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Volatiles in packaging materials Heasook Kim‐Kang
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Packaging Division, Libra Laboratories Inc., Room No. 330, 44 Shelton Road, Piscataway, NJ, 08854 Version of record first published: 29 Sep 2009
To cite this article: Heasook Kim‐Kang (1990): Volatiles in packaging materials, Critical Reviews in Food Science and Nutrition, 29:4, 255-271 To link to this article: http://dx.doi.org/10.1080/10408399009527527
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Food Science and Nutrition Volatiles in Packaging Materials Heasook Kim-Kang
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ABSTRACT The increasing application of complex natural and/or synthetic polymers to food packaging has required definitive information on the characteristics of the finished products. High temperature encountered during the manufacturing process may induce thermal decomposition products that can migrate into the packaged product and cause undesirable flavor. A general methodology for testing polymer odor and odor contributors is discussed in this article with examples representing the odor of a variety of packaging materials. The precursors and the mechanisms of the major volatile components of each packaging material are presented.
I. INTRODUCTION Food packaging has been dominated in the last few decades by innovations in plastic packages. These include the development of different types of polymers, copolymers, the laminates, and packaging materials suitable especially for microwave heating. Considerabe improvement in design effectiveness has been achieved with the introduction of new materials and processes, making this area one of the most active in food product development. This growth has been paralleled by an increasing attention to the interaction between plastic packaging materials and foods. One of the practical concerns related to a polymeric package is the presence of compounds in toxicologically insignificant amounts, but at levels affecting aroma and/or taste of the packaged food. 13 Although the pertinent Food and Drug Administration (FDA) policy mainly emphasizes the total nonvolatile extractives, there has been an extensive history of odor and taste problems experienced in the development of new packages.4"9 A package can directly disrupt the flavor balance of a food in three ways: (1) subtraction, (2) reaction, and (3) addition. Subtraction occurs when components contributing to the desired flavor of the product are absorbed by the package.10"15 Reaction takes place when package components chemically interact with the food product to produce flavor artifacts. Sometimes the components in a packaging material, such as metal components, can act as a catalyst to accelerate the decomposition of food ingredients, resulting in undesirable flavor.16-17 Addition occurs when the package releases compounds that alter the flavor balance of the food.4'1820 Addition is by far the most common. The manufacture of packaging materials is often conducted under conditions of high temperature. These conditions can
easily induce thermal degradation with the formation of volatile compounds in packaging materials.2126 For example, a burnt polyethylene odor has been experienced in the paper/foil/ polyethylene laminate field. The increasing demand for single-serving meals, often referred to as "TV dinners", prompted the food companies to upgrade the quality of these meals and created what they refer to as gourmet dinners. The higher-quality dinners also required a package that presented a new and upscale image, but also had to meet the criterion of being dual-ovenable. This new trend led flavor and packaging chemists to develop an appropriate test to determine the levels of specific compounds migrating under intended-use situations, so that the optimum design for the packaging materials can be achieved.3 In a conventional oven, as in a microwave oven, food heated on plastic trays occassionally develops an off-taste or odor. These undesirable odors are considered to be derived from the breakdown products of the plastic material and/or residual solvents used in the manufacturing of the plastic trays.27-28 Packaging materials are of such a versatile composition that the precursors for volatile compounds can be from many sources. These can include residual monomers and oligomers, residual solvents from printing inks, adhesives, coatings, breakdown products of polymers, and additives, etc. This article summarizes volatile compounds identified in a variety of packaging materials, as well as the precursors of the major volatile components.
II. MIGRATION THEORY The fundamental processes by which trace amounts of solvents, reaction byproducts, additives, and monomers migrate from polymer barriers used in food and beverage packaging applications into the contents of a package have been discussed by a number of scientists.29"31 The classic theory of diffusion of gases is based on Fick's laws.32 Diffusion in homogeneous substances is based upon the assumption that the rate of transfer, R, of a gas passing perpendicularly through the unit area of a section is proportional to the concentration gradient through the section. This is expressed as: R = - D(C) ^ dX
(1)
where D(C) is the diffusion coefficient in cm2/s, in general, D can be a function of the local diffusant concentration, C is the
H. Kim-Kang, B. S., M. S., Ph.D., Packaging Division, Room No. 330, Libra Laboratories Inc., 44 Shelton Road, Piscataway, NJ 08854.
