.9?» .13.th $71». 7 1 Mr??? H . I, ‘ , . .. . n... . .. . . .5..1P.Lit!v! emvgri. . L: . .. . , . ,: aif. 2mm» . . 5 . . .I‘Y‘MJL . 4””er .35.. ”mutt... ”EM... furrrfiflh if!!! 2 53!. . . . .tlni 3 . rrmmp. . Z .. ,. “giryr.tv..a 6N... . .. ~ .. ., . e. fihWQILEIHI: :1» r . . ., r {Nigbhfm . rt...€rt.tt J. 23.53.. .n. rf 0....IEFi61 \L‘fqtt. .51.... It .r .2... , Swain... .. 1? $1.5? at». , {—g’hzz 1‘... I (£5.21... I g , , . z.“—§uh 11;: ivy-Ii h... Ifi?§w.i§f fiffliqflmfigfitaftilfiw: $4.“?! . ...~‘b«§ne.&u.uflru§qesé. $3.4; ta... .mettr.§ . 12.x? unqkwuwmfitfirfimuftu? . gift 5 1%.? than} ($0.0. 9*} .5 . . .3. an»: ,. n8. . “afifNMHHAFELVJ. A . Zr! «(at v5.01?— f}. gm; . {Wipti « {OE . téiflk9§9§5t til 5 ti! .. . . tt.£§rflnfltv§'§.b§rg [INK I ? 2?;lé}. . .u. 3., . :fixfikfi. i. .2... In... . 0%!“ Pin £31.. .90.” «Pfl‘?.£\hu~ .70. . . ifs? o g . . K. .b 1‘. t. , a.» .. v vii... .9. ivzwfighfldfisgfii mmw... . . , I 3 \.\\8‘ ., .. .. . . .. . . s. , «$.52 . .. . {Withfiz.£‘§.£: . {it‘lp‘x £v‘ O cg“... .L . art . fut» . .VWH’NEW-Ssvnfld. trq’rvsfihumawdus 1 £1 5... §1" ’ . I . fez. ._ . . figuhfiflgg 5%,“? H _ 2.4753 «”5. « $09.??th nan» , ,. ..\ , gfi’ its? . . . r it t l..." 5‘"? €59. .zEFFluwp . . 1‘ hi . . . .. %~.zfiuflfiy , ,gdix M\u§531n§53 flaw... Hwy... lg“; . .3. 3:31, . IBM)» . , n . gift. . unit 334““. Vgghfii L. at. «Q . . .33? 3222 $1.. a 2.34.. 343. ganwfifiiim .huméwflwq‘e... . . “In“:nifia. . . 33:71.9 «1530113,!3; “1(533 5 Kflfiufisxlfi «4&4... .. liaile Ett‘ r... ., .z!g31s . A 1512’?» .. 351% {.tbwt..xih::.t+t a... . 1453.!réhézittbilfixafiawgabflnfl 3»? . . ~ . ,, . .. ,4 .._.v.R...,YI , . . . , 9.9.2.5.”...‘0. :1, .9 r . . 333.111.“ 3&3? t}... N. .iiliits .rL.” 2.0L. . kl. . , . KW , n if \§ 9. 7 1... My 0.3.511... g 3 . a» . a». .3», . 35.x. L A &.§ §“3.l\‘\lln ig I . if}, . IN A 75.33;.) kg: .2 5"“ f‘NJr s: I to), 7,. 1‘. . 5’73 4.1!}?! 3.1 , 91! zigfitigg iwzfiafl 13.; 34.4.... .V .XL.’ ¢ .. . . f \ L: .> It», ‘7 e ,2... . . A: MU. . . . x... . . fir V. m7fi$flvflvvfi)1\2 flit)! . . . heviigyrnflc . . i . . . .. , . . . 2.1 2. , . . . itsifgif uh; . @159...- t. . . u . 533...... . V. 5.5 \WWA‘ ». gtuxfigfifi. . . .5{ wruflyfig ,2 :31 71 \ : .33....wwv $.Nb.§fi$u£c€\ 381.. , 74.. n , _ :5? ~ 5 . g 4&7“. , L. in. 55.3“. 5...}? It; t. h l. J . . . 5.111331hy .» , 351315. Lift .. ~ . 7 wig-y’V 3531)! 5‘56... 1%,]: {5.7 1‘ r . $15)}; _. , Wildivp. . 1.2. Wuflflvhivh‘hurlflflvfiwxw 3...; H. .5: . .5. is... 1...? . .{2 NJ... 2. w . 53:545....41. 282 s... it a, «.fimfiafi. .v . $411,511...- .tfigxuvflhgz ain’tné‘ 1» M»... .. >93 33?... #25 5313;. 5:11. .3ka .c. ., , )wx...n.x,sh 1! ‘11 . sihisgirifiyi s k“. :9... Sign!“ .333flfiusilu. vivx... . .. . . . . , . 57):? a .3. . ., . . . . . 115.35 v . . J‘safinfiw .5 ., h... 1%.... hfiflzluwl~ «333:»...7. £3... . 11:. w .., 1.! .. v \;\|tp.n.7lfljlflr‘r> In 3.99:)... 4325,14... 7‘1. ‘k . 55551555 $5; . .3173. lll LIBRARY Michigan State University This is to certify that the thesis entitled DETECTION OF VOLATILES EVAPORATING FROM A PLASTIC CONTAINER AFTER BEING MICROWAVED presented by LYNN ELIZABETH DIXON has been accepted towards fulfillment of the requirements for M.S. PACKAGING degree in Wflwfi Dr. Bruce R. Harte Major professor July 23, 1987 Date 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution 47y'3éfix MSU LIBRARIES m. RETURNING MATERIALS: Place in book drop to remove this checkout from .your record. FINES will be charged if book is returned after the date stamped below. 35363 -': - ~13$ .PTC‘ 91 .363 3% N; 9033 DETECTION OF VOLATILES EVAPORATING FROM A PLASTIC CONTAINER AFTER BEING MICROWAVED BY Lynn Elizabeth Dixon A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE School of Packaging 1987 Copyright by LYNN ELIZABETH DIXON 1 8 ABSTRACT DETECTION OF VOLATILES EVAPORATING FROM A PLASTIC CONTAINER AFTER BEING MICROWAVED. BY Lynn Dixon A coextruded plastic container was cut up and placed into a glass vial which was then sealed. The Vial was suspended over the microwave floor and microwaved on full power for periods of time ranging from 3 to 7 minutes. The temperature the plastic surface experienced over the time range was approximated with the use of melting point standards waxes. Five major components were detected and quantified in the headspace of the vial using a gas chromatograph. The quantity of each component increased as the microwaving time increased. Using a mass spectrometer the five components were tentatively identified; four of the components were identified as hydrocarbons and the fifth component was positively identified as butylated hydroxytoluene. To Gene, my mother and father for their support. ACKNOWLEDGEMENTS I would like to thank the many people which helped me with my thesis research. I sincerely appreciate the time and guidance that Dr. Bruce R. Harte gave to me throughout the graduate program. I would also like to thank Ruben Hernandez for his advice on the devising of the experimental design and the analysis of the results. Many thanks to Kevin Slatkavitz from Pillsbury and Tom Cotton from Gerger for helping me with the Mass Spectrum interpretation. To Ball Plastics I extent an appreciation for donating the plastic containers used in this research. Finally, I would like to express a deep appreaciation to my family and my friends for their support during this endeavor. TABLE OF CONTENTS Introduction Literature Review A Brief Background on Electromagnetic Waves Explanation of Dielectric Constant, Dielectric Loss and Loss Tangent Description of Power Absorption, Depth Penetration and Change in Temperature Parameters Influencing the Dielectric Constant and Loss Mechanisms of Heating Via Microwave Absorption Effects of the Magnetic Field Influence of Electromagnetic Waves on Polymers Theory of Migration of Indirect Food Additives Packaging Materials for the Microwave Oven Containers on the Market Materials and Methods Description of Container Verification of the container Material The Power Level of the Microwave Oven Temperature Correlation and Sample Preparation Concentration vs. Time Microwaved Correlation Qualitative Anaylsis of Major Chromatograph Peaks iv 10 11 13 14 l6 19 21 24 24 26 28 31 33 3.6 Verification of the Mass Spectrometer Identification Results and Discussion 4.0 Verification of Container 4.1 Power Output of the Microwave Oven 4.2 Microwaving Time vs Temperature Correlation 4.3 Heat Transfer Via Conduction 4.4 Microwaving Time and Presence of Volatiles 4.5 Mass Spectrometer Identification 4.6 Origin of the Hydrocarbons and the BHT Conclusion Appendices Appendix Appendix Appendix Appendix Appendix Appendix Bibliography Temperature Scale Sample Gas Chromatograph Sample Calculation Used for Converting Area Response Units into Grams of BHT per Grams of Plastic Gas Chromatograph Raw data for Five Different Retention Times Temperature Data for Different Microwave Times Instructions for Attenuated Total Reflectance 33 35 39 39 41 43 50 60 64 67 68 69 70 75 79 81 TABLE OF TABLES Table 1. Dielectric Constant for Plastics Table 2. Power Output of the Microwave Oven Table 3. Summary of Tenetative Identicification of Mass Spectra Table 4. Speculation of the Correlation between the Mass Spectrometer and the Gas Chromatograph Appendix D Table 5. Gas Chromatograph Data for Microwaving 3.0 Minutes Table 6. Gas Chromatograph Data for Microwaving 3.5 Minutes Table 7. Gas Chromatograph Data for Microwaving 4.0 Minutes Table 8. Gas Chromatograph Data for Microwaving 4.5 Minutes Table 9. Gas Chromatograph Data for Microwaving 5.0 Minutes Table 10. Gas Chromatograph Data for Microwaving 5.5 Minutes Table 11. Gas Chromatograph Data for Microwaving 6.0 Minutes Table 12. Gas Chromatograph Data for Microwaving 6.5 Minutes Table 13. Gas Chromatograph Data for Microwaving 7.0 Minutes Appendix E. Table 14. Temperature Data for 3.0 Minutes vi 14 39 50 59 70 7O 71 71 72 72 73 73 74 75 Table Table Table Table Table Table Table Temperature Temperature Temperature Temperature Temperature Temperature Temperature Data Data Data Data Data Data Data for for for for for 4.0 4.5 5.0 5.5 6.0 6.5 7.0 Minutes Minutes Minutes Minutes Minutes Minutes Minutes 75 76 76 77 77 78 78 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure H C N J:- U) I D U1 o 6 a o 6 b. 7 8. 9. 