This is to certify that the thesis entitled LD MICROWAVE HEATING FOOD UTILIZATION OF A HOUSEHO FOR THE DETERMINATION OF MOISTURE IN presented by UNIT PRODUCTS SEBASTIAO CESAR CARDOSO BRANDAO has been accepted towards fulfillment of the requirements for M.S. degreein FOOD SCIENCE W Major professor 0’7 639 2%: 25¢ per day per item RETURNING LIBRARY MATERIALS: -————_.__._________ Place in book return to remove charge from circulation records LIBRAR Y Michigan State University UTILIZATION OF A HOUSEHOLD MICROWAVE HEATING UNIT FOR THE DETERMINATION OF MOISTURE IN FOOD PRODUCTS By Sebastiao C. C. Brandao A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Food Science and Human Nutrition 1978 6*. c0 ABSTRACT UTILIZATION OF A HOUSEHOLD MICROWAVE HEATING UNIT FOR THE DETERMINATION OF MOISTURE IN FOOD PRODUCTS By Sebastiao C. C. Brandao A common household microwave oven (2450 MHZ; 1.5 KW) and a simple analytical balance were used to evaluate the possibility of determining gravimetrically the moisture content of a variety of foods. As support of the sample, a 9 cm Whatman glass microfiber GF/C filter was used, which was held in a modi- fied weighing boat of polystyrene by a 12.7 mm width "scotch magic transparent adhesive tape." Drying curves were determined for each product which were used to determine the time of complete drying in the microwave oven. Specific weights for each product were required and some products also required a second filter to avoid spattering. Results with the method were evaluated by com- parative analysis with the official methods of Analysis of the Association of Official Analytical Chemists. Sebastiao C. C. Brandao The mean difference relative to standard error between both methods were not significant for the t-test at 95% probability level for the following products: milk (whole and skim), butter, Half and Half cream, and ice cream mix. The required drying time for these products were 7.0 minutes for milk (whole and skim) and Half and Half cream, and 8.0 minutes for butter and ice cream mix. However, for cheeses (cottage, cheddar, and Parmesan) and for nonfat dry milk, this method was found to be less suitable since the drying curves presented a prolonged operating time. ACKNOWLEDGMENTS The author wishes to express his sincere gratitude and appreciation to Dr. C. M. Stine for his encourage- ment, guidance, and help during the course of this study. Thanks are also expressed to Dr. R. C. Chandan, Dr. P. Markakis, and Dr. R. C. Nicholas for their assist- ance and aid in preparing this manuscript. Grateful acknowledgement is also due to Dr. H. A. Lillevik for his advice and effort in reading this manu- script. The author feels grateful to the Departamento de Tecnologia de Alimentos da Universidade Federal de Vicosa (Brazil) and to the Programa de Ensino Agricola Superior (PEAS—Brazil) for the opportunity and financial assistance granted to him. Very special gratitude is also due to the author's wife, Livia, and daughter, Ive, for their devotion, encouragement, patience, and understanding. ii TABLE OF CONTENTS Page LIST OF TABLES . . . . . . . . . . . . v LIST OF FIGURES . . . . . . . . . . . vi INTRODUCTION 1 LITERATURE REVIEW . 3 Microwave Heating . . . . . 3 Microwave Heating of Dielectric Materials . 3 Interactions of Microwave with Dielectrics . . 5 Dielectric Properties of Food Stuffs . . 7 Penetration of Radio- -Frequency Energy . 10 Power Adsorption in a Microwave Field . ll Microwave Moisture Determination . . . 12 MATERIALS AND METHODS . . . . . . . . . 20 The Microwave Oven . . . . . . . . 20 The Sample Holder . . . . . . . . . 21 The Analytical Balance . . . . . . . 22 The Filter Support . . . . . . . . 22 The Adhesive Tape . . . . 23 Preparation of the Products Analyzed . . 23 Preparation of Cheese Samples . . . . 24 Preparation of ”Half and Half" Cream Samples . . . . . . 24 Preparation of Milk Samples . . . . . 24 Preparation of Butter Samples . . . 24 Preparation of Ice Cream Mix Samples . . 25 The Standard Methods . . . . . . . . 25 The Microwave Methods . . . . . . . 25 The Drying Curves . . 26 Procedure to Determine Total Solids by the Microwave Oven Method for White Milk, Skim Milk, and "Half and Half" Cream . . . . . . . . . . 28 iii Page Procedure to Determine Total Solids by the Microwave Oven Method for Butter and Ice Cream Mix . . . . . . . 30 RESULTS AND DISCUSSION . . . . . . . . . 32 Interactions Between Solids and Water . . 32 Determination of Water . . . . . . . 36 The Oven Drying Method . . . . . . 38 Microwave Drying . . . 40 Drying Characteristics of the Dairy Products . . . . . . . 41 Comparison of the Methods . . . . . . 52 SUMMARY AND CONCLUSIONS . . . . . . . . 56 LITERATURE CITED . . . . . . . . . . . 57 iv Table LIST OF TABLES Classification of the Samples According to Recommended Sample Weights and Number of Filters . . . . Comparative Determinations for Percent Total Solids in Some Dairy Products by the Microwave Oven and by AOAC Oven Method Comparations of the Methods AOAC vs. Microwave . . . . . Page 26 53 54 LIST OF FIGURES Modified weighing boats Drying Drying Drying Drying Drying Drying Drying Drying Drying curve curve curve curve curve curve curve curve curve for for for for for for for for for deionized water skim milk butter cream "Half & Half” ice cream.mix cottage cheese cheddar cheese . Parmesan cheese non-fat dry milk vi Page 22 42 44 45 46 47 48 49 50 51 INTRODUCTION The determination of total solids in food products is one of the most common analyses in food processing plants. The technique best suited for a particular prod- uct is dependent on several factors, including time, sensitivity, the precision and accuracy required, facili- ties available, and the nature of the material itself. The vacuum and convection over methods officially recognized by the AOAC for determining the total solids content are much too time consuming for product control in food plant operations. The need for a rapid method for determination of moisture for quality control work has stimulated research to develop such techniques. Among these methods the microwave drying technique is the one which shows more versatility, speed, accuracy, and repeatibility. A bench-top microwave oven with a built-in elec— trobalance and a micro-processor has been described for total solids determination of fluid milk (Hamada et al., 1977), and another one with much more versatility is available commercially (Apollo Microwave Products). However, these sophisticated microwave ovens are very expensive. The research reported in this paper was intended to evaluate the possibility of using a household micro- wave oven (2450 MHZ:1.5 KW) and a simple analytical balance to determine moisture content of a variety of dairy products. LITERATURE REVIEW Microwave Heating Decareau (1972) and Justesen (1975) defined micro— wave energy as a kind of radiation, the process of emit- ting radiant energy in the form of wave particles, which is part of the electromagnetic spectrum. Microwaves travel at the speed of light and are classified as non- ionizing waves because they cannot cause chemical changes; however, a temperature rise in the object subjected to microwave radiation may occur. There are two microwave frequencies usually used for microwave ovens (Copson, 1975). 