tables 15 and 16 are shown graphically in fig- ures 5 and 6. 2hg_effect of sunlight upon the oxidation-reduction potential and the development of off flavor in unhomogenized and homogenized milk after various storage_periods. In this experiment oxidation-reduction potentials measurements and flavor studies were made on unhomogenized and homogenized milk in an effort to determine the relationship between oxidation potentials and the deve10pment of activated flavor. The oxidation-reduction potential determinations and flavor studies were made on the milk immediately after processing and after exposure to sunlight for O, 15, 30, 45 and 60 minutes, and after storage in the refrigerator for 1, 6, 24, 48 and 96 hours. The data obtained are presented in tables 17 and 18. An examination of the data shows that the oxidation-reduction poten- tials of the samples after exposure to the sun usually decreased on stor- age. Ebwever, in some cases the oxidation-reduction potential increased slightly depending upon the individual sample of milk. The unhomogenized milk not exposed to the sun showed the smallest decrease in potential on storage. The unh0m0genized samples exposed to the sun for 15 minutes showed practically the same trends as did the unex- posed sample. The unhomogenized samples exposed to the sun for 30, 40, and 60 minutes showed considerable decreases in oxidation-reduction poten- tials. The decrease in potential was usually noted at the six-hour ob- servation after being exposed to the sun. A further decrease occurred when the milk was stored for 24, 48 and 96 hours. The average oxidation-reduction potentials immediately after process- ing of the unhomogenized samples exposed to the sun for 0, 15, 50, 45, and 60 minutes were 0.3181, 0.3218, 0.3188, 0.3190, and 0.3153 reapectively, - 78 - decreasing to 0.2513, 0.2413, 0.1398, 0.1217 and 0.1383 reSpectively after 96 hours storage. The oxidation-reduction potential of the homogenized samples exposed to the sun decreased on storage but the decrease was not so great as in the unhomOgenized milk except in the sample exposed to the sun for 15 min- utes when the homogenized sample showed a lower potential than did the un- homogenized sample exposed to the sun for the same length of time. An oxidation-reduction potential as low as -0.20 volts was noted in the un- homogenized milk exposed to the sun, whereas the lowest potential noted in the homogenized milk exposed to the sun was -0.05 volts. . An examination of the develOpment of off-flavor in the homogenized milk shows that an activated flavor develoPed in some milks after only 15 minutes exposure to the sun. Exposure for longer period of time increased the intensity of the activated flavor which could be detected immediately after the exposure period. The intensity of the activated flavor in- creased on storage in all the homogenized samples which had been exposed to the sun. No activated flavor occurred in any of the homogenized sam- ples not exposed to the sun except in the trial 3 which had been exposed to diffused light for several minutes before storing. The off-flavor which deve10ped in the unhomogenized milk exposed to the sun was not a definitely oxidized or a true activated flavor but seemed to be a blend of the two flavors. The off-flavor did not develOp as quick- ly or become so intense in the unhomogenized milk exposed to the sun as in the homegenized milk exposed to the sun for a similar period. The off- flavor that developed in the unhomogenized milk exposed to the sun for 45 and 60 minutes was not much greater in intensity than the milk exposed to the sun for only 15 or 30 minutes. - 79 - The summarized results of the oxidation-reduction potentials pre- sented in tables 17 and 18 are shown graphically in figures 7 and 8. The effect of sunlight upon the oxidation-reduction_potential and the development of off-flavor in unhomogenized milk after various store Table 17. gegperiods. The oxidation-reduction potential and off-flavor develOpment when homo- xposed to the sun for nized milk was s jaggt Storage : 60 Minutes 45 Minutes __ : 30 Minutes 15 Minutes l 0 Minutes O. O. Fla- vor Eh Fla- vor Eh : Fla- vor 0. O. Fla- vor Eh July 1, 1939 : 0.3076 Trial I " : 003261 3 0.3241 : *00 *1! o " : (30:52:19 6 ? 0.3254 : 0.3042 1 0.3381 0.3096 : 0.1606 .0 0.2936 0.2896 0.3506 : 0.3496 : 0.3613 0.0996 : 0.0053 0.3191 : 0.1398 0.3511 : 0.3303 0 O 0 Trial II July 7, 1939 0.2995 : : 0.3007 : ? " 6 002857 - x 0.2750 0.2912 0.1894 - :-0.1283 : - :-0.2153 : - :-0.3406 : - :-0.0122 : 0. 0.3118 ‘. : C’.:3€fl323 3 O O. O O ? 0.1927 0.0092 ? :-0.0820 : + :~0.1264 : *0 :‘001230 3 + 0.0017 : - :‘000224 : " :"C).:32H51. : -001702 : §* : -0.1519 " :"().:LEN3E’ : Q 0275 : 0. 24 48 96 *4 Trial III July 12, 1939 0.3271 : 0.3351 : 0.3409 00 o e OOOOOO O. Q. .. 90°9- 0000.0 00+ 4 O. .0 O. O. O. O. bmmw¢l~ $§$8°8 (0910!) L0 0 O o o o 0 000000 .0 00 00 .0 .0 00 1 I c c e + 00 O. o. 00 .0 o. fi'NOl-‘K‘ to d' to 44 NO’iO @1003 [0531063 0 o o o 0 000°C 00 00 00 00 e. °~0sss *00 Determinations made before exposure to the sun. **0 Determinations made immediately after exposure to the sun. (Continued) Table 17. July 19, 1939 0.3236 : Trial IVl 0.3084 3 " 3 0.2647 0 0 0.3199 "’ 2 003037 : - : 0.3080 : ? : 0.3117 : 0.3131 0.2927 0.2996 0.3271 0.3254 " 3 003278 : ‘ 3 003299 3 0. 0.3200 : 0 O O .0 O. O. 0 0 1 +0 0.2240 : + : 0.2232 " : 001052 - :-0.0003 : 0.2475 3 0.1631 9'" 3'001321 : 4+ : 0.3154 : : 0.3113 : 6 24 *1- 0.3041 : 0.2706 ? 0.3250 : 0.3070 2 0.2272 +9 ? .0 - 81 - 0.2500 0 O 4 “0 o 1562 3 I. 0.2119 . 96 : Trial V July 28, 1939 O. .0 0.3007 : 0.2827 : 0.2952 0 O " 3 003292 ‘ : 003257 7 : 0.3437 9 0 0.3332 0.3337 7 2 0.3412 : - : 0.3437 : 1+ 0.3212 0.3117 0.3272 0.3397 ? : 0.3680 ' 3 003797 0 O O 0 O 0 0.2957 - 3 0.3092 0 O H O. O O .0 t0 ? + + 0.3542 : 0.3637 : ? 0' ? 0.3807 0 0.3472 : 0.3732 : O O O O O 0 trials 181 3 Averag 0.30 : 0.2269 : 0.30 : 0.3072 : 1.00 3 0.2728 0.2724 : : 0.2574 : 1.50 : 0.2621 : 0.80 x 0.1955 0. 0.2868 : 0.00 6 24 48 1.80 : 0.2375 : 0.60 : 0.1921 : 1.00 : 0.2155 : 1.60 0.80 : 0.1383 : 0.1799 1.40 : 0.1217 1.00 : 0.1165 : 1.20 : 0.2559 : : 0.2513 : 1.80 : 0.2413 : 1.00 1.40 0.2599 : O 0 2.00 O : 0.1398 : 96 The effect of sunlight upon the oxidation-reduction potential and the develOR: eriods. ment of off-flavor in homogenized milk after various storage Table 18. The oxidation-reduction potentials and off-flavor develOpment when milk .0 0. Was exposed to the sun for 60 Minutes 0 O 45 Minutes 30 Minutes 15 Minutes 0 Minutes O 0 (hours) Fla- Fla- Fla- Fla- : vor Fla- VOI‘ Eh 701‘ Eh YOI‘ Eh O. Eh VOI' Eh July 1, 1939 : 0.3341 : : 0.3355 : 0.3307 .0.3676 Trial I *00 O ##0 0.3626 0 0.3616 +4 : 0.2846 64- 0.3376 0.3086 0 O O O O O 0 0.3231 24 0.3931 : ? 0.2226 . -0.0367 9- O 0 O O + : 0.1436 " : 002678 0.1243 : 0. 0.2077 0.1452 0.0122 +*§ :-0.C$0 7 4. + f - x 0.3253 : " 3 003462 0.3255 : 0.3453 : 0.3305 : .0 O. 00 Trial II July 7, 1939 4+ 0.3640 ? 0.3494 : +++ : 0.1764 0 O 4. 0.3332 : 0.2262 0.0745 ‘ 3-0.0292 : - :-0.0569 : ++ ? : 0.3137 0.2972 0.1962 6 24 48 96 ++ 0 0 0.1935 0.1281 “0.0124 0.0762 : «6+ : 0.0363 0.1010 + 1‘ O 0 ++ +* :‘000027 +4 :-0.0426 0 O '04-: 0.2335 : 0.3377 Trial III July 12, 1939 '0'? O O o. + +++ +¢ + : 0.2667 ++ : 0.2755 : }++ 00 ? : 0.3472 : ++ : 0.3122 : ++4 +44 0.3349 : 0.3279 0.3325 0.3556 : 0.3851 : +++ +++. +++ 0.3451 4+ : 0.3525 +++ : 0.3499 0.2663 : +++ : 0.2032 0.1761 0.1302 0 0 0.3691 .0 ++ O O O O bf+ : +49 9 0 0.3191 0.3672 : +++ 44+ : 0.3629 : 24 +++ 0.3604 0.2576 0 +4 . O 0 +4". 3 003076 ++ 40- : ¢§§ 3 0.2574 Determinations made before exposure to the sun. ++§ : 0.3095 : ¢++ : 0.2817 : 96 I""‘Determinati ODS made immediately after exposure to the sun. (Continued) Table 18. 