10. ll. 12. LIST OF FIGURES Electromagnetic Spectrum A Plane Monochromatic Electromagnetic Wave Triboelectric Series of Polymers Coextruded Plastic Container Container Cross—Sectional Area Apparatus in the Microwave Oven Apparatus for Suspending a Vial Infrared Spectrum of Polypropylene Infrared Spectrum of Vinylidene Chloride/ Vinyl Chloride Copolymer Infrared Spectrum: x — Ethylene Vinyl Acetate, o — Polypropylene Power Output vs Microwave Oven Level Settings Microwave Time vs Temperature Experienced by the Plastic Strips Microwaving Time vs Concentration for a Component with a Retention Time of 7.98 minutes Microwaving Time vs Concentration for a Component with a Retention Time of 15.28 Minutes Microwaving Time vs Concentration for a Component with a Retention Time of 15.50 Minutes viii 15 25 27 30 30 36 37 38 4O 42 44 45 46 Figure 12. d. Microwaving Time vs Concentration for a Component with a Retention Time of 22.87 Minutes e. Microwaving Time vs Concentration for a Component with a Retention Time of 30.86 Minutes Figure 13. Relationship between the Concentrations of all Five Components vs Microwaving Time Figure 14. Mass Spectrum Figure 15. a. Mass Fragmentation for Retention Time of 1.13 minutes b. Mass Fragmentation for Retention Time of 3.19 minutes 0. Mass Fragmentation for Retention Time of 5.00 minutes d. Mass Fragmentation for Retention Time of 5.48 minutes e. Mass Fragmentation for Retention Time of 6.00 minutes f. Mass Fragmentation for Retention Time of 12.45 minutes 9. Mass Fragmentation for Retention Time of 17.58 minutes Figure 16. Standard Calibration Curve for Butylated Hydroxytoluene (BHT) Figure 17. Grams of BHT per Gram of Plastic vs. Microwaving Time Appendix A Figure 18. Sample Gas Chromatograph ix 47 48 49 51 52 53 54 55 56 57 58 62 63 68 INTRODUCTION Microwaving is becoming more commonplace in both the industrial and home sectors. Market penetration of micro- wave ovens in the home sector has reached about 60% and by 1990 it is estimated that 70 to 90% of all homes will have at least one microwave oven (Perry 1986). "In industry, microwave processes are slowly gaining acceptance by the food industry for various processing operations such as blanching, cooking, drying, pasteurizing, sterilizing and thawing of bulk food products. Perhaps the most widely accepted microwave food processes are the precooking of some meat products and the tempering or thawing of frozen foods" (Mudgett 1986). There exists a large market potential for microwave products as people look for ways to save time in food preparation. The typical owner of a microwave is a woman who is well educated, sophisticated and leads an extremely active life. The microwave oven fits into her life by enabling her to prepare food quickly and easily (Nelson 1982). Because of this market potential food companies are continuing to develop and market microwaveable products. Therefore a real need exists for microwaveable packages. The packaging engineer must design packages which can overcome potential problems due to microwaving. Perry (1986) suggests that areas to be evaluated specific to microwave food preparation in a package are venting, arcing, uniform heating, handling, eruption, and indirect additives. Adequate venting must be provided to avoid possible steam burns. Arcing between the packaging struc- ture and the oven should be avoided to prevent damage to the oven. Controlling the energy conversion is important to accomplish proper and uniform heating. Physical hazards may arise if eruptions occur as Viscous materials reach their boiling point. Packages should be designed so that they may be handled when hot. Migration or evaporation of indirect food additives from the package might be a problem due to the elevated temperatures of the microwave environment. Consumers have been warned never to place metal inside the microwave oven, because the metal could cause the reflection of energy that might damage the magnetron. Arcing may also occur, creating a fire hazard. However, microwaving of metal containers can be accomplished, as long as some precautions are taken. For example, the metal can be coated, left in its paperboard package if it has one, or the metal container can be placed in the center of the microwave away from the walls. Microwave ovens can also be designed to minimize damage to the magnetron. Food manufacturers need to educate the public on usage of metal in the microwave oven, in order for consumers to accept products in metal containers. Presently many manufacturers are making food containers out of plastic, paperboard, or a combination thereof. When using plastic containers there exists the potential problem associated with indirect food additives. Migrants can affect the quality of the food product; migrants may also have toxicological manifestations if the concentration exceeds an established safety level. Published information which describes the possibility of migration/ evaporation of indirect food additives from plastic containers to food as a result of microwaving in the container is limited. This research investigation was made based upon the following hypothesis: during heating of a plastic container in the microwave oven indirect food additives may volatilize from the container. These volatiles may then be absorbed by the contained product. Absorption of indirect food additives by the product could create a potential legal problem if they exceed the level allowed by the FDA. Even if the indirect food additives are not a safety hazard, absorption by the product could have an adverse effect on product quality (i.e. odor and taste). The objectives of this study were as follows: 1. To develop a method of detecting volatiles from plastic containers during microwave heating. 1h To determine the quantity of these it function of microwaving time. 3. To identify the major components. LITERATURE REVIEW 2.0 A Brief Background on Electromagnetic Waves Microwaves are a form of electromagnetic radiation. Electromagnetic waves travel at a velocity of 2.997 x 10 meters per second in a vacuum; the waves are characterized by two parameters, frequency and wavelength. (See Figure (1) for the location of the microwaves in the electro— magnetic wave spectrum.) The wavelength (X) is related to velocity (c) and frequency (f) by: A: c / f (1) The energy (E) in electromagnetic radiation varies with frequency according to: E = h x f (2) where h is Plank's constant. In a practical application such as in a microwave oven, a wave frequency of 2450 MHz is commonly used, which corresponds to a wavelength of 0.1223 meters and an energy of 1.624 x 10- J. Electromagnetic waves consist of an electric field (El) and a magnetic field (H), which oscillate in a plane perpendicular to one another. The wave depicted in Figure 2 is described by: Eduuoomm oauocmeouuoon AEov £mco.m>m>> .H oudwwm ow.” 0—.” 0:.” on gum 0:” 9.0 7 n. . . c o. W a . _ _ . a . . _ _ _ a _ a _ . a . . _ . — _ . _ . - .— . . . . _ _ _ — _ _ _ . . w _m_ _ com . _ _ o>m>noeuom¢r . _ _ _ . _ . . _ m>mm.x n . _ B. .> . SE. . _ _ . . .ofimm. . _ _ $301 o< . . _ . _ _ _ m . _ _ . . _ _ . . _-— . . _ . _ —.— _ _ _ _ . _ _— _ _ p _ h . h n _ _ p ~ _ . p p e _ L _ 2 no— 29 .9 .9 .9 F ANtoIV Ocoacmi o>m3 ofluodmmaowuooam owumaouaooaoz macaw < .N “$3me E1 = Elosin (wt-kx) (3) H = Ho sin (wt-kx) (4) where El and H are the amplitudes of the electric and magnetic fields respectively, and w = 2wf and k = 21 /x (Vankoughnett 1973). 2.1 Explanation of a Dielectric Constant, Dielectric Loss and Loss Tangent When materials are exposed to electromagnetic radiation the energy may be absorbed, reflected, or trans- mitted through the material body. The amount of microwave energy absorbed by a material depends upon its dielectric properties. Dielectric properties are defined by three parameters that are a function of the electromagnetic wave frequency. These parameters are the dielectric constant (EDthe dielectric loss factor (8“), and the loss tangent (tans). "The dielectric constant relates the value of the electric field within the material to the value of the externally applied electrical field" (Tinga 1970). In more simplifed terms the dielectric constant is a measure of a material's ability to store electrical energy. The loss factor, dielectric loss, ( E) is a measure of a material's ability to dissipate electrical energy. The ratio of the dielectric loss to the dielectric constant defines the loss tangent. tané = 678‘ (5) The loss tangent is related to the material's ability to be penetrated by an electrical field and to convert the electrical energy to heat (Mudgett 1986). 2.2 Description of Power Absorption, Depth Penetration and Change in Temperature The dielectric constant and the dielectric loss determine the amount of energy absorbed, transmitted, and reflected by the material. Shiffmans (1986) states that power absorption by a material can be expressed by the following equation: 9 = ka’e‘ (6) where P is the power developed in a volume of material; k is a constant in units of power and volume (watts/cm ); f is the frequency of the microwave system; E is the electric field strength (volts/unit distance); and E‘ is the dielectric loss factor for the material. Increase in the temperature (AT) of a material when microwaved is expressed as: AT = k'fEE“ (7) PCP Shiffmans (1986) defines the variables in the above equation as: p is the density of the material; CP is the specific heat; k‘ is a constant used to express heating (C/minutes). Both the power absorption and the rate of increase in the temperature equations are based on an assumption that there exists a uniform electric field within a unit volume of the material. Perry (1986) defines a penetration depth (d) as the 10 depth at which 1/e or 37% of the initial power remains as: ya d ==~I 1 \ (8) Earn + tan”: W2 - 11/ where d is the penetration depth, xois the wavelength in free space (.1223 m in this case), 8' is the dielectric constant and tana is the loss tangent. At a depth d into the product, 63% of the incident power has been absorbed while 37% remains. For the next thickness into the product, 2d, only 23 % of the incident power will be absorbed. For a depth of 3d into the product, 9% of the incident power is absorbed. Because of the significant power difference in power absorption between depths, the outside surface will heat much faster. 