915 ME A z 32.8 cm or 12.9 in 2450 MHz 1 12.2 cm or 4.82 in Microwave Heating of Dielectric Materials Tinga and Nelson (1973) and Copson (1975) described the dielectric properties of a material as the ' 0 I! complex dielectric constant e = e — JE . The real part, 5', directly influences the amount of energy that can be stored in the material in the form of electrical field. The loss factor, a , is an imaginary part which is a direct measure of how much energy a material can dissipate in the form of heat. The relation e"/e' is called the loss tangent. Copson (1975) observed that the basis for dielec— tric loss is the relaxation time, which usually is a fraction of a microsecond during which the particles delay in responding to the field change, or the time for them to revert to disorientation. Decareau (1972) described polarization as the phe- nomenon responsible for the microwave heating. Dipole molecules tend to orient themselves in a direction oppo- site that of the applied field; however, when the electric field is changing millions of times each second, the mole- cules magnets are unable to keep up because other forces acting to slow them down. Such forces which restrict their movement may be mechanical such as in the case with ice or solid fats, or viscous as in the case with a syrup- like molasses. The energy of the microwave in trying to overcome these forces is converted to heat. Roebuck et al., (1972) reported that the dielec- tric constant 5‘ decreased as the water content decreased and that for materials with the same water content (below 30%), the e' value depended on the concentration of solute. However, for a water content varying from 40- 100%, the 5' values are similar for the mixtures analyzed (starch, ethanol and glycerol). They also reported that the dielectric loss factor (8”) increased as the solute concentration increased from 0 - 50%. Goldblith (1972), and de Loor and Meijboom (1966) noted that there are two types of dielectric loss, dipole loss (e"d) and ionic loss (5"i), which contributed addi- tively to the total dielectric loss: 8 II + E” I = 8 total d 1 Roebuck and Goldblith (1972) cited that at 2,450 MHZ there is a predominance of dipole loss (8"d), while at 915 MHZ the ionic loss predominates. Interactions of Microwave witE’Dielectrics Aurand and Woods (1973) stated that water molecules are polar because oxygen is more electronegative than hydrogen, and the two 0 - H bond polarities add up to give a net polarity to the molecule as a whole. The amount of polarity of the molecules depends not only on the differ- ence in the electronegativity of the atoms, but also on the angle between bonds. The bond angle for water is 104,52° and if it were larger, the polarity of the water molecule would be smaller. Proctor and Goldblith (1953) and Copson (1975) stated that if a volume of water is placed between two metallic plates which are connected to a switch and battery in a circuit, and the swtich is open, the water molecules are randomly oriented. 0n the other hand, if the switch is closed, and a strong electric field is applied, the water molecules tend to align themselves with the electric field causing net polarization of the mass. Weil (1971) cited from Badger: This degree of order within the volume of matter is a form of potential energy. When the switch is opened the field is relaxed. The molecules revert to random orientation and the stored potential energy is converted to heat. Tinga and Nelson (1973) asserted that all dielec- tric phenomena can be attributed directly to the polari- zation of the material under the influence of an exter- nally applied electric field. They also pointed out that the major mechanism of interest at microwave frequencies is that of orientation polarization, which means that molecules with displaced positive and negative charges (dipoles) will tend to orient themselves in a direction opposite of the applied field. Roebuck et al., (1972) and Stuchly and Hamid (1972) mentioned that bound water has a different behavior than free water in an electromagnetic field. Free water has a dielectric constant of 60 - 80, while bound water has a much lower dielectric constant, about 4, due to its lower rotation. Ice at -14°C has a behavior similar to bound water (Copson, 1975), as both have a dielectric constant of about 4. According to McIntosh, 1966, (cited from Roebuck et al., 1972), the stronger the binding forces in the bound water, the smaller the dielectric constant and the dielectric loss factor. Dielectric Properties of Food Stuffs The dielectric constant (e') and the dielectric loss factor (6") are properties of a foodstuff which determine microwave power absorption as well as microwave energy penetration. For these reasons there has been an extensive effort to study these properties for many food products in the last few years. Tinga and Nelson (1973) reported values of the dielectric constant (e') and the dielectric loss factor (6”) for agricultural, plant material, biological, foods, and other products. This extensive review was tabulated as function of frequency, temperature, moisture content, and composition. To et al., (1974), Goldblith (1972, 1974), Tinga and Nelson (1973), and Ohlsson and Bengtsson (1975) found that the dielectric constant (e') of beef products increases with decreasing frequency at constant tempera- ture, and decreases with increasing temperature at con- stant frequency. The dielectric loss factor (e") increases with decreasing frequency at constant temperature and increases with increasing temperature at constant frequency. For pineapple syrup and pineapple pieces it has been reported that the dielectric constant (5') has the same behavior as beef products with respect to tempera- ture changes, but the dielectric loss (3”) has the oppo- site behavior (Ohlsson and Bengtsson, 1975). In the same work it was also found that other foodstuffs like ham, ham salad, frankfurters, and macaroni and cheese have both the dielectric constant (e') and dielectric loss (5”) increasing with temperature. Goldblith (1972) cited from Goldblith and Pace (1967) that dielectric loss of potato chips increases with the temperature and also increase with the moisture content. Roebuck et al., (1972) found that the dielectric loss factor (6") of a mixture of carbohydrate-water decreases as the water content decreases. Dairy products have received considerable atten- tion in order to determine the product parameters during processing (heating, drying, pasteurization, etc.) and in analytical determinations such as moisture. The dielectric properties of dehydrated whey, nonfat dry milk, and butter, and their variations with moisture and temperature were reported at 2540 MHz by Rzepecka and Pereira (1974). They found: 1. For all these dairy products both the dielec- tric constant (e') and the dielectric loss (8”) increase with temperature and with moisture. 