7 : 0.322 - : 0.3312 : - x 0.3489 : July 19, 1939 0.3269 0.3225 : 0.3184 : O 0 00 Trial IV 0 1 0.3327 : 7 ++ : 0.3545 7 0.2941 : 7 0.3518 : ++ 003607 3 0.2202 : 0.2127 : 0.2129 x 0.1772 : 6 24 48 96 Trial V +0 ‘H' h? 0.3570 : 0.3097 +4 : 0.1327 ‘9 + ++ : 0.3649 : 1788 044’ 3 00 +++ : 0.0835 : 0.3596 0.3292 0.2184 0 O 0. 0.3426 0. ++ +++ 0 0.2537 : ++ : 0.2184 : -83- July 28, 1939 00 0.3497 : ? ++ : 0.3422 +++ : 0.3542 : 0.3427 7 +9 0.3177 : O O O 0 0.2817 0.2832 0.2952 1 *9 ++ 0.3502 : q. 0. 0.3267 : ++ : 0.3512 : 6 24 0.3662 : 0.3887 : 0.3747 0.3342 4* Q“? +- + 3 *+§ 3 0.3887 0.3837 5+ ++ 3917 : 0.3827 : +4"! . 777 : 0.3797 : 0.3 ++ ... 0.3832 : O O 6 Average of all trials 9 : 0.3411 : 0.00 0.00 0.3318 .0 : 0.3332 : 0.00 0.00 261 '2 1.. : 0.3285 : 0.00 : 00 2.20 2.20 2.60 3.00 3.40 3.00 0.3048 3 1.20 : 0.3452 : 1.60 : 0.3104 0.2994 : 2.20 : 0.2815 2.60 : 0.2774 2.40 0. 2.20 0.2850 0.80 : 0.2888 : 3.20 : 0.2420 : 3.00 : 3. : 0.3158 0.2917 : 0.80 : 0.2766 0.2554 0.3025 : . 0.80 : 0.2655 : 3.20 : 0.2083 : 2.80 : 0.2657 : 24 48 60 : 0.1846 : 2.40 : 0.2046 : 2.80 : 0.1732 : 2. : 0.1894 : 0.80 0.2493 : 0.34 0 24 48 72 HOURS figure 7. The effect of GXpoéure to sunlight upon the oxidation-reduction potential or unhomOgenized milk after Vtrious storpge periods. [H -39- 0.36 \ \.r‘-60 Mil/V. \. x. ‘\ \ /5M//V./\‘\ 02¢ ' “‘ 0.20 0./6 .2 0/0 24 48 72 96 HOURS figure 8. The effect of exposure to sunlight upon the oxidation-reduction potential of homogenized milk after vsrious storpge periocs. - 86 - The develogment of rancid flavors and increases in titratable agigity due to lipolysis in milks made by mixinggraW'milk with homogenized pasteurized, homogenized raw milk with homogenized pasteurized and raw milk with homogenized raw milk after various storage periods In the study of the development of rancidity in homogenized milk the following mixtures of milk were made: 1, unhomogenized raw milk was mixed with homogenized pasteurized at a rate so that samples containing 0, 1, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 95, 99, and 100 per cent of raw milk were obtained; 2, homogenized raw milk was mixed with homogenized pasteurized milk in the same preportions as stated above; and 3, unhomo- genized raw milk was mixed with homegenized raw milk in the same ratios as was the other two series. These samples were titrated for increases in acidityycalculated as lactic acid,and were studied organoleptically for the development of rancid flavor immediately after processing and prepar- ing the various mixtures and after 1, 3, 7 and 10 days of storage at 35° to 40° F. The increase in titratable acidity was determined by subtract- ing the acidity of the raw milk after the various storage periods from the titratable acidity of the‘mixtures after similar storage. flghomogenized raW'milk in homogenized pasteurized milk. The data ob- tained regarding the milks made by mixing unhomogenized raw milk with ho- mogenized pasteurized milk are presented in tables 19, 20 and 21 with the average results presented in table 22 and plotted on figure 9. An examination of the data shows there was a slight increase in acidity in the homogenized milk after three to five days of storage when it contained as little as one per cent of added unhomogenized raw milk. An increase in acidity of the homogenized pasteurized milk containing 5 per cent of added unhomogenized raw milk could be detected in some samples after one day of storage and as the percentage of unhomogenized raw milk - 87 - in the homogenized pasteurized milk was increased a corresponding increase in the titratable acidity occurred until a milk containing 50 per cent un- homogenized raw milk and 50 per cent homogenized pasteurized milk was ob- tained. The maximum increase in acidity occurred when the ratio of unho- mogenized raw milk to homogenized pasteurized milk was approximately one to one. As the percentage of unhomogenized raw milk was increased above 50 there was a constant decrease in acidity from the maximum. Small quan- tities, five and one per cent,of homogenized pasteurized milk in unhomo— genized raw milk, sufficient to produce and increase in acidity after one to three days of storage, seemed more effective in producing this increase than similar quantities of unhomogenized raw milk in the homogenized pas- teurized milk. The largest increase in acidity usually occurred in the samples containing 50 per cent homogenized pasteurized milk at each of the various storage periods. The increase in acidity due to lipase action upon the fat are apparently equally dependent upon the increased surface caused by homogenization and upon the amount of lipase added by the raw milk. A rancid flavor could be detected in some samples of homOgenized pasteurized milk containing one per cent of unhomogenized raW'milk after 7 to 10 days of storage. When the samples contained five per cent of un- homogenized raw milk the rancid flavor developed more readily. A.further increase in the percentage of unhomogenized raw milk added to the homo- genized pasteurized milk caused a more intense rancid flavor to develop. All the samples containing from 10 to 90 per cent of unhomOgenized raw Milk deve10ped a very strong rancid flavor especially on storage. When the sample contained less than 10 per cent of homogenized pasteurized milk in unhomogenized raw milk the intensity of the rancid flavor decreased. - 88 - Homogenized raw milk in homogenizedgpasteurized milk The data obtained by mixing homogenized raw milk with homogenized pasteurized milk are pre- sented in tables 23, 24 and 25, with the average results presented in table 26 and shown graphically in figure 10. An examination of the data shows that the presence of one per cent of homogenized raw milk caused a slight increase in the titratable acidity of some samples after 7 to 10 days of storage. When five per cent of ho- mogenized raw milk was present definite increases in acidity could be de- tected after the first and third day of storage. As the percentage of homogenized raw milk was increased the titratable acidity increased, with the samples containing 100 per cent of homogenized raw milk showing the largest increases. The acidity increased slightly more rapidly than in the samples studied previously where the raw milk was not homogenized. This is reasonable because all the fat in the latter mixtures had been sub- Jected to homogenization and therefore most of the fat globules would be decreased in size, thus,there would be more surface upon which the lipase could act, whereas, in the samples composed of unhomOgenized raw milk and homogenized pasteurized milk only a part of each sample had been subjected to homogenization. Consequently, all the fat globules had not been de- creased in size so there was not as much surface exposed to the activity of the lipase. Another important difference between the milk composed of unhomogenized raw and homogenized pasteurized milk and the milk com- posed of homogenized raw milk and homogenized pasteurized milk is the fact that in the former the maximum acidity occurred when a l to 1 mixture was present after which the increase of titratable acidity decreased; whereas, in the latter the acidity reached a maximum when the sample consisted of - 89 - 100 per cent of homOgenized raw milk. The develOpment of a rancid fla- vor closely followed the changes in titratable acidity. ggppmogenized raw milk in homogenized raw milk. The data obtained regard- ing the develoPment of rancidity in mixtures of unhomogenized raw milk and homogenized raw milk are presented in tables 27 and 28, with the average results presented in table 29. An examination of the data show the same general trends in the de— ve10pment of a rancid flavor and increases in acidity as in the milk com- posed of homogenized raw milk and homogenized pasteurized milk,except that the acidity increased slightly more rapidly as the percentage of homogen- ized raw milk in unhomogenized raw milk was increased up to approximately 50 oer cent, after which the increase in titratable acidity was not so rapid. The average results in table 29 are presented graphically in figure 11. vor 44 444 +++ 10 days Fla- : Acidity: Fla- : Acidity: Fla- Ml 0.0000 : 0.0000 0.0245 : 0.0378 : 0.0512 : 4. 