2.3 Parameters Influencing the Dielectric Constant and Loss Moisture content affects a material's dielectric constant since water has a very high dielectric value. At low moisture content levels, the water is bound and thus cannot freely rotate in the electromagnetic field. Beyond the critical moisture level the loss factor increases and the product is more receptive to microwave heating (Shiffman 1986). The density of a material can affect its dielectric constant. Air is transparent to microwave radiation having a frequency of 2450 MHz. Therefore air spaces within a material reduce the dielectric constant below that of the 11 same material without air spaces. As the density of a material increases, the dielectric constant increases often in a linear fashion (Tinga 1970). During heating of a material the dielectric loss changes with temperature, depending upon the moisture content of the material (Tinga 1970). For example, ice has a much lower dielectric constant than water; ice (3.2) is relatively transparent to microwaves where as water (80) absorbs microwave energy. In the frozen state the water molecules cannot move freely to orientate the dipoles when electromagnetic waves are applied. 2.4 Mechanisms of Heating via Microwave Absorption Several mechanisms exist by which matter absorbs microwaves. The two most useful mechanisms are ionic conduction and dipole rotation (White 1973). In ionic conduction the ions are attracted by electric fields, because the ions are electrically charged. This movement constitutes a flow of current. The velocity of the ions represents kinetic energy as a result of their attraction to the electric field (White 1973). The ion particles (i.e. OH: Nat NHt, Nog, Cl: HJO+) collide with nonionized molecules such as H2 and release their kinetic energy. Any particle may collide several thousands of times during microwaving. In each collision the kinetic enegy of the particles is converted into heat. The field may be either continous or intermittent; ionic conduction will occur in 12 either case. This heating process is not dependent to any great degree on either temperature or microwave frequency (White 1973). In the mechanism of dipole rotation, the molecules will become oriented when an electric field is applied and will become random when the field is removed. In dipole rotation, the energy from the electrical field is stored as potential energy and then converted to random kinetic or thermal energy in the material. The energy conversion is efficient when the frequency coincides with the time required for the build up and decay of the induced order. As the temperature of the material increases, the faster orientation and randomization occurs; this process of buildup and decay is known as the relaxation frequency. Dipole rotation has a large dependency upon the microwave frequency and temperature (White 1973). Also the molecule's size and shape influence the dipolar rotation. Since dipole molecules are not spherical in shape, they are not completely free to rotate; thus there exist a tendency for the orientation of the molecules to be random. Small molecules have a relaxation frequency which is higher than the microwave frequency. The relaxation frequency will increase as the temperature of the material increases, which slows down the conversion of energy into heat. Polymers, large molecules, have relaxation frequencies which are lower than the microwave frequency. Therefore as the temperature increases in the polymer, the energy 13 conversion into heat speeds up, which may result in a runaway heating phenomenon (White 1973). The runaway heating may result in hot spots and degradation of the polymer. Yet, polymers do not easily absorb the microwave energy as liquids and.monomers do; hence, the chance of runaway heating of polymers is reduced. 2.5 Effects of the Magnetic Field When a material is placed in a magnetic field, magnetic resonance occurs because energy is absorbed by the small magnetic particles in the material. VanKrevelen (1976) explains that two kinds of transitions may be responsible for magnetic resonance. The first transition involves the reorientation of the magnetic moment of the electons in the field. This transition is called electron spin resonance and is observed in the microwave region of 9 to 28 GHz. The second transition, nuclear magnetic resonance, occurs when the reorientation of the magnetic moment is in the nuclei of the material. The most widely studied nucleus is the proton. Nuclear magnetic resonance occurs in the radio frequency region of 10—300 MHz. White (1973) suggests that the heating mechanisms induced by the magnetic field play a very minor role in heating microwave energy. As previously mentioned, ionic conduction and interface polarization have the greatest influence on microwave heating. 14 2.6 Influence of Electromagnetic Waves on Polymers Polymer's influenced by electromagnetic waves will depend upon crystallinity. The density difference between the crystalline and amorphous regions affects the dieletric constant. The amorphous regions will allow more dipole rotation to occur. The dielectric constant and dielectric loss also depend more on unbalance or asymmetrical dipoles than on the presence of polar groups (Billmeyer 1984). Polyethylene has a lower E and. E‘ , as compared to polyvinyl chloride. (See Table 1 for the dielectric constant of polymers and Figure 3 for how moisture can influence the E of some polymers (Vankrevelen 1976).) Table 1. Dielectric Constant for Plastics Polymer 5‘ calculated polyethylene polypropylene polystyrene poly(o-chlorostyrene) poly(vinly chloride) poly(vinlyidene chloride) poly(tetrafluoroethylene) poly(chlorotrifluoroethylene) poly(vinyl acetate) poly(ethyl methacrylate) polyacrylonitrile poly(ethylene terephthalate) polycarbonate nylon 6,6 0 DNNMNNNHNNNNNN o u . o o o o o . o o . O\D\D\J\IOON\I\DWO\\IJ>UJ l5 I A ........ _ - _ .. _ _ _ I / conditions 8........._____— g ' ‘l I ,/” C -- - - - - A} _r 7 I I ,,” I / F I I , wet conditions o.--- I x Es” / F .“. . I / ‘ ‘ . ‘ -: I T. 11:r_ ”7‘ G_.-, . .n':_-_-~. / H.‘ . . . . . : : : r_ 7 I. ------ ' 7‘: I: :7 / ._. .. _. .1 _7 I / I / L . _ _ f I M )- - 7, / I N . , »—/l O . . I P. ......... 2 3 4 5 ‘ . 8 9 10 (Dielectric constant) A. Nylon 6.6 G. Polyacrylonilrite L. Poly (2.6 dimethyl polyphonylane oxide) 8. Cellulose H. Poly (vinyl chloride) M. Polyslyyene C. Cellulose acetate 0. Poly (methyl methacrylate) E. Polyacetale l. Poly (blsphenol carbonate) J. Polychloroethet K, Poly (vinylidine chloride) F. Poly lethylene terephthalate) Figure 3. Triboelectric Series N. Polyethylene O. Polypropylene P. Poly (telralluoroethylene) of Polymers 16 2.7 Theory of Migration of Indirect Food Additives Global migration is a measure of all compounds which can transfer to a contained product, both toxic and nontoxic. The migration process depends partly on the diffusivity of the migrant. If the migrant interacts with the polymer then the process of migration is more complex. Diffusivity, or the diffusion coefficient (D), is defined as the tendency of a substance to diffuse through the polymer bulk phase (Giacin 1980). The driving force for migration is due to a concen— tration gradient, where the dissolved species diffuses from a high concentration area to an area of lower concentration (Giacin 1980). The rate of diffusion is defined by Fick's law: 8m = —DA 5c (9) where m is the mass of the component transferred, t is time, c is the concentration, D is the diffusion constant and A is the area of the plane across which diffusion occurs (Crosby 1981). The negative sign in front of the diffusion constant arises from the fact that the concentration decreases as mass transfer occurs. The flux (F, quantity/unit time/unit area) is the rate of diffusion perpendicular to the concentration gradient; x is the direction of diffusion; c is the concentration per unit volume. Crosby (1981) illustrates the diffusion through the 17 first plane (A) and the second plane (B) as: (A) dx ‘ (BI F=-D5c F2=F1+5Fdx X X The change in the flux, with distance = (- ac/ at), therefore is a_c = _§(D 5c) (10) Fick's second law defines D as constant and independent of the concentration: _ 2 The effect of temperature on diffusion is often described by an Arrhenius type equation: k = Aexp —(E/RT) (12) where k is the rate constant; A is the frequency factor; B is the activation energy of the reaction; R is the gas constant; T is the absolute temperature. Crosby (1981) expresses diffusion in terms of the Arrhenius equation by: D = Doexp -(ED/RT) (13) where ED is the energy of diffusion which is dependent upon the system. Briston and Katon (1974) identify three packaging material/contact phase migration systems; (1) nonmigrating, (2) independently migrating, (3) leaching. For system 1, 18 nonmigrating, the transfer of the component occurs only from the package surface. In this case the diffusion coefficient approaches zero and cannot be measured. In system 2, the component evaporates from the surface of the polymer and is replaced by component diffusing through the material. The diffusion coefficient of the migrant is measurable under the specific time-temperature conditions. "The rate of migration and the amount of migrant transferred is dependent upon the contact phase volume, boundary layer resistance in the extracting phase and the time scale for desorption (Giacin 1980)." In leaching, system 3, the contact phase penetrates the polymer causing swelling, thus acting as a plasticizer so migration will occur (Giacin 1980). Evaporation and diffusion of a component out of a material increases with temperature whereas the solubility decreases. Influence of temperature on migration of antioxidants out of films has been studied. Han (1984) profiles the migration of the antioxidant, butyl—4-methoxy phenol (BHA), out of high density polyethylene film at different temperatures. Little research is published on the influence of microwave heating on the migration of indirect food additives out of films. Heath and Reilly (1981) described the migration of acetyl—tributylcitrate (ATBC) from a plastic film into chicken during microwave cooking. ATBC was found to migrate out of the film and into the chicken. The rate of migration increased until the l9 chicken had been cooked for eight minutes. After eight minutes of cooking the rate of migration plateaus. The experiment had a cooking limit of ten minutes, so there was no data to indicate if the plateau represented the maximum absorption or if maximum extraction had been reached during the practical microwave cooking time for chicken products (Heath and Reilly, 1981). The experiment does indicate that the migration rate of ATBC is faster during the first part of the cooking period. Heath and Reilly (1981) show with their work that ATBC will migrated out of a film and I». .4... i into a chicken product after undergoing microwave cooking. 