2. Powdered milk of moisture content 3.3% can be considered as having all water "bound," while at 4.8% moisture, the water appears in both free and "bound" forms. 3. The permittivity of "bound" water between 0 - 30% has a positive temperature coeffi- cient, while the permittivity of free water decreases vfixfli temperature. The net coeffi- cient is positive. Borisio and Buy (1976) also noted an increase of dielectric properties, in arbitrary units, of butter with the moisture content. Mudgett et al., (1971) chemically simulated an aqueous solution of nonfat dry milk (NFDM) and found that the actual loss factor (6") at 3000 MHZ and at 25°C are substantially lower than those predicted, based on a linear additive relationship of the component parts. They attributed this difference to possible solute- solvent and solute-solute interactions. They suggested a model in which the salts, protein (casein) and lactose are those that contribute to the relative dielectric loss of milk. However, the salts have a reduction in their contribution to the total dielectric loss due to association (solute-solute interaction) and solute— solvent interaction (about 33% of the milk salts are in "bound" form and 26% involved in solute-solvent inter- actions). 10 To et al., (1974), using the same approach as Mudget et al., (1971), extended their findings to other temperatures (25°C to 55°C) and other frequencies (300 to 3000 MHZ). The results again indicated the same solute—solute (33%) and solute-solvent (26%) interactions of the salts, which affect the contribution of the salts to the dielectric loss. Sodium caseinate showed a change of dielectric constant (5') with frequency similar to that of water, that is, a slight rise between 970 and 1123 MH followed Z by a decline and tendency to level off from 1123 to 2973 MHZ. At a given frequency, the dielectric constant (e') tended to decrease as solute concentration increased (Weil, 1971). De Loor and Meijboom (1966) reported the dielec- tric constant (e') and the dielectric loss (8") of milk (3% fat and 12% T.S.) between 1.2 and 9.4 GWZ. They found that semi-circles may be extrapolated in a plot of dielectric loss (8") and dielectric constant (e'). Penetration of Radio- Frequency_Energy The penetration of radio—frequency energy is infinite in perfectly transparent substances (nonabsorb- ing glass, ceramics, some plastics, etc); zero in reflec- tive materials such as metal, and for most absorptive 11 material such as food stuffs, the penetration is a finite value (Decareau, 1972). The attenuation of energy from the incident wave by a sample thickness is (Puschner, 1966): P = P (1 - i'zax) o where: P = absorbed power P0 = incident power a = attenuation factor (cm-l) X = sample thickness (cm) Goldblith (1972) asserted that the choice of fre- quency is a matter of desired depth of penetration. For large objects, one needs the penetration of the lower frequency. But, this occurs at the expense of power input to the material being heated. However, when one is using thinner samples one may present more power to the sample by the higher frequency. Thus, the matter of fre- quency to a large extent becomes a trade of energy input versus penetration. Ohlsson and Bengtsson (1975) found that penetra- tion depth in most of the cases decreased slightly with increasing of temperature. Power Absorption in a Microwave FieId The amount of power (P) that can be absorbed by a sample in a microwave field is a function of the 12 electrical field and the equivalent dielectric conductiv- ity (Goldblith, 1972). 14 P = 55.61 x 10’ r - e" - E2 ttS where: P = power (HE—3—) cm r = frequency (EZ§%%§) dielectric loss factor 00 II E = voltage gradient ( volts) c Microwave Moisture Determination There are several different approaches to deter- mine moisture using microwaves. Only three general methods will be mentioned due to the complexity and diversity of the subject. These three methods are: 1. Microwave attenuation technique 2. Microwave resonance technique 3. Gravimetric microwave heating The microwave attenuation technique. In this method the measurement involves the determination of the attenuation suffered by the microwaves while passing through the test material, what can be visualized as an absorption coefficient. Usually the test material is introduced between a transmitting and a receiving horn, and the corresponding attenuation is measured by suitable methods (Wlodarski and Kostyrko, 1968). 13 The attenuation is a function of the nature of the material structure, dependent on moisture content (mainly), concentration of each constituent, interaction between constituents, temperature, frequency, etc. (Kraszewski, 1974). Therefore, it is indispensible to make a preliminary study of the properties of the mate- rial analyzed and have knowledge of the influence of moisture and composition change over the attenuation measurement (Tomek, 1971). Kent (1972) found that in powders, where density fluctuation can be large, the attenuation technique is difficult to apply. Another reason, besides density fluc- tuation, would be low attenuation of powders that would require the path length to be relatively long to allow for a reasonable degree of attenuation at normal moisture content. This method may be used for liquids (water solu- tions, suspensions, etc.), but at the normal moisture content of most liquids, the attenuation would become very large and would require a reduction in the length path of the sample. Kent (1972) suggested that for water the path length should be equal or less than 9 mm, at 10 GHz to keep attenuation within the dynamic range of most measuring instruments (0.1 to 50 dB). Therefore, this method may present some difficulties when used for l4 viscous liquids, and would be suitable for liquids with intermediary moisture content, homogeneous composition and non-viscous. Botsco (1970) reported the utilization of several sample holders for granules, powders, liquids, etc., and found it possible to determine moisture content of grains, such as wheat, barley, coffee beans, dried corn, etc. However, no data were presented to show the precision of the method. Kraszewski (1974) constructed and described a ”moisture detector” using microwave attenuation, where the moisture content may be shown on a meter graduated in units of moisture content, or recorded on paper. He claimed that this method had been applied to foodstuffs such as Italian paste, marzipan cake, milk powders, etc. No data were presented. Kent (1972) described a technique where a strip- line configuration was used to determine moisture. This method would allow the use of normal sample lengths and it was recommended for very high moisture content