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Critical Reviews In concentration of diffusant in mol/cm3, and X is the thickness of the material in centimeters. Fickian diffusion can be visualized as a series of jumps in which Brownian motion of the polymer chain segments produces transient voids in the vicinity of the diffusant, enabling it to move through the polymeric matrix. The size of the void required to permit a jump is, of course, related to the size of the diffusant. The thermal energy that must be localized near the diffusant is related to the volume of the fluctuation, the cohesive energy density of the polymer, and the presence of nearby penetrants, which can facilitate the motion of the surrounding chains.32 Particularly at high concentrations of sorbed diffusant, the plasticizing effect of neighboring diffusants can cause pronounced concentration dependency of D. The temperature dependence of the diffusion coefficient is generally described by an Arrhenius form.33 The slope of such plots is related to the amount of energy required to form the transient voids required by the diffusion process. However, the activation energy is not always constant over the whole range of temperatures. For many organic vapor-polymer systems above the glass transition temperature of the polymer, Tg, the apparent activation energy for diffusion decreases with increasing temperature.29 Furthermore, for a series of homologous organic compounds in a given polymer, the activation energy is often nearly independent of the size of the penetrant molecule if the diffusant is greater than or equal in size to the mobile segment of the polymer.29 At the low concentrations typical of most indirect additives, diffusion coefficients approach limiting values that are only a function of temperature, and mathematical description of the transport processes becomes simple. Non-Fickian transport is known to occur at relatively high-diffusant activities for water in some polymers and can complicate the otherwise Fickian transport processes of indirect additives. The amount of package components that may migrate from a plastic packaging material into solid or liquid food or foodsimulating solvents depends on the physical and chemical properties of both the food and polymer.34-35 The controlling factors for the degree of migration would be the original concentration of the migrants, the solubility in the contacting phase, and/or the partition coefficient between the polymer and the contacting phase, the temperature, time, and the morphological structure of the polymer.31-36 Polymer properties affecting migration can include molecular weight, percent crystallinity, chain branching, density, and affinity for migrants.34 Most experimental observations on the migration of small molecules from polymeric packaging materials into food or food-simulating solvents showed some non-Fickian behavior. Solvent adsorption . and swelling of the polymer have often been observed when the behavior is non-Fickian. A model for a solute diffusing in a swelling polymer was used to explain this phenomenon.36 In another case, where the migrant is sparingly soluble in the solvent, a stagnant solvent layer at the polymer surface may give rise to an initial migration behavior that is linear in time instead of linear in square root of time. In certain cases where 256
the solvent is not stirred or is highly viscous, the quiescent migration is not found to depend on the diffusion coefficient of the migrant in the solvent.36 Either alone or in combination, these models can be applied to describe most migration behavior in rubbery or semicrystalline packaging material.36 Migration was divided into three classes, based on the limiting control mechanism30-34-37 Class 1— Nonmigrating materials, with or without the presence of food Class 2— Independently migrating not controlled by the food, although the presence of food may accelerate the migration Class 3— Leaching; migration controlled by the food; negligible in the absence of food, significant in its presence Even though there is no absolute cutoff between these various classes, Class 3 systems were described by Bristori and Katan37 as those that have diffusion coefficients less than 10" 12 cm2 s" 1 in the absence of food, but 10~9 cm2 s" 1 or more in its presence. Their definition of nonmigrating is diffusion coefficients of less than 10~12 cm2 s" 1 . Residual, unreacted monomers from most addition type polymers will have significant volatility at normal conditions of storage and use. This leads to the interesting but seemingly contradictory situation