2.8 Packaging Materials for the Microwave Oven When a containers is placed in a microwave oven, the container material may absorb the energy, be transparent to the energy, or reflect the energy. The materia1(s) chosen for the package will depend on the desired heating of the product. A transparent material (i.e. most polymers currently used for packaging) will allow the microwaves to pass through with little absorption occurring. A transparent material is chosen when the entire product just needs to be heated. If the product has a large percentage of moisture then the package may experience temperatures around 1000 C. Higher temperatures may occur as the product dries out or if the product contains a large percentage of fat or protein. The product may reach temperatures which could 20 melt the plastic material. Therefore, a plastic that has good thermal stability should be chosen for products containing a high percentage of fat or protein, or products having a long cooking time. Paper products will absorb microwave energy if placed in a microwave with nothing else. Because paper products contain water in their structure as well as some mobile ions, paper products will heat in the microwave oven (Perry 1986). Usually the food product inside the paper container will absorb the microwave energy more readily than the paper container. Metals are materials which reflect the microwave energy. Metals reflect the energy back into the microwave cavity and do not allow the product to heat. If water is placed into an aluminum can and then is microwaved, the water temperature inside the can will not increase. Since these materials are good conductors of electricity, a large electrical potential energy may build up. If the material with the potential energy comes into contact with another piece of foil or a grounded surface an electrical discharge can occur. "This sparking (arcing) can occur between two packages, between layers of foil insulated from each other, across tears and wrinkles, and between foil packages and the oven walls (Perry 1986)." Arcing may be eliminated if precautions are taken such as centering a package in the oven or coating the foil with a non—conducting electric insulator. 21 A material which absorbs the microwave energy is called a susceptor. A susceptor is a metallized polyester laminated to a film, paper or paperboard (Perry 1986). The amount of metal on the polyester is so minute that arcing will rarely occur in the microwave oven. The construction of the susceptor causes localized heating to occur at the package/product contact surface. This localized heating causes higher temperatures which results in a drying and a browning effect on the food product surface. 2.9 Containers on the Market In early microwave ovens the use of metal containers was discouraged because of the potential damage the reflected energy might do to the magnetron. Therefore alternative packaging materials to metal were developed. Crystallized polyester (CPET) is a common nonmetallic material used in today's microwave market. The CPET is thermoformed into a tray which may be used in both the microwave and conventional oven. Collier (1986) explains that residual compounds can often exist in the thermoformed tray (CPET) and be released during heating. These residual compounds can affect the odor and taste of foods. As microwave ovens become more sophisticated and they are equipped with safeguards for protecting the magnetron, the use of modified metal containers becomes feasible. One method of modifying an aluminum tray is to coat the tray 22 with a polymeric coating, such as Alcoa's vinyl—matrix and epoxy coating, which enables the aluminum to absorb rather than reflect the energy (Aluminum Company of America). A plastic dome fits over the tray to provide extra protection against arcing between the sides of the tray and the sides of the microwave oven. Food will not brown in the microwave oven as it will in a conventional oven. A special package using a susceptor has been developed to help aid in browning by causing localized heating (Perry 1986). The susceptor material can be found in the microwave pizza packages and some of the microwave popcorn bags. Just as metal containers are being developed, plastic containers for microwave use are being improved. A new plastic container recently introduced into the market contains a shelf stable microwavable soup. The plastic bowl is constructed of a seven layer coextruded sheet composed of polypropylene / adhesive / polyvinylidene chloride / adhesive / polypropylene / adhesive / polypropylene copolymer. The lid material is an aluminum foil laminate composed of the following: oriented polyethylene terephthalate, adhesive, aluminum foil, adhesive, and polypropylene. The lid is designed to be removed before the soup is microwaved (Iezzi and Toner 1986). As more people buy microwave ovens, more convenience type foods will appear on the market. These food products 23 will have packages which are microwavable; thus, there will be a growth in the development of new materials that can be used in the microwave oven. MATERIALS AND METHODS 3.0 Description of Container A local supplier donated the containers used in this work. The containers were made from a coextrusion of polypropylene / Sarann/ polypropylene. The manufacturer's specifications listed the thickness of the respective layers as follows, .010/.001/.010 inches. The manufacturer did not specify the composition of the two tie layers. The materials were converted into a cup style by thermoforming. The dimensions of the container were as follows; height 7.2 i 0.1 cm, diameter at the top 6.5 + 0.1 cm, diameter at the bottom 6.3 i 0.1 cm. (See figure 4.) 3.1 Verification of the Container Materials The material was separated into two layers by warming the material in a microwave oven for four minutes on full power. A Perkin-Elmer 1330 Infrared Spectrophotometer was used to qualitatively identify the two layers. The attenuated total reflectance (ATR) method was used, because of the thickness and opaqueness of the materials (See appendix F.) An ATR was run on both surfaces of each strip. The materials were identified by comparing the 24 25 Coextruded Plastic Container Figure 4. 26 sample spectrums against known standard spectrums ("Identi— fication and Analysis of Plastics," Haslam et a1 1972). Both strips of plastic were then soaked in Toluene for five days. By the end of five days, a thin piece of plastic film was visible and was easily separated from one of the plastic strips. A tranmission IR was run on the clear thin piece of plastic. Qualitative verification of the sample was made by using the source mentioned previously. The thicknesses of the layers in the coextruded cup were checked by using an ocular microscope (American Optical Company). A piece of the container wall was placed into a freezer. The sample was held at approximately ~15°C for one week. The sample was then mounted, so a cross— sectional View would be visible, and trimmed to fit the sample holder. Freezing the sample gave a sharp cut surface. Magnifications of 43X and 10X were used. The eye lense contained a micrometer having a scale of 1 unit equals 0.725 mils, which was used to measure the thickness of the different layers. (See figure 5 for a cross- sectional View.) 3.2 The Power Level of the Microwave Oven An Amana Radarange (model RRlOlO) microwave oven was used for all microwave experiments. Power levels could be selected from 1 to 9, with level one being the lowest power output and 9 being the second highest. If no power level mouse. Honounuoomim mono Hose mmmmmmmmmm a? //////////% ooooooo . 28 was chosen then the microwave ran at full power. The power output of the mircowave was determined by placing a 500 ml beaker containing 300 mls of deionized water in the micro- wave oven. The water was heated for 60 seconds. The water temperature was measured before and after microwaving. Five different water samples were microwaved at each of the following levels; full power, 9, 8, 7 and 6. The power (P) equation: AtP = mC,AT was used to calculated the power of the microwave output (Fennema 1975). M was the mass of the water; t was the microwaving time; C, was the specific heat of water; AT was the change in temperature the water experienced. 