where other normal attenuation measurement would be difficult to apply. Kraszewski (1971) reported the continuous determination of moisture in margarine by the microwave attenuation technique. He carried out pilot tests in a margarine factory and found that an average difference 15 between his method and a standard gravimetric method was less than 0.25% H20 over the range from 14.5 to 18.5% H O. 2 Microwave resonance technique. This technique uses a resonant circuit consisting of an inductance in parallel with a capacitance cell which changes with the dielectric properties of the sample. Changes in the die- lectric prOperties of the sample cause corresponding changes in the cell capacitance, affecting the resonant frequency of the circuit (Neelakantoswamy, 1975; Doughty, 1977). A variable frequency oscillator is used along with a cavity to measure the permittivity and loss tangent of samples. The cavity resonator is excited in the dominant (lowest resonance frequency) mode of oscillation and the permittivity of the dielectric is determined from the resonant frequencies of the cavity when completely air- filled and when containing the dielectric sample (Doughty, 1977; Weil, 1971). Neelakantoswamy (1975) using a Fabry-Perot resona- tor reported the determination of water in papers. Doughty (1977) described a method using the microwave resonance technique to determine water in oil emulsions over a wide range of water content. However, as he 16 pointed out, the precision is reduced on samples of high water content above 40%. Gravimetric determination of moisture by micro- wave heating. The basic principle of this method is that of evaporating the moisture from a known quantity of sample, weighing the residue and computing it as percent of the original. Cotterill and Delaney (1959) studied the possi— bility of using this gravimetric method for determining total solids in liquid egg products. They adapted a resistance coil to supply supplemental heat for browning or broiling. They compared the microwave gravimetric method with a conventional oven method and found that the removal of moisture was 96% for egg white and 100% for egg yolk. Kemeny and Chazin (1973) studied a microwave gravimetric method in which they used ethyl acetate to eliminate charring by forming a suitable azeotropic mixture which distilled. They reported that the new microwave drying technique was found "to meet the desired criteria" for analytical use, particularly in the greater speed and ease of performance. Palmer and Pace (1974) found that the gravimetric microwave drying of wildland fuels offered an economical, rapid, simple and mobile method for determining fuel l7 moisture. They observed that residual moisture devia- tions indicate that the microwave drying process is not effective in quickly removing the water held in cellulose by hydrogen bonding. Successful gravimetric microwave drying of slur- ries and wet cakes of several inorganic chemicals (car- bonates and sulfates of several metals), and some organic compounds (saccharin and potassium biphthalate) was reported by Hesek and Wilson (1974). i Pettinati (1975) developed a procedure to deter- mine moisture in meat by the gravimetric microwave drying technique, in which samples were prepared for drying by a mixture with ferrous oxide and sodium chloride. These salts were added to accelerate the drying: ferrous oxide is a strong absorber and sodium chloride promotes dehydra— tion or ”salting-out” which in turn promoted water release. He concluded that the rapid microwave oven method was as accurate and precise as the AOAC method for the estimation of moisture content of fresh meat. Carter et al., (1976) compared the gravimetric microwave drying for two microwave power levels, with the thermal oven and found that the total solids as deter— mined by microwave oven values were slightly greater than those of the termal oven for solids determination of waste water. They attributed this difference to 18 volatilization of some material during the thermal oven treatment and concluded that the higher values obtained from the microwave oven were more correct. The microwave oven results were also found to be more precise when the coefficient of variation was used for comparation. They also reported that the larger microwave power input was the most precise when comparing coefficient of variations for both power levels. Chevalier and Retailleau (1970) reported a method to determine moisture content by gravimetric microwave drying of skim milk, whole milk, whey, and Camembert cheese. Results for total solids were similar to those obtained by drying on a boiling water bath for 7 and 12 hr. for milk and cheese, respectively. Marchart and Hoffer (1975) found that total solids of whole and skim milk determined by a microwave heating method were 0.2% lower relative to values obtaineth'drying during 0.5 hr. on a water bath, followed by 2.5 hr. in an oven at 104 i 2°C. They attributed this to absence of atmospheric oxidation of susceptible components. Hamada et al., (1977) proposed a new instrument to determine total solids in milk in three minutes in which an electrobalance placed inside a microwave heater was connected to an electronic circuit to calculate and to display the result in percent total solids. The l9 reproducibility of measurements obtained by this instru- ment was found to be superior to those obtained by the A.0.A.C. reference method. The accuracy was also found to be excellent. However, the new instrument employs volumetric sampling (0.973 ml at 20°C 2 1.000 g) and, consequently, it is affected by density (temperature and solids concentration) of the sample. MATERIALS AND METHODS The Microwave Oven The household microwave oven used throughout this experiment is commercially available and it has the fol- lowing specifications: 1. Trade name--Sears Kenmore Microwave oven Model No.--747.9947510 Frequency--2450 MHz Power--1.5 kw 2 3 4 5. E1ectricity--120 V at 60 Hz 6 DimensionSe-(H,W,D,) - 14 3/4, 24/, 17 1/8 in. 7 Weight--70 lbs 8 Cooking positions--cook and defrost Timer capacity--0 to 20 minutes The two operating positions mentioned above per- mit the microwave oven to work continuously (cook posi- tion) or to work for 30 seconds, stop for 30 seconds (defrost position). This pattern in the defrost posi- tion repeats itself until the end of the time set. How- ever, the fan is always on during operation. The unit was modified with flexible tubing to permit circulation of water in and out of the microwave oven to absorb some of the energy emitted. This 20 21 prevented excessive heating of the microwave oven itself and heating of the magnetron because the microwaves that are not absorbed by the sample would return to the magne- tron causing it to increase its temperature rapidly that may cause failure of the power generation. The water was regulated to run at about 30 liters per hour in operating conditions. The temperature of the inlet water was about 15°C and the outlet below 38°C. The tube must not touch any wall of the microwave oven. One may use a plastic holder to avoid the contact of the tube with the oven walls. The glass tray located in the internal base of the microwave oven was removed because it was observed that after some time of operation it became warm, trans- ferred heat to the base of the sample holder. Therefore, the glass tray was substituted by a polystyrene weight- ing boat. The Sample Holder Weighing boats of polystyrene were used because they did not absorb microwave energy. The boats were 140 x 140 x 22 mm (Fisher), and were cut to permit better air circulation below the filter, thereby improving vapor removal from the sample. The cut was done in the way shown in Fig. l. 22 \__/ Front View Top View Figure l.