that the equilibrium concentration of such a substance in the food product is expected to be essentially zero by theoretical considerations. At the outer, noncontact surface of the film some of the monomer escapes from the film as a gas. It is reasonable to think of the atmosphere as an infinite sink with respect to this tiny amount of gas so that, over time, the concentration of the monomer in the atmosphere remains essentially zero. At some point, a dynamic equilibrium will be achieved based on the partition coefficient and the ratio of the solubility of the monomer in the food product and in the film. If there was an artificial barrier that prevented escape of the gas into the atmosphere, this would result in a maximum concentration of migrant in the food and no further migration would occur. No such barrier exists, however, and therefore the gas molecules that escape into the atmosphere after this dynamic equilibrium has been achieved reverse the concentration gradient driving force. The monomer will now begin to migrate from the food into the film and into the atmosphere. Vom Brack et al.38 designed a mathematical equation for migration of volatile additives in packaging materials, and the measured value was found to fit the theoretical value well. Gilbert39 proposed an "essential zero" theory, where any migrant on the active site is not available for migration. The theory supported the idea of minimum limit of VCM (vinyl chloride monomer). The diffusion mechanism also depends on the volatility of the migrants. The more volatile the components are, the higher the concentration in the headspace. The migration does not
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Food Science and Nutrition require a direct contact between food and packaging material. However, if the migrant is a high-boiling compound such as parabutylphenol, it requires a direct contact to result in offflavor in the food product.4
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III. METHODOLOGY A. Sensory Evaluation When evaluation techniques were compared with each other, organoleptic analysis worked better than instrumental analysis, since some off-flavors could be tasted but did not show up in headspace or in extracts of products.40 In addition, the threshold of human sense is often lower than that of the instrument. The odor quality of test plastic materials such as resins, films, or laminates can be estimated directly by panelists using a rating scale evaluation of intensity, or by forced choice methods such as triangle test, as the test forms.4'4142 The extent of migration of residual volatile compounds in packaging materials to the contacting phase can be evaluated after exposing water, food simulant, or actual food to packaging materials at accelerated conditions, at ambient temperature, or at actual cooking condition. I-4-21-28 Food can be exposed to packaging materials by direct contact or by vapor phase transfer to determine if the adverse odor/flavor problem is caused by any of the more volatile package constituents or caused by constituents, which can only be transferred by direct contact solvation. Glass containers are generally used as control packages. Sensory evaluation can be paralleled with objective instrumental analyses and the correlation between the sensory and instrumental results can be obtained. 171843 B. Instrumental Analysis Conventional methods of determining flavor concentration in materials such as food and confectionary products require lengthy solvent extraction, followed by gas chromatographic analysis. Sugisawa44 catagorized the methods used for the isolation of flavor concentrates under three general headings: (1) distillation techniques, including vacuum distillation;1845 steam distillation and carbon dioxide distillation; (2) extraction techniques, including solvent extraction, simultaneous steam-distillation-solvent extraction,4*48 carbon dioxide, and supercritical fluid extraction; and (3) miscellaneous techniques, including gas entrainment in open and closed systems,20-4952 and adsorption on charcoal and porous polymers,53'57 etc. Both the chemical composition and physical state of the sample matrix can affect the headspace composition. The analysis of flavor in packaging materials generally involves headspace gas chromatographic techniques. It is simple and reproducible. However, sensitivity of the analyses is too low. Although concentration of headspace onto polymers can increase the sensitivity, it requires the use of correction factors because individual volatile compounds have different affinities for the adsorbent.