3.3 Temperature Correlation and Sample Preparation The plastic samples were held in glass hypo—vials (Pierce) having a volume of 39.008 t 0.148 ml with a standard deviation of 0.148. The volume of the vials was determined by weighing the vial with and without distilled water. The assumption was made that the density of the water used was .9982 gram/m1 at 20°C. Ten vials were filled with distilled water and weighed. Teflon/silicone septums and aluminum crimp caps were used to seal the vials. All vials were cleaned and baked at 120°C prior to usage. The plastic cup was cut into strips approximately 1 cm wide by 6.7 cm long. Five strips weighing a total of 2.00 t 0.05 grams were selected (Mettler AE160 balance). 29 Melting point standard waxes, obtained from Omega, were used to determine the temperatures of the plastic strips during microwaving. The waxes came in liquid form and were applied by capillary action using pipets. The drops of waxes were no larger than 6 mm in diameter, which were significantly smaller in size by comparison to the plastic srtips. The waxes dried five minutes after the solvent evaporated. Five different drops of melting point standard liquid waxes were placed on the surface of three of the plastic strips. The melting temperatures of the waxes were as follows; 52°, 66°, 79°, 93°, 107° + .2°c. All five plastic strips were then cut in approximately half and put into a vial. The strips with the waxes were randomly oriented in the vials. The microwave oven was preheated before runs and cooled between runs to maintain an oven temperature of approximately 27°C. The preheating was done by placing 2 grams of plastic in an unsealed vial and microwaving on high for 4.0 minutes. Eight minutes were allowed between each microwave run to permit the oven to cool down. An apparatus was constructed to suspend the vials above the glass plate in the microwave oven. (figure 6 a & b) The apparatus was placed in the microwave oven so that each vial would hang over the center of the glass plate. The distance of the vial above the glass floor was maintained between 1/8 and 3/8 inch. All experiments were run on full power. The vials 30 R \\\\\-e s». L X Figure 6.a. Apparatus in the Microwave Oven 3““ °:‘\\\ .. Figure 6.b. Apparatus for Suspending a Vial see as 31 were microwaved for different periods of time, ranging from 3.0 to 7.0 minutes in 30 second increments. After microwaving, the plastic strips with the waxes were examined to determine which ones melted. The waxes were considered melted if they were partly or completely glossy. The melt condition of the waxes for each different melt standard were recorded, as a yes or a no. A scale was developed to convert the yes or no recordings into tempera- tures. (See appendix A for the scale.) 3.4 Concentration vs Time Microwaved Correlation To quantify the volatiles evaporating from the plastic containers, samples were prepared as followed, 2.00 i 0.04 grams of the plastic Container were weighed and placed in a clean vial. No waxes were applied to the plastic samples. The vials were flushed with nitrogen gas before sealing to rid the vials of all contaminants. The vials were sealed with teflon/silicone septums and aluminum crimp caps. A variation of the hot jar technique was then used. Hamilton 0.5m1 gas tight syringes were placed in a conven- tional oven maintained at 70°C and allowed to heat for 30 minutes before being used. The microwave oven was preheated as before. The vials were suspended from the same apparatus used in the temperature calibration section and kept a distance of 1/8 to 3/8 inches from the glass plate. The vials containing the plastic samples were 32 microwaved for time intervals ranging from 3 minutes to 7 minutes in 30 second increments. Ten runs were done for each time interval. Following microwaving, a 0.5 m1 aliquot of the headspace was drawn from the vial using a hot syringe. The sample was injected into a Hewlett Packard 5890A gas Chromatograph equipped with dual flame ionization detector (FID). Approximately 7 seconds was necessary to inject the sample, to prevent a surge of pressure in the capillary column. The G.C. conditions were as follows: capillary column 60 m with an I.D. of .32 mm packed with SPB-S (Supelco), 5% diphenyl / 94% dimethyl / 1% vinyl polysiloxane. To elute the sample, the G.C. was temperature programmed. The initial oven temperature was 75°C with an initial time of 5.0 minutes; the temperature was increased at the rate of 5° C per minute until the final temperature of 175°C was reached. The G.C. was then held at 175°C for 25 minutes. The injection port temperature was 200°C; column head pressure was 11 psi. A splitless injection port was used with the purge on after 0.70 minutes. The helium flow through the column was 1.02 cc/minutes. The flow rate was determined by injecting a sample of butane and recording the retention time (4.73 minutes). The following equation was used: 2 flow rate = (I.D) x (L) x .785 (14) rt of butane where I.D. was the inner diameter of the column and L was 33 the length of the column. 3.5 Qualitative Anaylsis of Major Chromatograph Peaks The samples of plastic used for the mass spectrometer were prepared as stated in the previous section. The vial was heated for 7 minutes in a microwave oven to produce a detectable concentration level. A 0.5 ml aliquot of head- space was taken with a hot syringe and injected into a (Jeol JMS-HXllO) mass spectrometer. The column was a DB1 megabore (J & W Scientific) with a bonded stationary phase of methylsilicone having an I.D. of 0.53 mm. The temperature program started at 70°C for 5.0 minutes and increased at a rate of SOC/minute until the final temperature of 175°C was reached. The flow rate through the column was 5 cc/minute. The chromatograph of the mass spectrum was obtained for each of the peaks broken down into mass fragments. The reference text, EPA/NIH Mass Spectral Data Base, was used to identify the peaks of the mass chromatograph. 3.6 Verification of the Mass Spectrometer Identification Using the above source, one peak from the mass spectrum was identified as Butylated Hydroxytoluene (BHT). To verify the BHT, a pure sample of BHT was placed in a vial and heated to 50°C. A headspace aliquot of .5 mls was injected into the gas chromatograph. (See section 3.4 for the G.C. conditions). 34 The BHT peak was found to have the same retention time as the retention time from the microwave chroma- tograph. Following the matching identification, a standard curve for BHT was constructed. 0.104 grams of BHT was weighed and placed into a 100 ml volumeric flask which had been partially filled with petroleum ether. Petroleum ether was added to bring the level to the 100 ml mark. A series of dilutions were made using the 100 ml stock solution: 2 mls into a 25 m1 volumeric flask, 4 mls into a 10 ml volumeric flask, 2mls into a 50 m1 volumeric flask and 2 mls into a 10ml volumeric flask. The volumes of the above series were brought up to the correct heights by adding petroleum ether to individual flasks. From the different dilutions 0.5 pl aliquots were taken and injected into the gas chromatograph. The conditions of the G.C. were the same as used for section (3.4). The area response units for each dilution were recorded. RESULTS AND DISCUSSION 4.0 Verification of container Using the ocular microscope technique three layers in the container wall were distinguishable. (See figure 6 for the depiction of the cross sectional area.) The thick—ness of the layers starting from the outside and moving inward were 8.3/1.5/10.2 mils, as compared to the manufacturer's specfication of 10/1/10 mils. Analysis of the infrared spectrums using the reference "Identification and Analysis of Plastics" showed that the outside and inside layers of the container were polypropylene (figure 7). The spectrum of the container's middle layer appeared to be Vinylidene chloride/vinyl chloride copolymer (figure 8). The percentage of each component remains unknown; the two most common ratios of vinyldene chloride/vinyl chloride are 80/20 and 90/10. An adhesive layer was found on both of the polyproplene layers which did not show up when the ocular microscope technique was used. The two adhesive layers had the same IR spec- trums as ethylene vinyl acetate. (See figure 9 for the IR spectrum.) 