--Modified weighing boats. The Analytical Balance All weighings were performed on a Mettler H6 Analytical balance to the nearest 0.1 mg. The Filter Support The initial phase of this research was involved with the selection of a suitable filter support to be used. It was found that cellulose filter papers were not suitable due to its water loss in the microwave field. This water loss and subsequent adsorption after exposure to a microwave field was found to cause errors in the analysis. The filter supports used for all analyses were 9 cm diameter Whatman glass microfiber GF/C. Filters of this diameter were the largest size which fit in the analytical balance used. Some properties of the Whatman glass microfiber filter are: 23 1. Very high resistance to chemical attack 2. Unaffected by and will not denature bio- logical/biochemical fluids 3. Can be used at temperatures up to 500°C 4. Excellent gas filtration performance 5. Composition (typical) 8102 = 57.9% B203 = 10.7% Fe203 = 5.9% A1203 and Na20 = 10.1% K20 = 2.9% CaO = 2.6% MgO = 0.4% BaO = 5.0% ZnO = 3.9% F = 0.6% The Adhesive Tape The filters were held in the plastic holder by using a 12.7 mm width Scotch ”Magic transparent tape.” The adhesive tape should cover about 1.5 to 2.0 cm along the filter, at both sides of the holder. Preparation of the Products Analyzed All products tested throughout this experiment were commercial products. 24 Preparation of Cheese Samples All cheese samples were ground before every samp- ling in a Waring blender for about 5 minutes, placed in a closed container, and held under refrigeration (1°C) for no more than 2 hours prior to analysis. Preparation of Milk Samples Grade A, homogenized, pasteurized milk was warmed to 20°C, mixed, then cooled and kept in a closed container under refrigeration for analysis. Preparation of "Half and Half” Cream Samples Homogenized, ultra pasteurized Half and Half cream.was warmed to 20 C, mixed thoroughly, cooled and kept in a closed container under refrigeration for analysis. Preparation of Butter Samples Lightly salted, sweet cream butter was carefully softened under agitation in a water bath. The mixing was continued until the butter had a thick consistency. Then it was put in an ice water bath and mixed vigorously until it had a hard consistency. After that the sample was kept under refrigeration (1°C) in a closed container for no longer than four hours prior to analysis. 25 Preparation of Ice Cream Mix Samples Vanilla ice cream was melted at temperature below 12°C and mixed thoroughly for about 5 minutes, transferred to a container and kept under refrigeration for no longer than 4 hrs. Before sampling the ice cream mix was again mixed for about 2 min. The Standard Methods The standard methods for moisture used throughout this experiment were: The AOAC air oven method for milk (16.032), and for butter (16.187); the AOAC vacuum oven method for cheese (16.217), and for nonfat dry milk (16.174). When using the vacuum oven technique a small cur— rent of dried air was passed through the oven. The air was dried by passage through anhydrous calcium sulfate, rather than sulfuric acid. The vacuum was kept near 30 in of Hg and the drying time was at least 4 hrs. for both vacuum and air ovens. The temperature was maintained at 99 - 100°C. The Microwave Methods Each product was tested individually for its dry- ing curve, and the drying methods were based on these curves. Some products, therefore, involve identical pro- cedures for their drying method, as will be observed later. 26 The Drying Curves Each kind of sample requires specific weight because it was observed that too much sample could lead to difficulties in weighing, i.e., the sample has a tendency to run out of the filter. Some products (ice cream, NFDM and cheeses) required a second filter to avoid spattering of the sample. This was done by making a "sandwich” of the sample between two filters. The classification of the samples according to these weight variations and number of filters is pre- sented in Table 1. TABLE 1.--Classification of the Samples According to Recommended Sample Weights and Number of Filters Product Weight (g) Number of filters Whole milk 1.0 i 0.05 one Skim milk 1.0 0.05 one Half and Half 1.0 0.05 one Cheeses 2.4 0.1 two Butter 1.2 0.1 tWO Nonfat dry milk 1.2 0.1 tWO Ice cream mix 1.3 t 0.1 CW0 The method used to determine the drying curve was as follows: 1. The filter is positioned in the plastic holder with the help of the adhesive tape 2. In the case of samples using two filters, the second filter is placed in the base of the plastic holder. 27 3. Weigh the sample holder plus the filter(s) in a Mettler H6 Analytical balance to the nearest milli- gram. Record the weight. 4. Arrest balance after weighing 5. Increase the number of grams necessary to weigh the product (Table l) 6. Quickly place the sample on the filter 7. Immediately release balance fully and allow it to come to rest 8. Record the very first weight 9. In the case of samples using two filters, put the second filter over the first one held by the adhesive tape (and which contains the sample) in order to make a "sandwich" of the sample. 10. Dry in the microwave oven for increasing periods of time: for butter, cheese and water use the ”cook” position and for milk, cream, and ice cream, use the "defrost” position. During the phase where the sample loses weight rapidly, one may use the same sample for two or three successive analysis. However, when the drying is in the falling-rate, one should use a new sample for every analysis. 11. After the sample has dried for the specific time, weigh it immediately in the analytical balance. However, in the case of butter, cheeses and water, 28 one should wait for constant weight, by leaving the sample in the balance with both doors closed. 