4-4- 7 04‘" Hugh) vor 7 days (fl 0.0000 0.0000 0.0111 0.0222 : 0.0378 0 O O 0 O ? VOI‘ ++ 3 days Acidity (% - : 0.0000 : 0.0102 : 0.0348 : 0.0041 : 7 : 0.0225 : O O O O O O O Increases in acidity and rancidity after + 1 day Acidity: Fla- E 4%) 0.0000 0.0000 0.0041 0.0061 : O O C O O O 99 95 90 a) 1m The develoyment of rancid flavors and increases in titratable acidity due to lipolysis in milk made by mixing_unhomogenized raw milk with homogenized pasteurized milk after various storage periods. Hemo. :Pasteurized Table 19. Sample (E_ 10 Raw Unhomo. +0+9+ +4-094- +++§f 000000000. 29823 oRgss OO 00 00000 O 000 O O O O O O O O O O 444 444 +44 444 444 O. O. C. O. O. d'EJD'UD§~ racecocuco tntoroao‘o (DCDCDCDCD O O O 0 O c><>c><3<3 O. O. O. O. O. 4 4 4 4 4 4 4 4 4 4 + 4 4 4 4 O. .0 .07.. O. H H (\2 Q‘ ‘1' K>s>ozb-b- <4u> c>c>c>c>c> O O O O O <3<3c3c>c3 O. .0 O. .0 O. 4 4 4 4 4 + 4 4 O. O. .0 O. O. q)b-Q)b-b- d'G102d'di OIOIUDFDU) O O O O O O O O O 0 c>c>c>c>c> O. O. .0 O. .0 (Dc) (3 smBSn O. O. .0 O. O. (3 (3 2888s 4‘ 444 q. ? 44 0.0589 : 0.0311 0.0156 : 0.0000 0.0000 : ‘ e e e o e o o 444 444 ... ? 0.0534 : ¢ 3 000222 3 0.0000 : 0.0000 : ? 0.0067 4'0- 0 O 0 0.0102 : 0.0041 0.0000 0 O O 4 : 0.0225 0'!- : 000471 3 ? O O O : 0.0307 : 0.0144 0.0041 0.0041 0.0000 : m m 5 1 o 80 90 95 99 100 10 days Fla- : Acidity: Fla- 7 days Acidity Fla- . 3 days Acidity Increases in acidity and rancidity after 1 day Pasteurized: Acidity: Fla— O O polysis in milk made by mixing unhomogenized raw milk with homogenized pasteurized milk after various storage periods.4_jTrial 2L4 The development of rancid flavors and increases in titratable acidity due to 11 Homo. Sample Raw Table 20. Unhomo.: -91.. .4444 +4444 444:: H 4 44 4 44- O 4 44444 p .0 O. 0. O. 0. O. O. .0 O. O. .0 .0 O. O. O. .0 002000 fl'HOO'bd‘ oer-moo A Od‘NNt‘ ‘OIQNEQ N'DCOCDO E OONd‘ln 405(1) 40 dim-#00 00000 00000 00000 00.00 .00.. 00.00 00000 00000 00000 0. O. O. O. O. .0 O. O. I. O. O. O. .0 .0 O. .0 I I o-4 4 4 4 4 + 4 4 4 u c u H 4 44444 4 0 44444 l> O. O. O. O. O. O. O. I. O. O. O. .0 O. .0 O. I. tor-raga: 80:30.0: 05 b A duo‘s (D (0me 00 «38 3a 80mm¢.mwwmm WNHO v 0000 00000 00000 .0... 0.0.. .0000 00000 00000 00000 9. O. O. O. O. O. O. O. O. O. O. .0 O. O. .0 O. Iota-o4 44444 4440: h 4444+ 4 o 4444 p I. .0 O. O. 0. O. O. O. O. O. I. O. O. O. O. .0 l‘O‘DCng' Fbmmm $053030 A tomh c0 town—{Hg Nd't‘DCDO coo-402:0 demon «cur-'00 00000 00000 00000 00... 0.... .0000 00000 00000 00000 O. O. O. O. 0. O. O. O. .0 .0 0. .0 O. O. .0 00 44404 44444- 44c... 44 44.44 4- o p O. O. O. .0 00 O. O. O. Q. .0 .0 O. O. O. O. O. n our-Hood. HCOHHO ... ggngfl mr-Hotono Ht‘v-IHO 35 OOHHN Neal-onto nr-IHHO 00000 00000 00000 .0... 0.0.. .0... 00000 00000 00000 .0 O. O. O. O. O. O. .0 .0 O. O. O. .0 .0 O. O. A 05000 00 00044-0 s2 gm mo) €0,933 N 0. O. .0 O. O. 00 00 O. O. O. .0 .0 O. .0 O. .0 a OHIOOO 03000 001000 at HN t0 tomb 030304043 The development of rancid flavors and increases in titratable acidity due to lipolysis in milk made by_mixipg unhomogenized raw milk with homo en Table 21. ed 1 (Trial_31 pasteurized milk after various storage_periods. .0 Increases in acidity_and rancidity after Sample 10 days 7 days Fla- : Acidity: Fla- : Acidity: Fla- : Acidity: Fla- C21. 1 day Hemo. : Unhomo.: 2 VOP _lfidi ’ : 000000 3 VOP V0? .13) vor : 0.0000 (%1_ : 0.0000 0.0000 0.0000 0.0022 0.0156 (20 Pasteurized: Acidity 100 O O Raw _1131_ : - 92 - 0.0000 0 O O 0 0.0000 0.0022 0.0178 ? : 0.0289 0.0000 : ' : - : 0.0045 : 0 O O O 99 95 90 80 C>F4 + 4+ 4+ 0.0311 : O C 0. 0.0534 : 0.0667 0 O 4’ 0* 0.0200 : 0.0289 f 10 20 0.0489 ++ O O 0. 0.0801 : +44 +44 : 0.0734 : +44- +4 : 0.0667 : 44+ : 0.0712 : 44+ : 0.0801 : + : 0.0445 0.0245 : 0.0311 0 0 4+4 +++ +44 +4 040823 : +++ +++ 0.0512 4 : 0.0534 : O O ? 0.0400 : : 0.0378 : O 60 50 0.0712 : 44+ : 0.0712 0.0778 +44 : 0.0689 : 3 fff: 0.0556 0.0534 + + 00 § 0 0.0311 : O 0 30 +4 +4 0.0512 + +4 : 0.0378 : 0.0467 : 4+ 0.0556 : +4 : 0.0356 4 : 0.0423 : 0.0222 § 4 0.0289 : x 0.0222 : 2O 0 O 10 90 95 99 100 0.0289 + 0.0356 : - : 000068 0 0 0.0134 0.0022 0.0000 0 O 5 0.0000 : 0.0000 : O 0 0.0000 : O 2 Apment of rancid flavors and increases in titratable aciditypdue The develo Table 22. :- to lipolysis in milk made by mixing_unhomogenized raw milk with homogenized (Average of all trials) pasteurized milk after various storagegperiods. Increases in acidit Sample y;and rancidity after i O. 0. e. L 10 days -Acidity: Fla- : 3 days 1 day Hemo. : Unhomo.: VOI‘ 131 0.0015 : 0.00 : 0.0000 0 O 0 O Acidity: Fla- 9 O O O Fla- vor Acidity: (31, z (90 2 Pasteurized: Acidity: Fla- Raw _l%Q_ - 93 - 0.00 : 0.00 : 0.0022 : 0.00 . 0.0015 : 0.0015 : 0.00 : 0.0044 : 100 .0 0.00 : 0.0030 : 0.33 : 0.0014 : 0.57 0.33 1.00 : 1.00 0.0181 : 2.00 : 0.0323 : 2.67 : 0.0452 : 99 95 2.67 2.67 3.67 0.0225 0.0178 . O 0 0.0058 : 0.00 : 0.0109 : 0.0087 0.0444 : 3.33 : 0.0585 : .33 3 0.0297 : O 0 0.0223 : 0.33 : O O 90 80 10 20 O. 3.67 4.00 4.00 4.00 : 0.0733 : 4.00 0.0699 : 0.0740 3.33 4.00 4.00 3.00 : 0.0500 : 2.00 : 0.0454 : 0.0260 : 70 60 30 0.0675 2.67 : 0.0512 : 4.00 : 0.0712 4.00 : 0.0503 : 2.33 : 0.0303 : 0.0814 2 .0 0.0361 : 0.0360 0.0331 0 5O 4O 30 O. 50 60 70 0 0.0697 4.00 : 0.0682 : 4.00 : 0.0711 : 2.67 : 0.0547 : 4.00 4.00 O 0 0.0532 2.33 : 3.00 : 0.0497 : 3.33 : 0.0540 : 3.67 O. : 0.0303 : 2.57 : 0.0454 20 10 80 0.0312 : 3.33 1.33 : 0.0192 : 3.00 O O 2.00 : 0.0304 : 3. 0.33 : 0.0200 : 0.0275 : 0.67 : 0.0153 : 0.0181 : 2.00 . 0.0095 90 95 O O 0.33 0.00 0.0029 0.0000 : 0.33 . 0.00 0.0045 0.0000 0 I 0.00 : 0.00 O 0 0.0073 0.0000 0.0058 2 0.00 : 99 100 0.0000 : 0.00 : 0 ACID/TY (PER CENT) -94- 0./4 01? 0./0 I I a. 0‘ ~ Q a.-. ‘. 0.619 ,x'~\,/—/0 DA rs \ ” .... .\ I 0 1- 0“ ' -"‘\—70AV5‘ 0.05 ’ I /00 80 60 40 20 0 HOMO. 10.435de 0 20 40 60 80 /00 UNHOMO. RA W06) ' Figure 9. The increases in acidity erter differ- ent storEge periods when unhomogenized raw milk wee added to homogenized pasteurized milk in different proportions. _pment of rancid flavors and the increase in titratable acidity*due to lipolysis in milk made by mixing homogenized raw milk with homogenized The develo —: Table 230 .L‘. f Easteurized e periods. lerial 1} E r milk after various stora Increases in acidity and rancidity after Sample O. 10 days Acidity : 3 days : 7 days 1 day 0 day Acidity Ebmo. Hbmo. Fla- 0 O dity: Fla- : Acidity: Fla- : Acidity: Fla- Aci Fla- 0 O O 0 Raw :Pasteurized V01‘ 0 O 331 0.0000 : 0.0022 . : 0.0200 : 1%Q#»: vor 0.0000 .0 701‘ (31 0.0000 "’ 3 000000 (31, 41%) vor : 0.0020 : (3) 1%) : - 95 O O o 0 O O 100 " : 000000 0 0.0000 0.0082 0 99 95 90 80 ? 0.0178 0. O # 0.0062 7 : 0.0369 0.0062 0.0102 0 O O O 0. 0.0378 : 4+4 +++ : 0.0534 : +++ +4 0.0289 : + : 0.0400 0 0 0.0062 : 0.0082 0. 10 O 0 0.0225 : 20 +++ 0.0623 : O 0 4+4 +++ ++ : 0.0534 : 4+ : 0.0756 : 3 0.0451 : ? «0‘ 4'4 +++ "’ 6 000348 0.0103 : O O 70 60 50 40 30 4‘1"} +4"? 0.0823 : 0.0912 0.1001 0.1112 0 0 0.0531 0.0615 0.0676 0.0800 0 0 0.0144 : O 0 ++ : 0.0823 : o++ : +++ 0 0 0.0451 0.0451 - : 0.0574 : 0.0185 : : 0.0205 .0 +++ +1”? 9 0 0.0867 : ++ : 0.1023 : 1"}. O O 2 O O +++ **+ 0.0185 : O Q 70 +++ : 0.1179 : +++ +++ : 0.1290 : 4++ +¢+ : 0.1290 : +++ : 0.1023 : ++e : 0.1090 : +++ : 0.1156 : ’T" : 001201 3 0.0840 : 4++ : 0.0902 : +44 : 0.0963 : +++ : 0.1004 : +++ - : 0.0531 : 0.0205 : 20 10 80 0.0246 : 0.0205 0.0226 0.0205 90 +++ #1“? +44 0 0 0.0635 0.0717 0.0758 0 9 95 : 99 100 0.1334 : 0.1334 +++ *1"? O O O O O O : ¢++ : 0.1156 : : +++ : 0.1004 0 O O O 0 ‘4— The deveIOpment of rancid flavors and the increase in titratable acidity due Table 24 o to lipolysis in milk made by mixingg homogenized raw milk with homogenized (TrialggL pasteurized milk after various storage periods. Increases in acidity and rancidity after Sample 3 days 7 days 10 days : 1 day 0 days Homo. H0mo.: 2 vor (%0 0.0000 : 2 V0? 