35 Kill mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm Ii"'"'.ll 'Il'fi' " .I lfl'll". .l'l'l'uuufll'lilluiflunnnhfiflmm mm 39 4.1 Power Output of the Microwave Oven To understand the difference in the level settings of the microwave oven, the power output at several of levels were calculated by using the equation stated in the experimental section 3.2. The calculated results are listed in table 2 and plotted in figure 10. Table 2. Power Output of the Microwave Oven Level Power Standard Coeff. of Setting (J/s) Deviation Variance 10 424.2 33.4 .079 9 379.3 17.8 .047 8 349.0 9.3 .027 7 305.1 15.0 .049 6 258.0 12.6 .049 The level setting of 10 was chosen to ensure the plastic would receive the most power the microwave oven could put out in the shortest period of time. 4.2 Microwaving Time vs. Temperature Correlation The microwaving time was measured via the digital screen on the microwave oven, which can measure to the nearest second. From observing the microwave oven, a cycling of power on and off could be seen. An assumption was made that the cycling always started from the beginning each time the microwave was turned on. Therefore this cycling was not measured. Power (Joules/sec) 4O POWER OUTPUT OF THE MICROWAVE 500 460 - 420 - 380 ~ 340_ 300 - 260 - 220 - 200 l l l l l l l l 1 Power Level Setting on Microwave Oven Figure 10. Power Output vs. Microwave Oven Level Settings l.... '7, 41 A scale was constructed to convert the 'yes' and 'no' results from the melting of the waxes into temperatures. (See appendix A.) From this data a curve was constructed to show the correlation between microwaving time and the temperature the plastic experienced. The relationship appears to be linear (see figure 11) for the range tested (3.0 to 7.0 minutes). Standard deviations are plotted on the same curve in order to show the temperature precision of the waxes. The largest deviation occured at 6 minutes 30 seconds and was 6.0°C. The time period from 0 to 2 minutes 59 seconds was not tested because the melting point standards have a lower temperature limit of 52°C. In this time period the temperature was not sufficient to cause melting of the waxes. 4.3 Heat Transfer via Conduction The glass tray in the bottom of the microwave is made with materials which are capable of absorbing the electro— magnetic waves. The glass tray will absorb the energy when the microwave oven is run with nothing in the cavity, or with a material with a low dielectic constant. Poly— propylene has a low dielectric constant (2.2) compared to water (80), therefore the polypropylene will not absorb microwave energy easily. The glass tray will absorb part of the energy and will become hot. The vial containing the plastic had to be suspended off the glass tray so there would be no conduction from the Temperature (00 ) 100.0 90.0 80.0 70.0 60.0 50.0 40.0 42 MICROWAVE TIME vs PLASTIC TEMPERATURE 0.0 Microwave Time (minutes) Figure 11. Microwave Time vs Temperature Experienced by the Plastic Strips 43 glass tray to the glass vial. The glass vial would get hot as the microwaving time increased. The plastic strips were placed into a vial randomly to determine whether the waxes were melting due to conduction from the glass. The waxes on the strips melted at random, which appeared to indicate that conduction from the glass vial did not influence the melting of the waxes. The waxes melt via the conduction of heat from the plastic. 4.4 Microwaving Time and Presence of Volatiles Several major peaks were found using the gas chroma— tograph technique. (See appendix B for a sample chroma— tograph.) Five components were chosen for further examination in this study. These five components had high enough concentrations so they could be detected by the mass spectrometer and the resolution on the G.C. was good. The components had retention times of 7.98, 15.28, 15.50, 22.87 and 30.86 minutes. The average concentration area units for each of the five major peaks was plotted against microwaving time. (See figures 12 a,b,c,d,e.) The concentration of volatiles in the headspace increased as the plastic was exposed to longer periods of microwaving. The plots of the concentration of the components at the retention times 7.98 and 15.28 minutes appear to be of an exponential function. The components at the retention times 15.50, 22.86, and 30.87 minutes increased at a slower rate over time compared to the components at 7.98 and 15.28 44 AREA RESPONSE vs MICROWAVE TIME (for retention time = 7.98 minutes) 100.0 90.0 - A 80.0 — 0 8 . 70.0 - \ I 3 5 60.0 ~ 0 I) L 3 50.0 — I) I S 40.0 ~ 0. I 0 ‘r 30.0 — 0 0 L < 20.0 — 10.0 — 0.0 l l l l l l I If I 0.0 2.0 4.0 6.0 8.0 10.0 Microwave fime (minutes) Figure 12.a. Microwaving Time vs Concentration for a Component with a Retention Time of 7.98 Minutes 45 AREA RESPONSE vs MICROWAVE TIME ' (Iorretention time = 15.28 minutes) 100.0 80.0 r 70.0 a 60.0 a 50.0 - Area Response (area unite/1000) 20.0 r 10.0 - 0-0 l l l l l l a 1 Microwave Time (minutes) Figure 12.b. Microwaving Time vs Concentration for a Component with a Retention Time of 15.28 Minutes 10.0 46 AREA RESPONSE vs MICROWAVE TIME (for retention time = 15.50 minutes) 50.0 A 40.0 ‘ a 0 0 \ II 1' § 30.0 - U 0 L 3 0 II 3 20.0 — 0. II 0 r: U 0 L < 10.0 ‘ 0-0 l l l 1 l l l l l 00 20 I0 00 00 100 Microwave Time (minutes) Figure 12.c. Microwaving Time vs Concentration for a Component with a Retention Time of 15.50 Minutes Area Response (area units/1000) 47 AREA RESPONSE vs MICROWAVE TIME (for retention time = 22.87 minutes) 50.0 40.0 - 20.0 ~ 0.0 l l l l l l l l l 0.0 2.0 4.0 6.0 8.0 10.0 Microwave Tlme (minutes) Figure 12 d. Microwaving Time vs Concentration for a Component with a Retention Time of 22.87 Minutes 50.0 A 40.0 0 0 O \ :3 § 30.0 0 0 L 3 0 I 3 20.0 0. I 0 K 0 8 < 10.0 0.0 48 AREA RESPONSE vs MICROWAVE TIME (for retention time = 30.86 minutes) 0.0 Microwave Time (minutes) Figure 12.e. Microwaving Time vs Concentration for a Component with a Retention Time of 30.86 Minutes 49 AREA RESPONSE vs MICROWAVE TIME (tor all five retention times) 100.0 90.0 '- 70.0 — 50.0 a 40.0 a 30.0 - Area Reaper-nee (area unite/1000) 20.0 - 10.0 — 0-0 I I I ' I I I I I I 0.0 2.0 4.0 6.0 8.0 10.0 Microwave Time (minutes) I] It 7.98 minutes + rt 15.28 minutes 0 rt 15.50 minutes A rt 22.87 minutes X rt 30.86 minutes Figure 13. Relationship between the Concentrations of all Five Components vs Microwaving Time 50 minutes. Figure (13) depicts the concentration relation— ship between all five components. A correlation may exist between the size of the molecule and the length of the retention time. The longer the retention, time the larger the molecule. This correlation maybe important in identifying the components and in understanding the mechanism of transfer out of the plastic. 4.5 Mass Spectrometer Identification The spectra obtained from the mass spectrometer were compared to standard spectra in the EPA/NIH Mass Spectral Data Base. (See figure 14 for the mass spectrum. See figures 15 a,b,c,d,e,f,g for the individual mass fragmentation schemes.) The spectrums are tentatively identified in table 3. Table 3. Summary of Tentative Identification of Mass Spectra Mass Spec compounds Molecular retention time Weight (minutes) 1.13 2,4 dimethyl Heptane 128 3.19 2,5-dimethyl Nonane 156 5.00 2 methyl undecane 170 5.48 4,8 dimethyl undecane 184 6.00 5 ethyl—5 methyl—decane 184 12.45 * ————————————— 212 17.58 Butylated Hydroxytoluene 220 * Not able to identify the hydrocarbon. 51 cmomaeaa EDHuomam mmmz .qa mpswflm our .mv r®m mg as mi w an ID H HEPUCH'HPZU U 's _; m m—«c¥-~ 52 N\Z a?“ mmuscflz mH.H mo mEHH COHquumm How COHumquEmem mmmz GNA Gad mm 4 .m.mH ohdmflm Gm LI _ .m: _ a _ a pm (II—HJC'OGEUQJ flim—-oe~:>m 53 mmuDGHZ mH.m mo wEflH coaucmumm How coaumquEmeh mmmz .n.ma mpswflm N\Z am." av.“ aNa am.“ am am h —I — .- uh u. — — > w P ~ — _ _ > p 4 c . mm“ # m2 = _ . hNa . mm m . .mm o C . K a I v 3 . :9» c v 3 mm m m .. 1mm m n w 3 . . s. . v 6 . .mm a . v .w . m mm 00H 54 mousse: oo.m mo mEHH COHquumm now coaumucofiwmum mmmz .o.mH mudwflm N\E S.NN afiN am." awe mm; GNH aw." am am. . _ I # _ __ __. _ __ . _ c . fie.“ mm . . ems -am 1 .8g . mm 160 L .G@ . C. . mm . CEJISC'OOCUQJ Ctm~nsv~>m C] C] *4 55 mmudcflz wq.m mo mEHH cowucmumm Mom COHumUEmemum mmmz .p.mH opnwflm Nxz 8N am: am: mm; mm am: am am a — . . u m » m > p u —b .4 p . _— . _. — f. a: mm - c. H m: .8 j . .9. - .am . .am R hm . CEJJC'UOEUM (rm—enhm-‘Dvu C] C] .4 ‘ I mmudcwz 00.0 mo 95H. coaucmuwm How SOHumucoEmmwm mmmz .o.ma 9:;me Nxz mam mm H mm“ 9.; am“ mama mm mm a. Mr ._ __ _. _ _ _ __ _ . mm . . pm.“ . m3 . . .8 M r r. A v C r . .0 mm . -mv n H , . . mm . c I -mm o A v > A V d- . «N . A. G I law a u U . . a . mm was 57 mmudcflz m¢.NH mo mEflH coauCoumm How Goaumucmfimmwm mmmz .w.ma ondwflm Nxz amm saw and am: 9. a snag am a am am - . . ’ - p p n p p“ . - u. p . .p p . u n .u _b I» i— . .1. — . ~L . m2 _ _ ___ _ _ __ . 4 mm“ . . RNA m: mm .8 ®.®m* . . _ _ _ w a . um; c. a New Em . m m . H mm . km E a CEJIJC'UGEUO 0:0—edv-«31: 58 mmuchZ mm.mH mo oEflH COHucmumm How COHumucmemwm mmmz .w.mH madman N\Z mmN flaw am“ am: Gm - r . — J 1 . J ‘— . _._ ___:_ _.__ __— — _ «=_ I: .1 :— 2 1H 41 J —<. .E . — Lin 41—. km. _ . Hm“ H H m a . mmfi find m H mm hm . o . Am -GN o . C r 6 . m3 3 - -aw c v 3 J m . .mm m > . .. I I ‘9. M I [am fl v M . . m mm mmm one 59 Positive identification of any the compounds listed above would have to be done by obtaining a pure sample of the compound and injecting a sample into the gas chroma— tograph. If the retention times were the same as previously found for the G.C. work then the pure compound should be evaluated by mass spectrometer. The two mass spectra could then be compared to see if the mass fragmen— tations were the same. The peaks with the retention times 1'13" through 12'45" appear to be hydrocarbons. As retention time increases so does the molecular weight of the hydrocarbon. The identity of these hydrocarbons was not verified using gas chromatography. Therefore no direct correlation was made between the G.C. and the mass spectrometer. The speculation made was based upon the order of retention times, the amount of area response units from the G.C., and peak heights from the mass spectrometer. The results of the speculation are listed in table 4. Table 4 Speculation on the Correlation between the Mass Spectrometer and the Gas Chromatograph. Mass Spectrometer Gas Chromatograph retention time retention time (minutes) (minutes) 3'19" 7.98 5'48" 15.28 6'00" 15.50 12'45" 22.87 17'58" 30.86 60 Butylated Hydroxytoluene was verified by injecting a headspace sample of gaseous BHT into the gas chromatograph. The G.C.peak from the BHT headspace sample matched the G.C. peak from the microwave sample. A standard calibration curve for BHT was then constructed. (See Figure 16.) The slope of the curve was found by using linear regression. Using the standard curve, the area response units from figure 16 were converted to grams of BHT per gram of plastic. (See appendix C for a sample calculation used for the conversion. See figure 17.) The data were then plotted against microwaving time. This curve has the same fit as when the data was plotted against area response units. 