12. Record the weight 13. Calculate the % total solids by the formula weight of solids x 100 A total solids = weight of sample The procedure described above was used to deter- mine the best drying time for each product, and coinci- dently, as previously noted, some products have the same drying time. Procedure to Determine Total Solids by the Microwave Oven MetEOd for Whole Milk,’Skim MiIk, and ”Half and Half” Cream 1. Put the filter in the plastic holder with the help of the adhesive tape. The weight of the holder plus the filter should be slightly above the integer weight (i.e., 3,0500 grams). This is done because after putting the product in the filter, it will start evaporating and under the conditions specified the time required to weigh the sample will be shorter than when the weight is much above the integer weight. 2. Weigh the sample holder plus the filter in the analytical balance to the nearest milligram 3. Arrest the balance 29 4. Increase 1,0 gram in the weight reading of the analytical balance. 5. Pipette in a fast delivery pipette about one gram (1.0 i 0.05 g) of milk or cream 6. At this point the left hand is kept in the release knob of the balance. 7. The sample is added at once, spreading as evenly as possible, taking care to avoid touching the filter with the pipette. 8. The balance is released just after the sample is added and the very first weight is recorded. Do not permit any delay in weighing because the sample begins to lose weight very rapidly. 9. Dry the sample in the microwave oven during 3.5 minutes at defrost position. Always use the same place in the microwave oven. 10. Remove the sample holder from the microwave oven just after it is turned off (automatically) and weigh it immediately. Take the very first weight. 11. Compute the percent total solids by the formula: weight of solids x 100 A Total Solids = weight of sampIe 30 Procedure to Determine Total Solids—by Ehe Microwave Oven Method for Butter and Ice Cream Mix 1. Put the filter in the plastic holder with the help of the adhesive tape. 2. Put a second filter in the base of the plas- tic holder (loose) 3. Weigh to the nearest ten thousandth of a gram in the analytical balance 4. First, for butter only, put a piece of butter weighing about 2.4 r 0.1 g in the center of the filter and weigh immediately in the analytical balance; second, for ice cream mix only, put about 1.3 i 0.1 g of ice cream mix in the upper filter. The sample should be put in drops to avoid it to run out of the filter due to its low adsorptivity by the filter. Weigh immediately in the analytical balance. 5. Put the filter which was in the base of the holder just above the filter with the sample, in order to make a I'sandwich" of the sample. 6. Dry in the microwave oven for 8 minutes in cook position. If temperature of the microwave oven is too high, one may use defrost position for about 3—5 min- utes and then cook position for the rest of the time. 7. Take the holder with the dried sample out of the microwave oven immediately after it is turned off 31 (automatically) and weigh as soon as it is at room tem- perature. For butter it will take about 2.0 minutes to cool down and for ice cream one should weigh immediately. 8. Record the weight. 9. Compute percent total solids by the formula: weight of solids x 100 A total SOlldS = weight OfISampIe RESULTS AND DISCUSSION Moisture content of a product is usually expressed as moisture mass (Mm) per unit weight of the wet product 04,) ILit-“3x100 A M = M S The moisture mass (Mm) designates only the water that is in homogeneous solution throughout the substance, in retention in small capillaries, in physical adhesion or adsorption on solid surfaces. It does not include chemically bound water. Interactions Between Solids and Water Most of the problems arising in moisture measure- ments are related to material properties. Food products are often composite systems, where the structure of mate- rial has a great effect on the interaction between parti- cles and water. Food products may be classified into three groups (Juhasz and Tomek, 1971): l. colloidal foods 2. foods with capillary pores 3. colloidal foods with capillary pores 32 33 Foods of colloidal nature are in the first group, such as gelatin, pastes, milk, etc. Into the group of food products with capillary pores are classified products being rigid after the extraction of moisture such as grains, etc. The last group combines the properties of both groups mentioned above. Materials of this group consists of capillaries with flexible walls and when absorbing or losing moisture they change their form. A considerable number of food materials in which moisture measurements are routinely determined belong to this group, i.e., cheeses, etc. Four types of bonds, and four types of water may be distinguished in terms of the amounts of energy invol- ved in binding of water (Wlodarski and Kostyrko, 1968; Juhasz and Tomek, 1971; Kuprianoff, 1958). 1 unbound water (physical or mechanical bondings) 2 surface water 3. physico chemically bound water 4 chemically bound water Free water molecules are the type least strongly combined with the matrix, namely, by physicomechanical forces. The water is found in voids, pores, and at grain boundaries. The tie-up force is related to the size of the space occupied by this water and the resistance of 34 the medium to the exit of this water. Unbound water is linked the most weakly to the material and it complies approximately with the concept of water independent of material and free of interaction. The name free or unbound water implies that the water has the properties of ordinary water and is not affected by the surrounding molecules. Surface water is not part of the product itself, but comes from outside (condensation, washing, etc.). Such water could be considered as free water only so long RN as it has not mixed or reacted in any way with the sur— face components. Two types of bonds may occur in physico-chemically bound water, osmotic, and absorptive. The osmotic bonds tie up only water contained in micropores. These forces are related to capillary size, and increase as the capil- lary diameter is decreased. Adsorption is the result of Van derWaals intermolecular forces and of permanent dipole orientation (Keeson effect), dipole induction (Debye effect) and dispersion forces (London effect). Adsorbed water forms the first layer adhering to the capillary walls. The energy of water adsorption is clearly by one order of magnitude lower than that of chemical bonding (Wlodarski and Kostyrko, 1968). Water molecules are strongly adsorbed on polar substances 35 surfaces because they have small permanent dipoles of a high dipole moment per unit molecular surface area. Energetically the most stable type of binding is the chemical bond. This bond develops if water is directly engaged in building up the chemical structure of the material and forms practically a part of the chemical structure. Crystallization and coordination water, i.e., water bound coordinatively by polar groups of certain biocolloids, fall into this category of water. The extraction of chemically bound water can only be carried out by breaking up the chemical structure, when materials properties also change (Kuprianoff, 1958; Juhasz and Tomek, 1971). Dehydration is usually effected by