031 - : 0.0045 : V0? (3) 0.0067 110 0.0045 (3) 0.0000 0.0000 Raw :Pasteurized 2 Acidity: Fla- : Acidity: Fla- : Acidity: Fla- : Acidity: Bla- : Acidity: Fla- (3) vor _£%) : 100 - 95 - 0.0042 : 0.0197 0.0353 ? ff #0 0.0089 : ? 2 000245 2 99 fé *2 0*? 0.0178 0.0245 0.0089 0.0133 0 0 0.0041 : 0.0021 0.0082 95 90 80 O 0 0.0334 : 0.0512 ? 9+ 6 O O 10 : 20 0.0531 : 0.0356 : O O O O O O §§§ r++ : 0.0709 : 0.0623 : +*+ : 0.0823 : 2 ++ 2 0.0512 7 : 0.0512 : O 0 ? O 0 0.0267 0.0445 0.0102 : O O 70 60 50 40 30 30 40 50 5 + 6 4. +++ 0.0842 : O O +9+ 00+ 0.0102 : : 0.0164 : 0 if 0.0953 : ++h : 0.0976 4+0 : 0.1109 : 0.0867 2*. 2 001001 2 0+? 0.0623 : I. ft 9* 0 0.0623 : 4+ : 0.0712 0.0467 : 0.0489 0 0 0.0123 0.0184 <3 ‘0 O 0 0.1198 : +++ 0.1243 0.1309 «+4 : 0.1332 : 0.1067 : 00+ : 0.1134 0.1179 4+4 : 0.1179 : 95+ - : 0.0578 : rt : 0.0778 : : 0.0205 : 20 10 80 : ft? ++§ 2 : ¢++ 2 #94 0.0823 tf 0.0600 : 0.0623 - : 0.0689 : 0.0205 : 90 : *0. 00+ +++ : 0.0845 : +++ : 0.0867 : 2* O 0 0.0225 : : 0.0184 : 95 : 99 : 100 : : +++ : 0.1179 : ++t : 0.1354 4+4 0.0845 2 *ktp2 0.0689 0.0205 0 0 vor 10 days Acidity: Fla- : Acidity: Fla— : Acidity: Fla- : Acidity: kla- : Acidity: Fla- (31 - : 0.0000 : V0? 7 days (40h 0.0000 : 0.0000 (Trial 3 : vor 3 days nized raw milk with homogenized L 031 O 6 1 dayg, vor (3) 0.0000 Increases in acidity and rancidity after pment of rancid flavors and the increase in titratable acidity dug 0 days _fi$)_ : vor 0.0000 : 0.0000 0.0000 0.0000 : 0.0000 : O O O O pasteurized milk after various storagegperiods. to lipglysis in milkgmade by mixing_hom The develo _—‘ .13) I'IOHIO o Pasteurized Sample Raw _i%) Hamo. Table 25. 100 4+ +4. +++ +++ +++ +++ ++ 0.0067 0.0311 0.0512 ++r : 0.0778 0.0934 v++ : 0.0978 : +++ : 0.1112 : O O O O 0 ++ +++ : 0.0867 : 1%? 0.0245 : 0.0356 : 0.0578 0.0712 .0756 +++ 2 0.0867 2 ¢++ : 0.0934 : ++4 : 0.1045 : O O O O O O ? O 4. ++ ... O O O O C O 0 0.0000 0.0045 0.0278 0.0311 0.0445 0.0534 : 0.0667 : 0712 +4 : 0.0778 : 4 9 ++ ++ : 0. '0? O O O 0 0.0000 2 0.0000 0.0067 0.0156 2 0.0311 0.0378 - 2 0.0445 : 0.0044 : 0.0111 : 0.0067 : O O 0 O O 99 95 90 80 60 50 20 001134 9 O O +r+ 0.0512 2 0 E? *Qf 0'9"? ¢++ § t? +++ 1379 0.1379 0.1223 : +4; : 0g1357 : +++ : 0.1201 : ' *2? +++ 2 0. ti"? Z..— 0.1112 1312 0. ‘04 : 0.1134 444-: 0.1201 : 44+ : 0.1245 : +++ +++ 0.0867 : +++ : 0.0912 : 04+ : 0.0978 : 90+ : 0.0934 : 004 : 0.0823 : +++ 0.0534 0.0601 - : 0.0623 : 0.0623 : " : 0.0755 0 : 0.0156 : 0.0133 : 0.0245 0.0267: 20 10 5 80 90 95 99 100 The development of rancid flavors and the increases in titratable acidity due to lipolysis in milk made by mixing homogenized raW'milk with_homogenized Table 26. gage period., (Average of all trials rious stor t pasteurized milk after va Increases in acidity and rancidity;after Sam 1e 0. O. O. 7 days 10 days Acidity: Fla- 2 Acidity: Fla- 2 Acidity: Fla- : Acidity: Fla- :.Acidity: Fla- 2 3 days 2 1 day 0 days Homo. RaW’2Pasteurized Homo.: VOI‘ 1(3) 0.00 : 0.0022 : 0.00 : 0.0014 : 0.00 : 0.0000 : 0.00 O 0 V0? 2 (30 V0 1' : (3) vor (3) 0.0014 (30 vor 0.0007 : 0.00 0.0000 0.0041 0.0027 0.0055 (30 _i3d : 100 98 - 983200 0.0000 : 0.00 : 0.0030 : 0.00 : 0.0044 : 0.6 0.0014 2 0.00 0.00 : 99 95 90 80 0 SW 0236 3.00 : 0.0414 : 0.0222 : 1.66 0.0326 : O 0.00 2 0.0095 2 1.33 0.00 2 0.0256 2 1.66 0.00 2 0.0050 2 3.3 .0 0.0101 . 0.00 2 0.00 10 4.0 0 0.0614 3.6 0.0497 2 0.0219 2 1.33 2 0.0345 2 2.67 O O 20 2 0.0469 2 3.00 2 0.0623 2 4.00 2 0.0733 2 4.00 3 2.00 2 0.0525 1.3 0.0288 : 0.0083 : 0.00 30 2 40 4.00 2 0.0866 2 4.00 4.00 4. 0.0778 : 0.0852 0.0912 3.67 2 .0 0.0390 : 0.0425 0.00 : 0.0462 : O. 2 0.0119 2 0.00 60 50 40 30 0.0948 2 4.00 0.1029 2 67 O : : 2.67 : 0.0635 2 3 0.0138 2 0.00 0.0139 50 2 4.00 O O O 0 3 : 0.0670 : 3.67 (:0 2") 0 0.1118 : 4.00 .0 3.67 : 0.1023 : 4.00 O. O L‘ 4.00 2 0.1193 2 4.00 4.00 2 0.1067 0 2 3.67 2 0.0814 : 0.0188 : 0.00 : 0.0538 20 10 80 0.1119 : 4.00 : 0.1252 : 4.00 4.00 0.0605 2 3.67 2 0.0864 . 0.0627 2 0.0676 2 4.00 0.0195 : 0.00 : 90 : 6 : 4200 0.0950 2 4.00 2 0.1208 2 4.00 2 0.1348 2 4.00 0.0928 CV 4.00 2 0.1178 2 4.00 2 0.13 0.0907 2 3.67 2 O O O 0 0.0225 2 0.00 0.0196 95 : 2 0. O O 1 0 99 : 100 : 0,1348 4.00 4.00 : CO 0.1 16 4.00 O O 4.00 0.0734 0.0226 2 0.00 —— ACID/TY’PER CENT! OM 012 0./0 /0 DA mm." ,- .1 '.l’ /./.,\‘7 DA VS . ’I’ ' /'0 O. ’0’ ./. o I ./ (7(38 ,' .fl . .1 . I, o, a. .’ --13 0A Y5 ./ o l '0' ’ id 00 .l. -—< l. e e- "' 0.0? ' . ' e—-""—'/. " ./0——-——’/\2 HOURS ii I l 00 /00 80 60 40 20 0 HOMO. PAS T. ("/01 O 20 40 6O '80 /OO' HOMO. RAW(%’ Figure 10. The incre:ees in acidity After differ- ent storage periods when homogeni?ed rrw milk was added to homegeni7ed pasteurized milk in different preportions. genized raw milk after increases in titratable acidity due to ilk with homo ‘— mixing_raw m (Trial l)__ Increases in acidity and rancidityoafter #17 milk made b ent of rancid flavors and _._._‘ _ "1 polysis in _l_. pm various storage periods. The develo 11 Sample Table 2 7e 8 V01‘ 10 da : Acidity: Fla- : Acidity: Fla— : Acidity: Fla- : Acidity: Fla- 130 V01‘ 7 days (3) 0.0000 - : 0.0111 vor 3 days (30 O O O 0 d2! 1 (3)_ : vor 0 O .. Unhomo. Raw (30 Homo. Fat 130 O 0 0.0000 0 O 100 0 0.0512 : 7 : 0.0400 0 0 0.0067 "’ : 000267 2 O O O 0 0.0067 0.0111 0.0267 - : 0.0378 : 0.0000 2 99 95 90 80 O. .... - 100 - .0 0.0378 : + : 0.0445 7 O 0 0.0000 : 0.0000 : 0.0000 : O 0578 : ++ 2 0.0867 0.0336 : 4"? 0. f 10 ¢++ 0001 4: he 0 Cd +++ 0.1134 : : +++ 2 0.0889 0.1045 0.1067 0.1156 +++ 2 0.1179 2 O 0 +++ 78 : 0.0712 0.0778 0.0956 +++ : 000890 2 If.) 0.0 70 60 59 49 0.0045 : 0.0067 : 0.0067 00+ 0+4 + 0.1134 : 0.1290 4+4 : +++ +++ 2 0.1179 O O +++ 2 +++ O 0 ‘++ 0.0489 : 0.0578 : O. O. 9 .0 O O O 0 4'4 0 O (0 ++ +++ +++ +++ +++ +++ +++ 0.1312 x 0.1423 : +++ : 0.1334 : +++ : 0.1357 : 0.1334 : 0.1357 : O O O O +++ +++ +++ +++ 12 "l w 0.1 O O 0 +++ : 0.1245 : +++ : 0.1268 2 +++ : 0.1290 : +4+ 2 0.1245 2 +++ ++¢ O O O O O O O 0 0.0845 0.0934 +++ : 0.0912 : +++ : 0.0962 2' 0.0934 0 O O O 0 ++ +++ +++ +++ 0.0512 : 0.0578 : 0.0601 : 0.0578 : "’ 2 000645 2 0.0645 - : 0.0756 0 e 0 0.0111 0.0156 0.0178 0.0178 : 0.0200 : 0.0200 : 0.0267 : 20 10 0 60 : 80 90 95 : 99 : 100 : e 19 The development of rancid_flavors and increases in titratable acidity du Table 28. lipolysis in milk made by mixing_raW'milk with homogenized raw milk after various storage periods. _(Trial 2) Increases in acidity and ranciditypefter Sample O. __.—..._. 10 days Acidity: Fla- 7 days 3 days Acidity: Fla- : Acidity: Fla- 2 Acidity (3) 1 day 0 days Acidity i3) - 101 - (3)p_: vor Fla- vor (3) - : 0.0000 : O. vor _(3) - : 0.0000 : Fla- vor 0.0000 2 0.0000 2 000000 2 100 O. O O 0 0.0000 2 0.0111 0.0045 : 99 O. .... 4' .4 +0 7 : 0.0156 : 0.0000 : ? 0.0067 2 0.0200 0 0 0.0067 : 0 95 90 80 2 0.0267 2 0.0467 .5 2'? 0.0133 : + : 0.0356 : + O O O O ‘9 0.0133 2 0 0.0089 : O. 10 O 0 0.0423 2 7 0.0089 : O. O (\1 4. 44 0.0734 : 0.0845 : +44 : 0.0934 : 44+ ++ 2 2+4- 0 O O 0 0.0600 0.0734 0 O O O 4 444 444 : 0.0867 : 0734 2 0.0600 : 0.0712 .5 4+ 04 0.0400 0.0467 0.0534 : 0.0111 0.0111 : 0.0156 : 0.0178 : 70 60 50 : 444': 0.0956 : +44 444 : 0.1023 : 444 0.0845 0*? 6 000934 2 : 0.0845 : 4+4 : 0.0934 44+ 444 40 30 60 70 O O " : 0.0556 0.0222 : O O 444 : 0.1023 : 444 444 : 0.1112 : +44 +44 : 0.0934 : 444 : 0.0978 : 0.0934 0.0912 44+ : 0.1001 0 0 M94 " 2 000623 2 20 2 0.0178 2 10 80 4+4 0.0600 2 0.0600 0.0200 0.0289 0.0267 0.0267 0. O at +44 4+4 0.1112 : 0.1112 44+ : 0.1201 :_444 +44 : +44 0 0 0.1001 0.1023 +44 : 0.0912 : O O 444 4+. 