4.6 Origin of the Hydrocarbons and the BHT The hydrocarbons could be coming from either scission of the polymeric chains or residuals remaining from the polymerization process. To determine whether the electro- magnetic waves could cause scission a plastic sample was placed into a conventional oven. A headspace aliquot was injected into the G.C. to see if the retention times from the conventional oven method compared to those of the microwave method. The chromatographs were the same. Therefore the hydrocarbons were not produced because of the electromagnetic waves but probably due to a thermal phnenomenon. The hydrocarbons were most likely residuals from the processing of the plastic cup because the energy 61 required to break a C—C bond is 347 Kj/mol. The tempera- ture to induce C—C bonds to break is higher than the melting temperature of polyproplyene (165°C). The plastic strips only obtained a maxium temperature of 90°C during microwave heating. Area Response (area uhlts/1000) 200 190 1&0 110 100 150 140 1&0 120 1L0 100 90 8O 7O 00 50 4O 30 20 10 00 Figure 16. 62 STANDARD CURVE FOR BHT GomsofBHTxE—B Standard Calibration Curve for Butylated Hydroxytoluene (BHT) 63 CORRELATION BHT CONCENTRATION vs MICROWAVE 11ME 15.00 14.00 — 13.00 ~ 12.00 - 11.00 ~ 10.00 ~ 8.00 - 7.00 _ 6.00 - 5.00 ~ (grams BHT >< EZ—B/grarne plastic) 3.00 - Cane. 1.00 ~ 0-00 r I I I I I I I I I 0.00 2.00 4.00 6.00 8.00 Microwaving Mme (minutes) Figure 17. Grams of BHT per Gram of Plastic vs. Microwaving Time 10.00 CONCLUSION Consumers have been taught never to place metal in a microwave oven and never to run the microwave oven empty. Since consumers shy away from foil packages, plastic containers have become the alternative packaging solution. How good are plastic containers? Plastics in general have low dielectric constants if compared to water. However, a plastic container can absorb microwave energy, if no other substances are placed in the microwave oven along with the plastic container. A plastic container, composed primarily of polypropylene, reached an approximate temperature of 88°C after 7.0 minutes of micro- waving. If an empty plastic container can reach a temperature of 88°C in 7 minutes then what temperature would the container experience if filled with a food product? Food products will readily absorb more energy than the plastic container, because of their high water content. Liquid water within the product absorbs microwave energy and converts the energy into heat. The heat produced will be conducted to the remaining mass of the food product and the plastic container. The plastic container in this case 64 65 is heated indirectly. An incorrect assumption would be that the temperature of the container would never exceed 100°C. A plastic container may experience temperatures higher than 100°C, the boiling point of water, if the product contains sugars, oils, or proteins. Sugars and oils also absorb microwave energy but not as readily as water. With temperatures higher than 100°C more indirect food additives may evaporate and migrate into the food product. The food additive regulations do not list microwave heating in the "Conditions of Use" set forth in Table 2 of 21 C.F.R. 176.170 (c). Many polymeric packaging materials are limited with respect to the temperature under which the materials may be used. The regulations do not make clear which of these materials are approved for use in packaging partially precooked foods intended to be microwaved. Components will evaporate from a plastic container which has undergone microwave heating. One of the components identified as evaporating from the plastic container was butylated hydroxytoluene. After 7 minutes of microwaving the plastic container, 13.734 x 10 grams of BHT/ gram of plastic was detected in the headspace of a sealed vial. The FDA permits .02 % of BHT by weight to migrate into a food product. The other components detected in the experiment were hydrocarbons. As microwaving time increased so did the concentration of these components in the headspace of the sealed vial. The FDA has no restriction on the amount of 66 hydrocarbons allowed to migrate into a food substance. If any work on the migration of hydrocarbons has been done it is very limited. Therefore little is known as to whether hydrocarbons influence the quality or safely of the food product. This experiment should be done using a different contact phase such as water or oil. The experiment could determine how the temperture of the container is influenced by the contact. The higher temperatures may induce more components from the plastic container to migrate into the contact phase. As to whether these migrating components would alter the food quality or exceed the regulation levels must also be determined. APPENDICES APPENDIX A Temperature Scale YYN = 52°C NNN, YNN, YYN = 66°C NNN, YNN, YYN = 79°C NNN, YNN, YYN = 93°C “ NNN, YNN, YYN = 107°C 67 APPENDIX B 6!! 7. 152’ 15 SO '10...» 22 I) I.” I!“ Figure 18. Sample Gas Chromatograph 68 mha APPENDIX C Sample Calculation Used for Converting Area Response Units into Grams of BHT per Grams of Plastic E1)(V -R} grams of BHT S A -P grams of polymer = .1172 grams BHT Area Response from Standard BHT Curve = Volume of the Vial (40mls) = Aliquot Size (.Smls) = Area Response from figure 12.e. for a given Microwave Time = Plastic Sample Size (2 grams) 69 APPENDIX D Gas Chromatograph Raw Data for Five Different Retention Times vs. Microwaving Time. Table 5. Gas Chromatograph Data for Microwaving 3.0 Minutes Area Response (area units) Vial # rt 7.98 rt 15.28 rt 15.50 rt 22.87 rt 30.86 minutes minutes minutes minutes minutes A/3.0 8432 7866 3862 1496 5128 B/3.0 7668 6357 2768 1404 2786 C/3.0 9762 7184 3270 1025 4283 D/3.0 6030 5031 2196 622 2056 E/3.0 6015 5263 1876 748 2934 F/3.0 6263 5131 2322 589 2345 G/3.0 5617 4459 2313 856 2927 H/3.0 3809 5680 3987 1876 2679 I/3.0 7070 6015 2907 333 2852 J/3.0 9282 7767 3581 1079 4674 Table 6. Gas Chromatograph Data for Microwaving 3.5 Minute Area Response (area units) Vial # rt 7.98 rt 15.28 rt 15.50 rt 22.87 rt 30.87 minutes minutes minutes minutes minutes A/3.5 11836 9711 4685 3897 4267 B/3.5 12870 9984 4886 1443 6207 C/3.5 4902 4740 2574 907 2748 D/3.5 11196 8855 4727 1296 6147 E/3.5 5186 3912 1825 650 6927 F/3.5 7218 5703 2463 739 3357 G/3.5 5015 5194 2444 1329 6027 H/3.5 8046 6195 2984 2336 7393 I/3.5 9175 5932 3109 ~--— 4765 J/3.5 12868 8646 4219 1167 1945 70 71 APPENDIX D (continued) Table 7. Gas Chromatograph Data for Microwaving 4.0 Minutes Area Response (area units) Vial # rt 7.98 rt 15.28 rt 15.50 rt 22.87 rt 30.86 minutes minutes minutes minutes minutes A/4.0 19051 12394 5992 1120 5659 B/4.0 16693 13705 6511 2104 9881 C/4.0 16146 10289 4968 1198 5792 . D/4.0 9414 5963 2763 790 3307 E/4.0 11195 6950 3367 980 5597 F/4.0 21272 11956 5645 1323 7166 G/4.0 10941 6519 3428 2105 4516 H/4.0 12463 9764 4988 1581 6077 I/4.0 16031 10188 4971 1361 6030 J/4.0 16571 11945 5860 1876 5594 Table 8. Gas Chromatograph for Microwaving 4.5 Minutes Area Response (area units) Vial # rt 7.98 rt 15.28 rt 15.50 rt 22.87 rt 30.86 minutes minutes minutes minutes minutes A/4.5 10881 10270 5185 2031 10094 B/4.5 19010 14245 7133 2565 9369 C/4.5 28833 20887 10584 3855 14270 D/4.5 23388 16275 8028 2929 11849 E/4.5 20904 15186 8000 3165 8172 F/4.5 34389 22710 11682 4511 10275 G/4.5 42160 26887 13861 4279 8222 H/4.5 11397 8073 4152 6290 4971 I/4.5 14381 9802 4952 1752 3968 J/4.5 13791 10528 5406 1611 5773 Note rt = retention time 72 APPENDIX D (continued) Table 9. Gas Chromatograph Data for Microwaving 5.0 minute Area Response (area units) Vial # rt 7.98 rt 15.28 rt 15.50 rt 22.87 rt 30.86 minutes minutes minutes minutes minutes A/5.0 30633 22754 11498 3974 14253 B/5.0 34900 20890 10380 2861 14609 C/5.0 47394 27085 14227 5800 13981 D/5.0 26901 15102 7288 2270 9614 E/5.0 27748 19045 9346 3691 11555 F/5.0 57355 30484 14942 3652 10907 G/5.0 19890 13700 6778 3025 10118 H/5.0 26160 16887 8408 4238 12393 I/5.0 34367 22478 11020 4295 11860 J/5.0 23466 14031 6696 2222 6669 Table 10. Gas Chromatograph Data for Microwaving 5.5 Minutes Area Response (area units) Vial # rt 7.98 rt 15.28 rt 15.50 rt 22.87 rt 30.86 minutes minutes minutes minutes minutes A/5.5 66027 39819 19940 6745 15621 B/5.5 31102 21803 11650 4786 14466 C/5.5 30897 21909 11162 4514 11996 D/5.5 60266 28223 14274 4075 17452 E/5.5 26013 18739 9827 4038 11972 F/5.5 29713 20480 9804 4627 12908 G/5.5 51728 39606 19779 7285 22435 H/5.5 66109 46653 23601 8255 22907 I/5.5 31487 22315 11011 3780 10440 J/5.5 22604 16175 8123 5327 25099 K/5.5 26530 18208 8997 3506 11765 L/5.5 32698 