subject— ing the food to conditions such that water is removed by vaporization. This requires that the partial pressure of water vapor at the surface of the material due to its water content must exceed that in the environment for drying to proceed. However, in the case of chemically boundivaterthe vapor pressure of water accumulated in the material is reduced due to bonding existing as the over- all result of interactions of different types of bonds (Wlodarski and Kostyrko, 1968). The vapor pressure of the surface layer water molecules, (Psl), may be defined as: 36 PSI = P5 - Pb where: PS = saturated vapor pressure Pb = pressure due to bonding The surface of a material adsorbs water when the partial pressure of water vapor (PW) in ambient air is higher than Psl; if the reverse is true, water is desorbed from the material. Desorption occurs as long as a difference between the surface and the bulk moisture content continues to exist. Determination of Water The determination of water is one of the most common analyses that the analytical chemist is required to do. Most of the foods are regulated in water content and analyses for moisture are done frequently in order to check the regulatory needs and/or to define a product during processing. However, as Willits said: the determination of moisture in foodstuffs is not a simple procedure. Even today a common concept of moisture analysis is that implied by an old (1885) procedure of the AOAC, which spe- cified moisture as the loss in weight occurring when a substance was heated at 98° to 100°C (for some specific time). This simple method will give approximate moisture values, but can not be depended upon to produce accurate results because it fails to recognize the complexity of the process of water removal. The results obtained by the time- honored thermal drying methods may sometimes be 37 wrong, but nevertheless oven methods for moisture analysis generally are the standard for most other moisture method whether by distillation, electric moisture tests, chemical reaction or some other physical means (1951). A basic drying method should fulfill the follow- ing conditions (Juhasz and Tomek, 1971): 1. The composition of the material will not change qualitatively and quantitatively during the dry- ing process. 2. Water bound by physico—mechanical and by physico chemical forces to the material escapes from the material 3. No change will occur in the moisture content of the material during the necessary preparatory process Usually the moisture method used will only partly comply with these conditions and generally it is diffi- cult to know where water evaluation ends and where mate- rials begin to decompose. Sometimes decomposition is verified visually before all the water is removed, as is the case with cottage cheese, where browning is apparent after 1 h at 99-100°C but all moisture is not removed in as long as 4 hrs. at this temperature. In a given product there is an equilibrium between the different components at a given water content, depend- ing of the form in which this water is attached to the different substances. However, when some water is 38 removed, and especially, when only a small amount of water is left in the product, the concentrations of sub- stances dissolved in it (salts, etc.) is increasing, possibly leading to a nonuniform distribution of water within the product and to a stronger attraction for water by the moledules. This can be noted in a graph of moisture vs. time, where a straight line is not observed (falling rate) (see Figures 2 and 9). After the removal of sufficient water, buffering action of ionic and neutral molecules will cease and . there may be a change in pH. The high concentration of salts, and the high temperature of the product may cause denaturation coagulation and precipitation of proteins (Connell, 1958). This precipitation can entrap water molecules which will be more difficult to remove because the pro- teins are very dried and usually resist diffusion of water through the pores. The Oven Dryinngethod The classical oven drying technique consists of heating a sample in an oven for a specified time and temperature and calculating the moisture by difference in sample weights before and after drying (Wlodarsky and Kostyrko, 1968). 39 In this process of drying, the partial vapor pressure of water in the gas phase (air) is lower than the vapor pressure of the water in the sample. The pres— sure vapor of water in the liquid state rises rapidly with increasing temperature (Weast, 1977), increasing the mass transfer from the drying sample to the air. However, there are disadvantages of using very high tem— peratures, such as decomposition of the product. There— fore, the best technique is to remove moisture at the lowest possible temperature, a time consuming operation. On the other hand, if the temperature is not high enough, some water may be trapped in the solid matrix by physico- chemical forces (capillary and/or adsorption) resulting in a low moisture value. For most food products the AOAC standard methods call for a temperature between 98 to 100°C for some specified period of time or until con- stant weight is attained. Some products such as cheese must be dried under reduced pessure (usually 100 mm of Hg absolute AOAC 16.217) in order to increase the water vapor—pressure differential (air and liquid) which will increase the rate of drying. The vacuum technique also assists in removing residual moisture that is so small that its vapor pressure will not influence the vapor pressure of the vacuum chamber air (Willits, 1951). During drying by the vacuum oven method, it is necessary to admit into 40 oven 3 slow current of air (about two bubbles/sec.) dried by passing through H2804 or through a proper desiccant to prevent saturation of the air by the water evaporated by the sample, which would decrease the effi- ciency of the method. Willitz (1951) noted that using a drying agent inside the oven it will reduce diffusion time with con- sequent gain in efficiency. The combination of both methods, a vacuum oven and a desiccant, improves the dry— ing oven method to the highest efficiency possible by this general technique. Microwave Drying Ringle and David (1975) and Bobeng and David (1975) reported that uneven heating of food occurs in a microwave oven due to the cavity geometry. They attri— buted this to the fact that in the zone where the energy is reflected from the walls and floor of the oven cavity a geometrical factor must be assumed to the proper dis- tribution of energy, which would depend not only on the horizontal position, but also on the height of the food within the microwave cavity. Therefore, a study of the drying characteristics of deionized water (1.00 g) was done in this research to find out how this uneven distribution would affect the drying characteristics of a given amount of deionized 41 water. The results are given in Figure 2. The height of the sample is the height