95 9 0 0.1023 : 0.1023 1*? O O 1 0 99 100 444 0.0645 2 1;due to nized reW'milk after SE1 and increases in titratable acidit e n? .d a «P-fl «g h .p .Mud FiFi -g m 1. B o E m m ”29 O a H P-H 0 31:2; Has—4 '6 h -H.9 . o m a 0'6 arc o “ 2'2 4+ 0 0.! d -P:§ o a 29 a a H .H O o -P .4 m m 2'8 m o .348 o °'§ Jflfifiw 54.4 > 0 as oz 0 F4 .9 w 5* Increases in aciditx_and rancidity after Samgle 10 days Fla- : Acidity: Fla- _j%Q O O S 7 ds Acidity 1%) 3 days 1 day Fla- : Acidity: Fla- 1% vor 0 days vor 0 O Fle- vor dity 120 0.0000 Aci vor Acidity ($1 *1. O O Unhomo. Homo. - 102 - 0.00 2 0.0000 2 0.00 0.0000 2 0.0033 : 0.00 : 0.0056 : 0.00 0.00 : 0.0111 : 0.00 : 0.0167 : 1.00 : 0.0189 : 1.00 : 0.0278 : 2.00 0.00 2 0.00 2 0.0000 : 0.0033 : 0.00 0.0000 2 0.00 2 100 2.50 0.0289 2 0.00 0.0423 2 : 2.00 : 2.00 : 0.0289 0.0268 : 2 0.0200 2 1.50 : 0. 0. : 0.0022 0.0033 0.0045 99 5 90 80 O. C. 0. F1“) 10 50 0.0667 2 .00 RR 3.50 4.00 0.0934 : 0.0990 4.00 2 0.1112 2 4.00 3.50 4.00 : 3.50 2 0.0750 0.0589 2.00 2.00 0.0423 0.0478 0.0555 0.0523 0.00 2 000567 2 0.00 : 0.00 0.00 0.0145 2 0.00 0.0189 0.0078 0.0089 0.0111 70 : 60 50 40 30 0.0890 : 0.0967 4.00 : 0.1001 4.00 : 0.0723 : 2.00 2 0.0745 2 O. O O 00 . 50 0.1067 2 4.00 0.1168 4.00 2 0.0901 0.0912 2.00 : 4.00 : O O O O 60 70 4.00 0.1056 2 4.00 2 4.00 : O O 30 0.1223 : 4.00 4.00 : 0.1030 : 4.00 4.00 2 0.0900 0.0178 : 0.00 : 0.0612 : 0.00 0.0589 0.00 : 0.0623 : 0.0190 0.0245 20 10 80 0.1225 2 4.00 0.1123 2 4.00 0.1145 4.00 0.0923 2 4.00 4.00 90 95 0.1234 : 4.00 4.00 2 00 2 0.0936 : O 5 .0 ONO A C/D/ TY'PER CE/VTI OH 0 ./Z 0./O 0.68 006 I /00 80 60 40 / O\./. /,/' \2 HOURS o—-—‘ ' 20 0 UNHOMO. RAW I %' 0 20 40 60 60 /00 HOMO. RAW !%’ figure 11. The increbses in acidity efter differ— ent storage periods when unhomOgenizec rew milk wes added to homogenized rem milk in different prOportions. - 104 - DISCUSSION Milk from different sources studied in this experiment showed vari- ations in oxidation-reduction potentials immediately after processing and after storage for various periods. The potential of the milk in- creased on storage and reached a maximum on approximately the fourteenth day of storage after which there was a tendency for the oxidation-reduc- tion potential to decrease. Hewever, Tracy et a1 (1933) found that fresh- ly drawn milk had a tendency to go toward reduction on storage for 24 hours, but they did not carry their work over long storage periods. There was no significant difference in the oxidation-reduction potential of un- homogenized and homegenized immediately after processing or during the storage periods. This is in agreement with the work of Tracy et a1 (1933). A rise in the oxidation reduction-potential during storage of the un- homogenized milk was accompanied by the development of an oxidized flavor. Hewever, an increase in oxidation-reduction potential in the homogenized milk was not accompanied by the development of an oxidized flavor. The stability of homogenized milk against oxidized flavor cannot be explained on an oxidation-reduction basis. The homogenization of pasteurized milk at a pressure of 2500 pounds was sufficient to prevent the development of oxidized flavor in.milk stored for as long as 35 days. An oxidized flavor deve10ped in only a few of the homegenized samples which were studied. These results are in harmony with the work of Tracy et al (1933), Doan and Minster (1933), Ross (1937) and Trout and Gould (1938) who usually employed storage periods of only 48 to 72 hours . - 105 - The addition of cOpper to milk increased the oxidation-reduction potential of both the unhomogenized milk and the homogenized milk. The potential of the capper-treated milk increased rapidly on storage, reach- ing a maximum potential after one day of storage. After reaching the maximum the potential had a tendency to decrease. There was a close relationship between an increase in the oxidation- reduction potential and the deve10pment of oxidized flavor in the unhomo- genized c0pper-treated milk. The relationship was not very close between the develOpment of oxidized flavor and an increase in the oxidation-re- duction potential of cOpper-treated homogenized milk, again showing that the flavor stability of homogenized milk cannot be explained on an oxi- dation-reduction basis. The exposure of unhomogenized milk and homogenized milk to sunlight resulted in a decrease in oxidation-reduction potential. Whitehead (1931) observed similar changes in the oxidation-reduction potential when milk was exposed to the sun. An activated flavor developed more rapidly in the homogenized milk on exposure to sunlight than in the unhomogenized milk similarly exposed. The fact that the oxidation-reduction potential of the unhomogenized milk decreased as a rule when an oxidized flavor developed, whereas, the potential of milk decreased when an activated flavor deve10ped, seems to indicate further that there is a distinct difference in the oxi- dized flavor and the activated flavor. The removal of the membrane from the fat globule by churning the cream and washing the fat with water seemed to increase the stability of the remade homogenized milk against oxidized flavor development as compared with the original unhomogenized milk. The remade homogenized milk in which - 106 - the fat had been churned and washed three times was more resistant to oxidized flavor deve10pment than the remade milk in which the fat had been churned and washed only one or two times. The probable reason for the greater stability of the samples washed three times was the removal of most of the lecithin from the milk by the churning and washing. Con- siderable evidence has been presented in the literature indicating that lecithin is the substance which becomes oxidized and causes the oxidized flavor, and the removal of the lecithin from the milk results in a milk more stable against oxidized flavor. The lecithin content of the butter- milk from each of the churnings decreased with each successive churning showing that some of the lecithin was removed from the milk by churning and washing. 0n the other hand, the milk in which the fat had been churned and washed one, two or three times was more susceptible to the deve10pment of oxidized flavor than the normal homogenized milk. This greater sus- ceptibility of the washed samples toward oxidized flavor was probably due to the severe treatment such as churning, washing and homogenization to which the fat was subjected. Milk reconstituted from homogenized cream and homogenized skim milk was more resistant toward oxidized flavor development than milks made from homegenized cream and unhomogenized skim milk, unhomogenized cream and homogenized skim. milk, or from unhomogenized cream.and unhomogenized skim milk. The milk made from homogenized cream and unhomogenized skim milk was less susceptible to oxidized flavor develOpment than the milk made'from unhomogenized cream and homogenized skim.milk, indicating that the substance or substances affected by homogenization in such manner as ~107- to prevent oxidized flavor deve10pment are more closely associated with the cream than with the skim milk. However, the milk made from unhomo- genized cream and homogenized skim milk was more resistant to oxidized flavor development than the milk reconstituted from cream and skim milk which were not homogenized or than the original unhomogenized milk. This possibly indicates that some substance or substances in the skim milk are affected also by homogenization in such a manner as to retard oxidized flavor development. The normal homogenized milk was more stable against oxidized flavor develOpment than any of the reconstituted samples showing that maximum stability was obtained only when the cream and skim milk were in their original form as whole milk while being homogenized. The ascorbic acid content of