22077 11044 4417 14536 M/5.5 38767 24779 12963 3942 7775 Note rt = retention time 73 APPENDIX D (continued) Table 11. Gas Chromatograph Data for Microwaving 6.0 Minutes Area Response (area units) Vial # rt 7.98 rt 15.28 rt 15.50 rt 22.87 rt 30.86 minutes minutes minutes minutes minutes A/6.0 73815 50211 24417 8924 24353 B/6.0 70109 44507 21935 8624 23121 C/6.0 75146 52358 26525 9639 27658 D/6.0 49404 36599 18098 8048 30484 E/6.0 65704 46115 22611 9068 29127 F/6.0 57010 39872 20207 7353 24157 G/6.0 37214 25807 13298 5171 17956 H/6.0 42501 29644 14575 6442 24647 I/6.0 55233 30857 16308 5543 11641 J/6.0 31629 20312 10306 4124 9445 K/6.0 26644 16979 8415 3250 7673 Table 12. Gas Chromatograph Data for Microwaving 6.5 Minutes Area Response (area units) Vial # rt 7.98 rt 15.28 rt 15.50 rt 22.87 rt 30.86 minutes minutes minutes minutes minutes A/6.5 65519 45541 22445 7641 23121 B/6.5 111180 77076 38755 14016 29932 C/6.5 73810 50961 26038 7866 15427 D/6.5 58616 40688 20395 6246 11204 E/6.5 32987 22879 11702 4066 8499 F/6.5 71049 49291 23886 8989 17170 G/6.5 61446 39503 19446 7837 20273 H/6.5 50568 32633 16432 6207 15761 I/6.5 62849 41309 20688 7689 17006 J/6.5 93920 66559 34604 11781 33402 K/6.5 46000 29990 15307 5791 16387 Note rt = retention time 74 APPENDIX D (continued) Table 13. Gas Chromatograph Data for Microwaving 7.0 Minutes Area Response (area units) Vial # rt 7.98 rt 15.28 rt 15.50 rt 22.87 rt 30.86 minutes minutes minutes minutes minutes A/7.0 55062 37463 18304 6825 23277 B/7.0 75146 42054 20345 5496 16732 C/7.0 108390 69836 35672 16337 25003 D/7.0 65196 36628 17919 5592 18842 E/7.0 76269 42969 21415 5903 28005 F/7 0 66897 38148 18945 5627 19638 G/7 0 133800 77545 38863 14764 54058 H/7.0 121190 69869 34955 11709 46984 I/7 0 72152 40590 19899 6092 22743 J/7 0 95838 58133 29519 9771 41680 Note rt = retention time APPENDIX E Temperature Data for Different Microwave Times Table 14. Temperature Data for 3.0 Minutes Sample Number Temperature (°C) 180a 52 180b 52 180c 52 180d 52 1806 52 180f 52 1809 52 180h 52 Table 15. Temperature Data for 4.0 Minutes Sample Number Temperature 1°C) 240a 66 240b 66 240C 66 240d 66 240e 66 240f 66 2409 66 240h 66 75 Table 16. Table 17. 76 APPENDIX E (continued) Temperature Data for 4.5 Minutes Sample Number 270a 270b 270c 270d 270e 270f 270g 270h Temperature (00) Temperature Data for 5.0 Minutes Sample Number 300a 300b 300C 300d 300e 300f 300g 300h 3001 Temperature (00) 77 APPENDIX E (continued) Table 18. Temperature Data for 5.5 Minutes Sample Number Temperature(°C) 330a 79 330b 66 330C 66 330d 79 3308 79 330f 66 330g 79 330h 79 3301 79 330j 66 Table 19. Temperature Data for 6.0 Minutes Sample Number Temperature (°C) 360a 79 360b 66 360C 66 360d 79 3608 79 360f 66 360g 79 360h 79 360i 79 78 APPENDIX E (continued) Table 20. Temperature Data for 6.5 Minutes Sample Number Temperature (°C) 390a 79 390b 79 390C 66 390d 79 390e 66 390f 79 390g 79 390h 93 3901 79 390j 93 Table 21. Temperature Data for 7.0 Minutes Sample Number Temperature (°C) 420a 79 420h 93 420C 93 420d 79 420e 93 420f 93 4209 79 420h 79 420i 93 420j 93 APPENDIX F Instructions for Attenuated Total Reflectance The following instructions for the ATR method were modified from Perkin-Elmer. 1. 2. Remove the clamping plate and clamping block from the sample holder assembly afer removing the three knurled- head screws from the assembly. To insert the crystal (KBS-S) in the crystal holder assembly: a. Turn the knurled—head crystal release screw on top of the crystal holder assembly, to open the cell. b. Slide the crystal between the grooves of the three 'V' blocks on the holder until the crystal is seated against the limiting adjustment pin. c. Turn the crystal release screw counterclockwise until spring tension holds the crysal in place. Place one of the pressure pads on the clamping plate and place the film sample over the pad. (The sample must be as long as the pad. The pressure pad must not be in direct contact with the crystal.) Place the crystal holder and crystal down on the plate so that the holes in the crystal holder are centered over the holes in the plate. Place the sample and then a pressure pad in the cutout of the crystal holder. Place the clamping block over the gasket. Insert the three knurled screws, removed in step 1, in the clamping block and screw them through the clamping plate until resistance is felt. 79 10 11 12. 13 80 APPENDIX F (continued) Tighten each screw a small amount as evenly as possoble until each screw is finger-tight. Mount the crystal holder assembly on the accessory base so that the crystal holder is at a 45 degree angle from the third mirror. To clamp the holder onto the mounting pin, tighten the 1/8 inch Allen clamping screw on the side of the holder. Adjust mirror 2 with the adjustment setscrew until the exit face of the crystal appears the brightest. Adjust mirror 3 with the adjustment setscrew until maximum upscale deflection of the recorder pen is obtained. Use a reference beam attenuator to bring the maximum transmittance level to 80-90 %T. Operate the spectrophotometer according to the instrument instruction manual. BIBLIOGRAPHY BIBLIOGRAPHY 1 Agoos, A., "Serving Up a Better Package for Foods," Chemical Week, October 1985, pp 82-83. 2 Aluminum Company of America, Alcoa Opens the Door to America's First Microwave-Compatible Aluminum Tray: Wave'r Bake, Pittsburgh, Pa, 1986. 3 Billmeyer, F.W., Textbook of Polymer Science; 3rd Edition, New York, John Wiley & Sons Inc., 1984, pp 344 — 348. 4 Bishop, C.S. and Dye, A., "Micowave Heating Enhances the Migration of Plasticizers Out of Plastics," Journal of Environmental Health, March/April 1982. 5 Briston, J.H. and Katan, L.L., Plastics in Contact with Food, London, Food Trade Press LTD, 1974, pp 136-150. 6 Collier, J.C., "Practical Considerations on the Use of PET and Food Containers," Packaging Technology, May/ June 1986. 7 Copson, D.A., Microwave Heating; 2nd Edition, Westport, Connecticut, AVI Publishing, 1975, pp 1 — 28 8 Cotton, T.S., Personal Consultant, Gerber Products Company, Fremont, Mi, 1986. 9 Crosby, N.T., Food Packaging Materials; Aspects of Analysis and Migration of Contaminants, London, Applied Science Publishers LTD, 1981, pp 106 — 122. 10 Fennema, O.R., Karel, M. and Lund, D.B., Principles of Dood Science Part II; Physical Principles of Food Preservation, New York and Basel, Marcel Dekker Inc., 1975. 11 Giacin, J.R., "Evaluation of Plastics Packaging Materials for Food Packaging Applications: Food Safety Considerations," Journal of Food Safety, 1980 Vol. 2:4. 81 1 N 13 14 15 16 17 18 19 20 21 22 23 82 Gilbert, S.G., Miltz. J., and Giacin, J.R., "Transport Considerations of Potential Migrants from Food Packaging Materials," Journal of Food Processing and Preservation, 4(1980), 27-49. Han, J.K., "Evalutionof Loss of 2-Tertiary Buty1—4- Methoxy Phenol (BHA) from High Density Polyethylene Film," Thesis for M.S., Michigan State University, 1984. Hancock, G., "New Microwavable Packaging System Could Revolutionize Frozen Food Business," Packaging Technology, May/June 1986, pp 23-25. Haslam, Willis, Squirrell, Identification and Analysis of Plastics; 2nd Edition, England, Hazell, Watson & Viney Ltd., 1972. Heath, J.L. and Reilly, M., "Migration of Acetyl— Tributylcitrare from Plastic Film into Poultry Products During Microwave Cooking," Poultry Science, 1982, pp 2258—2264. Iezzi, R.A. and Toner, D.F., "Plastics Packaging Thrusts at Campell Soup Company," Packaging Institue Conferenc, Changing Patterns in Domestic and Interna- tional Packaging, Illinois, September 1986. Karel, M., Fennema, O.R. and Lund, D.B., Principles of Food Science; Physical Principles of Food Preservation, New York, N.Y., Marcel Dekker Inc., 1975. Kashtock, M.E., Giacin, J.R. and Gilbert, S.G.,"Migration of Indirect Food Additives: A Physical Chemical Approach," Journal of Food Science, Vol.45, 1980, pp 1008-1011. Mudgett, R.E., "Microwave Properties and Heating Characteristics of Foods," Food Technology, June 1986, pp 84—93. Nelson, E.B., "The Microwave Consumer : A Life Style Profile," Journal of Microwave Power, 17(4), 1982. NSRDS—NBS 63, EPA/NIH Mass Spectral Data Base, Washington, D.C., U.S. Government Printing Office, 1978. Perry, M.R., "Packaging for the Microwave Oven," 1986, Food Product Packaging Compatability Seminar, Michigan State University, July 1986. 24 ’25 26 27 28 2 \O 30 31 83 Rice, J., "New Mexican-Style Snack Products in Aseptic, Microwavable Packaging," Food Processing, September 1986, pp 82-83. Schiffmann, R.E., "Food Product Development for Micro- wave Processing," Food Technology, June 1986, pp 94- 98. Shapiro, R.G. and Bayne, J.F., "Microwave Heating of Glass Containers," Food Technology, February 1986, pp 46-48. Slatkavitz, K.J., Personal Consultant, Pillsbury Company, Minneapolis, Mn, 1986. Tinga, W.R., "Interactions of Microwaves with Materials," 1970, Proceedings of IMPI Short Course, Microwave Processing in the Food Industry, Chicago, 1979. VanKoughnett, A.L., "Fundamentals of Microwave Heating," 1973, Proceedings of IMPI Short Course, Microwave Processing in the Food Industry, Chicago, 1979. VanKrevelen, D.W., Properties of Polymers; Their estimation and Correlation with Chemical Structure, Amsterdam, Elsevier Scientific Publishing, 1976 pp 233— 256. White, J.R., "Why Materials Heat," 1973, Proceedings of IMPI Short Course, Microwave Processing in the Food Industry, Chicago, 1979. HICHIGR III 329 IIIIIIITIIIII