of an inverted weighing boat used to support the holder, instead of the glass tray which comes with the microwave oven, plus the height of the filter paper in the sample holder (modified weigh- ing boat). The horizontal positions were those which permitted the inverted weighing boat to be as close as possible to the concavity used to fit the glass tray (which was removed). Consequently, the positions used were more distant to the edges than it appears in Figure 2. The results again indicated that different posi- tions inside the microwave oven resulted in different drying characteristics and that position 5 was the posi- tion where more energy was absorbed by the sample. Posi- tion 6 was the area in which less energy was absorbed and positions 1, 2, 3, and 4 had a similar, intermediate pattern during drying. The tube with running water was used passing through position 5 and 6. Position 1 was used throughout all the analyses due to the ease with which one could insert and remove samples. Drying Characteristics of the Dairy Products The relationship between the length of time for drying each product and the apparent total solids 42 “”0“ mentions IN ' 0'0 THE MICROWAVE. 90.0 _ WEN _ [0. 0 80.0 — _ 20.0 DEIONIZED 0 mo- MMTER -30 2 _‘ _ _ o géoo ‘10. F" § 0 I: 500- — 500 8), =\ x U? o .1 \ .3 [(0 00 0 “540°” I~L~5~+ " 6' A ”1 E R 700 g 8900— 5 — ._. ... zao— _ 80.0 [0.0— _ 90.0 0— [00.0 l | l l 0 1 7— 5 4 drying time, minutes Figure 2.--Drying curve for deionized water. 43 variation is shown in Figures 3 through 10. These graphs indicate that milk, butter, cream "Half and Half” and ice cream mix can be dried to a total solids content which is in good agreement to the AOAC oven procedure. However, for cottage cheese, cheddar cheese, Parmesan cheese, and non-fat dry milk, the method is less suitable, since removal of the last traces of moisture would require too prolonged an operation, as compared to the AOAC vacuum oven procedure. This residual retention of moisture may be attributed to the water bound to the food proteins, since the amount of water retained increased directly with protein content. The water bound to proteins has a much lower dielectric constant due to its interactions with the proteins. The stronger the binding forces between adsorbate (water) and adsorbent, the smaller the die- lectric constant (Roebuck, Goldblith and Westphal, 1972; Juhasz and Tomek, 1971). The term ”bound water" is used here to define the portion of the total water which remains in the product in an unchanged (or bound) state after application of the microwave drying procedure and which can be expelled only by the AOAC oven procedures after a sufficiently long time. However, as different degrees of water binding may occur, the water with an obviously high degree of binding may also be referred to in this paper as bound water. 44 1/5 A 1 ‘ 88'5 SIiIM A110 _ MILK _ 89.0 2 3 - —+ \ g r— j/as .. — 39:5 8 5: Sn ”3 o\° E /00-— — 900g 8 \\ H E «3 <75- — 905 +2 0 a.) l I I l O 1 L 5 + drying time, minutes Figure 3.--Drying curve for skim milk (defrost position). 45 -150 3.. 9‘ Ln (M/M) °/. 'ssm 1M -160 g 8.0. BUTTER o\0 m\ :2 '6 U) 5 5‘5. 4— O +— 84.0— ! I I I I l I l 1 Z 5 4 5' b 7 8 drying time, minutes Figure 7.--Drying curve for butter. 46 24.0 A - 760 25.0 _ CREAM _ 770 ”HALF & HALF“ g —{ A 22.0 _ - 78.0 E 3 m E 21.0 - — 79-03:: .o f. m\ _ 800 \ s W ‘ s (D Ho — — 9’0 79 o *— 1e.o — 820 1 z b 4— drying time, minutes Figure 5.—-Drying curve for cream ”Half & Half" (defrost position). 47 IHCUE (SfiflAJNIIEIIIC 4&0— —570 420- -580 E E 41.0- -590 In“ _ 600 E 40.0-— 8 p—q 5".0" ‘ - 6’0 £53. 8 360_ _62.0 I I l l l l I 1 1 1 z. 0 4 6‘ a 7 5 1 drying time, minutes Figure 6.--Drying curve for ice cream mix. 1 48 MO _ L660 ,fio __ ‘ COTTAGE CHEESE 51.0 -— ~ 680 51.0 . E a90— _700 A f— ; 8 §Zfio - ‘m E o\0 o \ _ 2.0 ,. €200 — 7 g ”110.. IE 3?. 9 no 7 74 0 250 — 240-4 l-76I) 230~ AOAC-ZLH 0 i L 3 i E L ; drying time, minutes Figure 7.--Drying curve for cottage cheese. 49 mi 72.0- _ 2 8 O 71.0 _ CHEDDAR E 700— CHEESE — 30 0 -4 ,_ E 4,10- 5D , 5 .\° .= GOO— —32-OA e :1 (J70- E O w ill '5 4,4,0_ _ 34.0 ‘6 H 650- 64.0» AOAC. - 4.1.4,: 36.0 I l l o 5 10 If drying time, minutes Figure 8 --Drying curve for cheddar cheese. 50 94,0 -— - 4- 0 95.0 .4 PARMESAN 44.0- CHEESE r. 6.0 ‘75 0 —1 E 92 o- - 8~0 \ E °\° 41.0.. E”; E ‘00. - 10'0 a) B 0 § bio; 800— _. 12.0 870. 5co— AOWAC :- 5150 ,— HO 1 5 :0 75 Figure 9.--Drying curve for Parmesan cheese. drying time, minutes (Ni/M) % ’SSO'l .LM 51 ,1 loo-0 - NON-FAT * 0'0 3 DRY MILK \ e E a . I'— fig 6140 _ —- {.0 8 '5 (m "3 \° 3 e 3980- —20\\ A.0.A.C. = flier. E I I I I I I I 1. 5 5 7 ‘7 H 15 drying time, minutes Figure 10.--Drying curve for non-fat dry milk. 52 A comparison of Figures 2 through UJreveals that different products have different falling rate periods during dehydration which is brought about by the decreased rate at which evaporation occurs. After reaching a criti- cal moisture content (sometimes not very well defined), the drying rate decreases linearly with decreasing mois- ture content for the remaining portion of the process, until a horizontal curve is (ideally) achieved. The time to achieve the critical moisture content is longer and more difficult to define for those products which contain more protein. This may be explained by the fact that dielectric properties are influenced by the kind of bond between solids and water (Tomek, 1971), or better, some types of bonds may have a higher dielectric constant than others. Consequently, as the amount of protein increases for a given weight of sample, the amount of tightly bound water as well as water bound in a lower degree will also increase. The latter may be removed in part during the process, which may cause the higher and the less defined critical moisture content. Comparison of the Methods Moisture determination on milk (skim and whole), cream, "Half and Half," ice cream mix and butter are shown in Table 2. Resultscflfstatistical treatment for these products are also presented in Table 3. The mean 53 TABLE 2.--Comparative Determinations for Percent Total Solids in Some Dairy Products by the Micro- wave Oven and by AOAC Oven Method Microwave Oven (Percent t.s.) AOAC (Percent t.s.) Skim.Milk* 9.62 9.64 9.63 9.63 9 63 9.61 9.61 9.60 9.63 9.62 Whole Milk ll 89 ll 94 ll 91 ll 91 ll 93 11 91 11 93 ll 92 ll 92 ll 91 11 94 11 91 ll 92 ll 90 11 90 ll 92 Half and Half 18 38 18 38 18 36 18 36 18 34 18 34 18 38 18 34 Ice Cream Mix 38 08 . 38 13 38 15 38 08 38 13 38 04 38 14 38 10 Butter 84.36 84.35 84.39 84.34 84.40 84.42 84.32 84.30 84.29 84.39 84.31 84.38 *Commercial skim milk with added non-fat milk solids. 54 .SOHum«>m© wumwcmum w .menHOm anus 000.00: 00000 0003 mane anew Hmnonwaaooo .HGwUC—OU QHSUmHOE mewm OS“ wGHSHmuwp mposuoa o3u emu umfiu wcflumoflmcfl .Hm>mH huflaflnmnoua Nmm um moDHm>ru HmHsnwu wmooxm uos ow meHm>uu wwofiHQ .mudpoooua So>o cowuom>Coo o m>m30HoH2 unwouwm .meHom HmuOH m>mBOMOHZ .m> o