unhomogenized pasteurized and homogenized pasteurized capper-free milk immediately after processing was lower than that of the fresh raw milk. The ascorbic acid content of the raw and ho- mogenized pasteurized milk decreased more rapidly upon storage than that of the unhomogenized pasteurized.milk. The ascorbic acid disappeared more rapidly in the homegenized pasteurized milk than in the raw or unhomogenized pasteurized milk. In cepper-treated milk the rate of disappearance of the ascorbic acid in unhomogenized and homogenized milk were similar. Woessner and associates (1939) observed that homogenization tended to destroy the ascorbic acid in commercial bottled milk. The homogenized samples treated with capper were not very suscepti- ble to oxidized flavor development even though the ascorbic acid disap- peared very rapidly upon storage. The results indicate that ascorbic acid is apparently not a factor reaponsible for the flavor stability of homo- genized milk. - 108 - A rancid flavor and an increase in acidity were found to deve10p readily on storage when raw milk was mixed with homogenized pasteurized milk. The maximum increase in acidity occurred when the ratio of raw milk to homogenized pasteurized milk was approximately one to one. As the percentage of raw milk in the homogenized pasteurized milk increased above 50 per cent, the increase in titratable acidity was found to be correSpondingly less. When only a small per cent of the sample was ho- mogenized pasteurized milk very small increases in acidity occurred. These increases in titratable acidity were closely associated with the de-. ve10pment of a rancid flavor. Dorner and Widmer (1932) were the first to note the develOpment of rancidity when unhomogenized raw milk was mixed with homogenized pasteurized milk. The fact that the greatest increases in acidity occurred when the milk was approximately 50 per cent raw and 50 per cent homogenized pas- teurized indicates that the amount of increased surface caused by homo- genization and the amount of lipase added by the raw milk are of approxi- mately equal importance in the develOpment of rancidity in homogenized milk. If this were true, then the increase in acidity and the develOp- ment of rancidity in homogenized raw milk are dependent upon the in- creased surface and not upon the activation of lipase by homogenization as has been suggested. Further evidence of the equal importance of the amount of fat sur- face exposed and the amount of lipase present is shown by the fact that when homogenized raw milk was added to homogenized pasteurized milk the rate of increase in acidity was only slightly greater than when unhomo- - 109 - genized raw milk was mixed with homogenized pasteurized milk. If lipase were activated by homogenization these increases would seem to have been considerably faster than those noted. The more rapid increase which did occur in the homogenized raw and homogenized pasteurized milk mixtures might be explained by the fact that all the fat had been subjected to ho- mogenization so there was more fat surface exposed upon which the lipase could act than in the raw milk and homogenized pasteurized milk mixture where only a portion of the fat had been subjected to homogenization. The amount of lipase added by the raw milk seemed to be the limiting fac- tor in the deve10pment of rancidity in homogenized raw milk. The lipase added to the homegenized pasteurized milk in the form of unhomogenized raw'milk was just as effective in causing rancidity as was the lipase added by the homegenized raw milk. - 110 - SUMMARY Pasteurized milk homogenized at 2,500 pounds pressure withstood the develOpment of oxidized flavor over storage periods extending from.28 to 35 days at a temperature of 35° to 40° F. The oxidation-reduction potentials of both unhomogenized and homo- genized pasteurized capper-free milk showed parallel trends during stor- age. The addition of cOpper to the milk caused an increase in the oxi- dation-reduction potential Quite similar in both the unhomogenized and in the homogenized milk. An increase in oxidation-reduction potential was accompanied by the deve10pment of an oxidized flavor in the unhomogenized milk but not in the homogenized milk. The flavor stability of homogenized milk cannot be explained on an oxidation-reduction basis. Unhomogenized and homogenized milk exposed to sunlight and subse- quently stored showed a decrease in the oxidation-reduction potential and was accompanied by the development of an activated flavor. However, homogenized milk deve10ped the activated flavor more quickly than did the unhomogenized milk. The churning and washing of the fat for three times resulted in the removal of more lecithin and a more stable remade homogenized milk than those milks in which the fat had been churned and washed only one or two times. The substance or substances affected by homogenization, thus prevent- ing or retarding the develOpment of oxidized flavor, were more closely associated with the cream than with the skim milk. However, some evi- dence was obtained indicating that some substance or substances in the - 111 - skim milk were also affected by homogenization. Homogenization caused a slight decrease in the ascorbic acid content of fresh pasteurized capper-free milk. The ascorbic acid content of the homegenized pasteurized milk decreased more rapidly during storage than did that of the unhomogenized pasteurized or the unhomogenized raw milk. HomOgenization did not have much effect upon the rate of disappearance of the ascorbic acid content of cepper-treated milk during storage. Rancidity developed readily in mixtures of milk composed of unhomo- genized raw milk and homOgenized pasteurized milk, with the greatest in- creases in acidity occurring when the ratio of unhomogenized raw milk to homogenized pasteurized milk was approximately one to one. The development of rancidity seemed to be equally dependent upon the amount of lipase present and upon the amount of fat surface exposed by the homogenization process. The lipase of unhomogenized raW' milk when added to homogenized pasteurized milk was apparently just as effective in causing rancidity as a similar quantity of lipase in homOgenized raw milk. 1. 2. 3. 4. 5. 6. 7. 8. - 112 - LITERATURE CITED Anderson, E. 0. 1937 variations in Susceptibility of Milk as Secreted by the Cow Proc. 30th.Ann Conv. Internatl. Assoc. Milk Dealers, Lab. Sect., pp. 153-168 Anderson, E. 0., Dowd, L. R. and Stuewer, C. A. 1937 Relation of Acidity of Milk to Oxidized Flavor. Food Res., 22(2), pp. 143-150 Anderson, E. 0. 1939 Preventing DevelOpment of Oxidized Flavor through the Addi- tion of Small Amounts of Pancreatic Enzyme Milk Dealer, 29:(3), p. 52 Anderson, J. A. 1936a The Cause of Oxidized and Rancid Flavor in Raw Milk. Proc. 29th.Ann. Conv. Internatl. Assoc. Milk Dealers, Lab. Sect., pp. 117-134 Anderson, J. A. 1936b The Influence of the Ration on Milk Flavor. 25th Rept. In- ternatl. Assoc. Milk Sanitarians, pp. 227-238. (Abs. Jour. Dairy Sci. 21, p. 183, 1938) Anderson, I. A., Wilson, L. T. and Hardenbergh, J. G. 1937 The Causes of Off-flavor in Milk. The Facts and a Theory. Proc. 30th Ann. Conv. Internatl. Assoc. Milk Dealers, Lab. Sect., pp. 177-183 Babcock, C. J. 1934a Some Considerations in the HomOgenization of Milk. Abs. Proc. 29th Ann. Meeting Amer. Dairy Sci. Assoc., p. 74 BEbCOCk, C. J. 1934b The Effect of Homogenization on Certain Characteristics of Milk. U. 8. Dept. Agr. Tech. Bul. 438 Babcock, C. J. 1939 Hbmogenized Milk. Jour. Milk Technol. 22(1), pp. 26-31 Baldwin, H. B. 1916 Some Observations on Homogenized Milk and Cream. Amer. Pub. Health, 6: (8), pp. 862-864. (Abs. Exp. Sta. Rec. 36, p.275, 1917) Beck, G. H., Whitnah, C. H. and Martin, W. H. 1939 Relation of Vitamin C, Lecithin, and Carotene of Milk to the Development of Oxidized Flavor. Jour. Dairy Sci. 22: (1), pp. 17-29 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. Bell, R. W. 1939 Effects of the Cold Storage Temperatures, Heat Treatment and Homegenization Pressure on the Properties of Frozen Condensed Milk. Jour. Dairy Sci., 222(2), pp. 89-100 Bodansky, Aaron 1932 Phosphatase Studies. I. Determinations of Inorganic Phos- phates. Jour. Biol. Chem., 99 (1), pp. 197-206 Brown, W. Carson, Thurston, L. M. and Dustman, R. B. 1936 Oxidized Flavor in Milk. III. The Time of Copper Contamina- tion during Production and Processing, and Aeration versus Ho Aeration as Related to Oxidized Flavor DeveIOpment. Jour. Dairy Sci., 202(12), pp. 753-760 Brown, W. Carson, Dustman, R. B. and Thurston, L. M. 1937a Oxidized Flavor in Milk. IV. Studies of the Relation of the Feed of the Cow to Oxidized Flavor. Jour. Dairy Sci., 20: (3), pp. 133-145 Brown, W. Carson, Dustman, R. B. and Thurston, L. M. 1937b Oxidized Flavor in Milk. V. The Effect of Metal-DevelOped Oxidized Flavor on the Iodine Number of the Milk Fat. Jour. Dairy Sci., 20: (9), pp. 599-604 Brown, W. Carson and Dustman, R. B. 1939 Oxidized Flavor in Milk. VI. A.Study of the Relation of Titratable Acidity to Metal-DeveIOped Oxidized Flavor in Milk. Jour. Dairy Sci., 22: (1), pp. 31-35 Brown, W. Carson, Vanlandingham, A. H. and Weakley, Chas. E. Jr. 1939 Oxidized Flavor in Milk. VII. Studies of the Effect of Carotene and Ascorbic Acid in the Feed of the Cow on the Susceptibility of the Milk to Metal-Induced Oxidized Flavor. Jour. Dairy Sci., 222 (5), pp.345-352 Buruiana, Lascar 1937 The Action of Sunlight on Milk Biochem. Jour., 31, p. 1452 Charles, D. A. and Sommer, H. H. 1934 Sedimentation in Homogenized Milk. Abs. Proc. 29th.Ann. Meeting Amer. Dairy Sci. Assoc., p. 74 Chilson, William.Harley 1935 What Causes Meet Common Off-flavor of.Market Milk? Milk Plant Monthly, 24: (11), pp. 24-26: (12), pp. 30-34 Clayton, William 1935 The Theory of Emulsions and their Technical Application. 458 pp. plus IX illus. P. Blakiston's Son and Co., Philadelphia. pp. 350 23. 24. 25. 26. 27.\ 28. 29. 31. 32. 33. 34. 35. - 114 - Dahle, C. D. and Palmer, L. S. 1937 The Oxidized Flavor in Milk from the Individual Cow Pa. Agr. Exp. Sta. Bul. 347 Dahle, C. D. 1938 Preventing the Oxidized Flavor in Milk and Milk Products Milk Dealer, 272 (5), p. 68 Dahlberg, A. C. and Carpenter, D. C. 1936 The Influence of Method of Sterilizing Equipment upon De- velopment of Oxidized Flavor in Milk Jour. Dairy Sci., 192 (8), pp. 541-551 Davies, W. L. 1931 The Action of Strong Sunlight on Milk Cert. Milk, 5: (61), pp. 4-5 Doan, F. J. and SwoPe, W. D. 1927 Studies on the Viscolizing or Homogenizing Process Pa. Agr. Exp. Sta. Bul. 213, p. 22 Doan, F. J. and Minster, O. H. 1930 The Effect of the Homogenization Process on Fat Diapersion and Casein Stability of Milk and Cream Pa. Agr. Exp. Sta. Bul. 258, p. 28 Doan, F. I. and Minster, C. H. 1933 The Homegenization of Milk and Cream Pa. Agr. Exp. Sta. Bul. 287 D0811, F. I. 1933 Critical Preheating Temperatures for the Inhibiting Rancidity in Homogenized Milk. 'Milk Dealer, 232(2), pp. 40-42, 64 Doan, F. J. and Meyers, C. H. 1936 The Effect of Sunlight on Some Milk and Cream Products Milk Dealer, 262 (1), pp. 76-87 D0811, F. I. 1938 Problems Related to Homogenized Milk Jour. Milk Technol. 2: (6), pp. 20-25 Dorner, W. and Widmer, A. 1932 HomOgenization and Milk Rancidity Milk Plant Monthly, 21: (7), pp. 50-57 Flake, J. 0., Weckel, K; G. and Jackson, H. C. 1939 Studies on the Activated Flavor of Milk Jour. Dairy Sci. 222 (3), pp. 153-161 Fox, Wm. K. 1937 The Relationship of Lecithin Content of Milk to the Development of Oxidized Flavor. Thesis, Degree of M.S., Mich. State College 36. 37. 39. 41. 42. 43. 46. 47. - 115 - Frazier, William C. 1928 A Defect in Milk Due to Light Jour. Dairy Sci., 112 (5), pp. 375-379 Garrett, O. F., Tucker, H. H. and Button, F. C. 1938 Relation of Color and Ascorbic Acid to Flavor in.Milk from Individual Cows. Jour. Dairy Sci., 212 (3), pp. 121-126 Garrett, 0. F., Hartman, G. H. and Arnold, R. B. 1939 Some Factors Affecting the Stability of Certain Milk Proper- ties. I. Effect of Succulent Roughages on Flavor Jour. Dairy Sci., 22: (9), pp. 717-728 Garrett, O. F. and Bender, C. B. 1940 The Production and Control of Good Flavor in Milk Milk Plant Monthly, 29: (1), 23-25 Garrett, O. F. 1940 The Antioxidant Action of Finely Milled Oat Flour on Milk Milk Plant Monthly, 29: (2), pp. 40-42 Garrett, O. F., Arnold, R. B. and Hartman, G. H. 1940 Some Factors Affecting Certain Milk Preperties. III. Effect of Roughages on Ascorbic Acid Jour. Dairy Sci. 23: (1), pp. 47-52 Golding, I. and Feilmann, E. 1905 Taint in.Milk Due to Contamination by COpper Jour. Soc. Chem. Ind. 24, p. 1285 Gould, I. A. and Trout, G. M. 1936 The Effect of Homogenization on Some of the Characteristics of Milk Fat. Jour. Agr. Res., 522 (1), pp. 49-57 Could, I. A. and Sommer, H. H. 1939 Effect of Heat on Milk with.Especial Reference to the Cooked Flavor. Mich. Agr. Exp. Sta. Tech. Bul. 164 001110, I. A. 1939 Cooked and Oxidized Flavors of Milk as Affected by Ferrous Iron. Jour. Dairy Sci., 222 (12), pp. 1017-1023 Greenbank, George R. 1936 Control of the Oxidized Flavor in Milk. Proc. 29th Ann. Conv. International Assoc..Milk Dealers, Lab. Sect., pp. 101-116 Greenbank, George R. 1938a Detecting Milk that May Become Oxidized. Abs. Proc. 33rd Ann. Meeting Amer. Dairy Sci. Assoc., Jour. Dairy Sci., 212(5), p. 143 Greenbank, George R. 1938b The Relation of Oxidation-Reduction Potential to Oxidized Fla- vor in Milk. Abs. Proc. 33rd Ann. Meeting, Amer. Dairy Sci. Assoc., Jour. Dairy Sci., 212 (5), p. 144 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. {59. 60. -116- Guthrie, E. 3., Roadhouse, C. L. and Richardson, G. A. 1931 Corrosion of Metals by Milk and Its Relation to the Oxidized Flavor of Milk. Calif. Agr. Exp. Sta. Hilgardia, 5 (14) Guthrie, E. S. and Brueckner, H. J'. 1933 The Cow as a Source of Oxidized Flavor of Milk New'York (Cornell) Agr. Exp. Sta. Bul. 606 Guthrie, E. 8., Band, David B. and Sharp, Paul F. 1939 Expulsion of Air PrOposed to Prevent Destruction of Vitamin C and Deve10pment of Oxidized Flavor in Milk Milk Plant Monthly, 28: (4), pp. 26-28 Halloran, C. P. and Trout, G. Malcolm 1932 The Effect of Viscolization on Some of the Physical Preperties of Milk. Abs. Proc. 27th.Ann. Meeting, Amer. Dairy Sci. Assoc. p. 17 Hammer, B. W. and Cordes, W. A. 1920 A Study of Brown Glass Milk Bottles with Reference to their Use in Preventing Abnormal Flavors Iowa Agr. Exp. Sta. Res. Bul. 64 Hand, David B., Guthrie, E. S. and Sharp, Paul F. 1938 Effect of Oxygen, Light and Lactoflavin on the Oxidation of Vitamin C in Milk. Sci., 87: (2263), pp. 439-441 Henderson, J. L. and Roadhouse, C. L. 1934 Factors Influencing the Initial Induction Period in the Oxi- dation of Milk Fat. Jour. Dairy Sci., 172(4), pp. 321-330 Henderson, J. L. 1939 The Vitamin C Content of Milk and Its Relation to Oxidized Flavor. Internatl. Assoc. Milk Dealers Bul. 122 pp. 271-278 Hening, I. C. and Dahlberg, A. C. 1938a The Effect of Level of Feeding Cows upon the Flavor of their Milk. Abs. Proc. 33rd Ann. Meeting Amer. Dairy Sci. Assoc., Jour. Dairy Sci., 21: (5), p. 109 Hening, .T. C. and Dahlberg, A. C. 1938b The Effect of Feeding Mangels or Dried Beet Pulp to Cows on the Deve10pment of Oxidized Flavor in.Milk Jour. Dairy Sci., 212 (7), pp. 345-352 Hening, J'. C. and Dahlberg, A. C. 1939 The Flavor of Milk as Affected by Season, Age and the Level of Feeding Dairy Cows. Jour. Dairy Sci., 222(11), pp. 883-888 Henry, K. M. and Kon, S. K. 1938 V. The Effect of Commercial Sterilization on the Vitamin C of Milk. Jour. Dairy Res., 92(2), pp. 185-187 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 72. - 117 - Hollingsworth, J. B. 1931 Homogenized Market Milk - How It Is Increasing Consumption in Canada. .Milk Dealer, 202 (9), pp. 63-65, 90 Holm, George F., Greenbank, G. R. and Deysher, E. F. 1925 The Effect of Homogenization, Condensation and Variations in the Fat Content of a Milk upon the Keeping Quality of Its Milk Powder. Jour. Dairy Sci., 8: (6), pp. 515-522 Hood, E. G. and White, A. H. 1934 Homogenization of Market Milk. Can. Dept. Agr., Dairy and Cold Storage Branch, Mimeograph 25 Herrall, B. E. 1935 A.Study of the Lecithin Content of Milk and Its Products Ind. Agr. Exp. Sta. Bul. 401 Hunziker, O. F., Cordes, W. A. and Nissan, B. H. 1929 Metals in Dairy Equipment. Metallic Corrosion in Milk Products and Its Effect on Flavor Jour. Dairy Sci., 12: (2), pp. 140-181 Josephson, D. V. and Doan, F. J. 1939 Observations on Cooked Flavor in Milk. Its Source and Significance. Milk Dealer, 29; (2), pp. 35-36, 54 Kende, Sigmund 1931 Causes and Combating of Oily Milk and Similar Defects in [Milk. 9th.Ann. Internatl. Dairy Cong., Rep. to Sec. State, p. 63 Ken, S. K. and Watson, M. B. 1936 The Effect of Light on the Vitamin C Content of Milk Biochem. Jour., 30, p. 2273 Liebscher, Wilhelm 1937 The Influence of Feeding Cows Beet Tops Treated with Capper on the Amount and Quality of the Milk Proc. 11th Worlds Dairy Cong., p. 29 Mattick, A. T. R. 1927 Oiliness in Milk. Jour. Agr. Sci., 172 (3), pp. 388-391 Mueller, E. S. and “80k, 1‘. J. 1939 Cereal Flours as Antioxidants in Dairy Products. Food Res., 4: (4), pp. 401-405 Prewitt, Ed. and Parfitt, E. H. 1935 Effects of Feeds on the Oxidized Flavor in Pasteurized Milk Abs. Proc. 30th Ann. Meeting, Amer. Dairy Sci. Assoc. Jour. Dairy Sci., 182 (7), p. 468 73. 74. 75. 76. 7'7. 78. I, 4'79. 80. 81. 82. 83. 84. -118- Pfeffer, J. C., Jackson, H. C. and Weekel, K. G. 1938 Observations on the Lipase Activity of Cows Milk. Abs. Proc. 33rd.Ann. Meeting, Amer. Dairy Sci. Assoc., Jour. Dairy Sci., 21: (5), p. 143 Riddell, W. H.,'Whitnah, C. H. and Hughes, J. S. 1935 Influence of the Ration on the Vitamin 0 Content of.Milk Abs. Proc. 30th Ann. Meeting Amer. Dairy Sci. Assoc., Jour. Dairy Sci., 18: (7), p. 437 Riddell, W. B., Whitnah, C. H., Hughes, J. S. and Lienhardt, H. F. 1936 Influence of the Ration on the Vitamin 0 Content of Milk Roadhouse, C. L. and Henderson, J. L. 1935 Flavorsof Milk and their Control Cal. Agr. Exp. Sta. Bul. 595 Roland, C. F., Sorensen, C. M. and Whitaker, R. 1937 A Study of Oxidized Flavor in Commercial Pasteurized Milk Jour. Dairy Sci., 20: (4), pp. 213-218 Roland, C. F., and Trebler, H.1A. 1937 The Effect of Fat Content on Oxidized Flavor in Milk and Cream. Jour. Dairy Sci., 20: (6), 345-350 Ross, Harold E. 1935 Hemogenization as a Preventative of Oxidized Flavor. Milk Plant Monthly, 262(4), pp. 36-39, (5), pp. 40-44 Sharp, P. F. and de Tomasi, I. A. 1932 Increase in the Non-Lactic Acidity in Raw Cream and Its Control. Proc. 25th Ann. Conv. Internatl. Assoc. Milk Dealers, Lab. Sect., pp. 3-20 Sharp, Paul F., Trout, 0. Malcolm and Guthrie, E. S. 1936 Vitamin C, Capper, and the Oxidized Flavor of Milk. 10th Ann. Rep. N. Y. State Assoc. Dairy and Milk Insp., pp. 153- ‘ 154 Sharp, P8111 F0 1938 Rapid Method for the Quantitative Determination of Reduced Ascorbic Acid in Milk Jour. Dairy Sci., 21: (2), pp. 85-88 Smallfield, H. A. 1929 Is There a Future for Homogenized Milk for Retail Trade Can. Dairy and Ice Cream Jour., 8: (2), p. 31 Sommer, H. H. 1938 Market Milk and Related Products. 699 pp. plus XIV illus. Pub. by Author, Madison, Wisconsin - 119 - 85. Stebnitz, V. C. and Sommer, H. E. 19378 The Oxidation of Butterfat. I. The Catalytic Effect of Light. Jour. Dairy Sci., 20: (4), pp. 191-196 86. Stebnitz, V. C. and Sommer, H. H. 1937b The Oxidation of Butterfat. II. The Composition of the Fat in Relation to Its Susceptibility toward Oxidation Jour. Dairy Sci., 20: (5), pp. 265-280 87. Swanson, V. E. and Sommer, H. H. 1940 Oxidized Flavor in Milk. I. Effect of the Development of Oxidized Flavor on the Iodine Number of the PhOSpholipid Fraction of Milk. Jour. Dairy Sci., 23: (3), pp. 201-208 88. Theophilus, D. B., Hanson, H. C. and Spencer, M. B. 1934 Influence of homogenization on the Curd Tension of Milk Jour. Dairy Sci., 17: (7), pp. 519-524 89. Thurston, L. M. 1935 Oxidized Flavor in Milk. Proc. 28th.Ann. Conv. Internatl. Assoc. Milk Dealers, Lab. Sect., PP. 121-141 90. Thurston, L. M. 1935 Oxidized Flavor in Milk. I. The Probable Relation of Lecithin to Oxidized Flavor Jour. Dairy Sci., 18: (5), pp. 301-306 91. Thurston, L. M., Brown, W. Carson, and Dustman, R. B. 1936 Oxidized Flavor in Milk. II. The Effect of Homogenization Agitation, and Freezing of Milk on Its Subsequent Suscepti- bility to Oxidized Flavor Development. Jour. Dairy Sci., 192 (11), pp. 671-682 92. Thurston, L. M. 1938 Theoretical Aspects of the Causes of Oxidized Flavor Particu- larly from the Lecithin Angle. Proc. 30th.Ann. Conv. Internatl. Assoc. Milk Dealers, Lab. Sect., pp. 143-152 93. Tracy, Po H. and Rushe, E; An 1931 The Relation of Certain Plant Processes to Flavor Development in.Market Milk. Jour. Dairy Sci., 14: (3), pp. 260-267 94. Tracy, P. B., Ramsey, R. J. and Ruehe, H..A. 1933 Certain Biological Factors Related to Tallowiness in Milk and Cream. I11. Agr. Exp. Sta. Bul. 389 95. Trout, G. Malcolm and Halloran, C. P. 1932 Sediment in HDmOgenized Milk. Mich. Agr. Exp. Sta. Quar. Enle’ l5: (2), pp. 107-110 96. Trout, G. Malcolm and Halloran, C. P. 1933 Sediment Test not a Reliable Guide in the Selection of Milk for Hemogenization. Mich. Agr. Exp. Sta. Quar. Bul., l5: (4), pp. 271-274 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108. - 120 - Trout, G. Malcolm 1933 Physical and Chemical Effects of Homegenization on Milk Proc. 26th Ann. Conv. Internatl. Assoc. Milk Dealers, Lab. Sect., pp. 199-220 Trout, G. M., Halloran, C. P. and Gould, I. 1935 The Effect of Homogenization on Some of the Physical and Chemical PrOperties of Milk . Mich. Agr. Exp. Sta. Tech. Bul. 145 Trout, G. M. 1937 Off-flavors in Raw and Pasteurized Milk. Proc. 30th Ann. Conv. Internatl. Assoc. Milk Dealers, Lab. Sect., pp. 131-142 Trout, G. M. and Gould, I. A. 1938 Homogenization as a Means of Stabilizing the Flavor of Milk Mich. Agr. Exp. Sta. Quar. Bul., 212 (1), pp. 21-31 Trout, G. M. and Gjessing, Erland C. 1939 Ascorbic Acid and Oxidized Flavor in Milk. I. Distribution of Ascorbic Acid in Commercial Grade A, in Pasteurized Ir- radiated and in Pasteurized Milk throughout the Year Jour. Dairy Sci., 22: (4), pp. 271-281 “'ebb, R. E. 8nd Hileman, Jo Le 1937 The Relation of the Oxidation-Reduction Potential of Milk to Oxidized Flavor. Jour. Dairy Sci., 20: (1), pp. 47-57 Weckel, K. G. and Jackson, H. C. 1936 Observations on the Source of Flavor in Milk Exposed for Prolonged Periods to Radiation. Food Res., 12 (5), pp. 419-426 Weigner, G. 1914 The Change of Some Physical Preperties of Cow Milk with Changes in the Degree of Dispersion of Its Dispersed Phase Kolloid Z., 15, pp. 105-123. (Chem. Abs., 9, p. 668, 1915) Whitaker, Randall and Hilker, L. D. 1937 The Effect of Homogenization at Different Temperatures on Some of the Physical PrOperties of Milk and Cream Jour. Dairy Sci., 202 (5), pp. 281-287 Whitehead, Hugh Robinson 1930 The Reduction of Methylene Blue in Milk. The Influence of Light. Biochem. Jour., 24, p. 579 Whitehead, Hugh Robinson 1931 The Influence of Sunlight on Milk Biochem. Jour., 25, p. 1647 Woessner, Warren W., Elvehjem, C. A. and Schuette, Henry A. 1939 The Determination of Ascorbic Acid in Commercial Milka Jour. Nutr., 18: (6), pp. 619-626 I. . . . _ , A . --;.}sm ~ -.- ' . . ' . r’- . ‘ . 7 . .}N u ’. .- ‘, .. . .‘ ' l . I ‘ . I f” ., I” , ‘ \1- , ’ ‘ . ..- . (d ‘ -. ,I ‘ u . t 9’ ‘1 r-" . ’.- 1 . ' 1 ~ .‘X ‘ ‘. “ - .o .“v‘ i. ' ‘0 I I. .' 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