The analytical method of choice for monomers is undoubtedly headspace gas chromatography. The monomers of primary concern to date have been vinyl chloride (VC), vinylidene chloride (VDC), acrylonitrile (AN), and styrene. Headspace gas chromatography consists essentially of placing the sample for analysis in a closed vessel: allowing the compound of interest to equilibrate between the sample and the surrounding vapor, usually with heating in order to obtain rapid equilibration: and, finally, to inject a sample of this equilibrated "headspace" into a gas chromatograph. (The term "equilibration" has been emphasized because this is an essential condition for quantitative analysis.) It is possible to perform the necessary operations manually, but automated chromatographs are available dedicated to headspace chromatography, which combine the advantages of automated sample handling with enhanced reproducibility.58'59 The popular choice of detector is flame ionization detector. However, a specific detector such as an electron capture detector,46'47-49-51 or a thermal energy analyzer,60^2 can be used, either to eliminate interferences or to increase sensitivity. Styrene is not an ideal candidate for analysis by headspace chromatography, for with a boiling point of 145°C its volatility is low in comparison with VC, VDC, or even AN. However, the concentration of styrene in polymers can be achieved by a modified solution headspace approach.63 This modified technique involves dissolving the polymer in an organic solvent in the normal manner and then altering the composition of the solvent phase to decrease styrene solubility and thereby increase its equilibrium concentration in the headspace. For this purpose water was found to be the most effective solvent, since water is normally free of interfering organic contaminants and it will not be sensed by the GC/FID. The modified headspace approach has also been applied for the analysis of 2-ethylhexylacrylate (BP 214°C) in polymers, and to the determination of 1,1,1-trichloroethane (BP 74°C) in PVC resins and bottles.64 The technique of selected ion monitor (SIM) mode GC/MS offers both a powerful confirmatory test, and the added advantage of increasing the sensitivity of the analysis.52-65'66 The popular tools for the identification of unknown volatile compounds include gas chromatography/mass spectrometry (GC/ MS) — electron impact (El), chemical ionization (CI), mass spectrometry/mass spectrometry (MS/MS) — and gas chromatography/infrared spectroscopy (GC/IR). However, a rapid approach can be made for the tentative identification of foodpackaging-derived volatile residues in foods and containers, by use of headspace/gas chromatography employing dual capillary columns of different polarities, combined with a computer data search program.8 Direct headspace analysis has been generally used to analyze for residual solvents in packaging materials.1'5-6-8'67-70 In a typical method for the determination of residual contaminants, a sample is dissolved in a solvent such as dimethyl formamide, isopropanol, or diethyl ether, and the solution is sealed in a glass container, which is equipped with a septum. The con1990
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Critical Reviews In tainer is then placed in a heated bath at 70 to 80°C for 1 to 2 h, and a portion of the headspace is removed and analyzed using GC. Examples of this method include methylene chloride in polycarbonates31 and plasticizer in 2,4-diacetate cellulose.71 Headspace analysis using solid samples has also been applied to synthetic rubbers,72 vinyl chloride in polyvinyl chloride,73 and residual hydrocarbon solvent in polyacrylic acid.74 Kolb58 has used a "discontinuous gas extraction'' procedure and found that this procedure was useful in the determination of residual solvents in printed foil, but was too time-consuming for routine analyses. Just as the computer has become a valuable asset in mathematical modeling of migration studies, it has also become indispensable in multiresidue methods for the measurement of food-packaging-derived volatiles in food and containers. The procedure combines the sensitivity and repeatability of automated headspace sampling, the resolving power of capillary GC, and the speed of automated data reduction to handle the many replicate measurements required. A typical analysis of a given sample may require up to 10 chromatograms, including controls and duplicates. This procedure is most useful in screening large numbers of laboratory samples for several components at once.75 Vapor samples can be prepared by drawing the headspace of sealed packages through tubes containing sorbents such as TenaxR GC (Alltech Association, Deerfield, IL). Both filled and empty packages can be used in these analyses. A gentle vacuum is applied to one end of the trap using an aspirator with a needle attached at the other end of the trap to puncture the bag.76 A plastic material can be subjected directly to gas chromatographic analysis by introducing a sample of about 1 cm2 into an injection block on the chromatograph, using an insert designed to inject solid substances. However, the surface area of sample is so small that the variation in area can result in a false reading.77 Dynamic headspace/gas chromatography (DH/GQ has gained popularity as an effective and sensitive technique for the analysis of volatile compounds.78 Residual volatile compounds in plastic materials were investigated using similar procedures.21-24-25-41-79 Although there is a guideline for determination of nonvolatile migrants described in parts 170 to 189 in the Code of Federal Regulations (CFR),80 using heptane, 3% acetic acid, 8% alcohol, and water as testing medium, none of the test conditions are suitable to determine the degree of migration of volatile compounds from a packaging material during storage and/or cooking condition. Risch3-28 carried out an extensive study to select a fat simulant that was pure enough not to interfere with the identification of volatile compounds derived from packaging material as low as the ppb level and stable at temperatures equivalent to cooking in an oven, such as 350T for 30 min. Purified silicon oil was found adequate for both catagories. Oil absorption test and surface hardness tests also demonstrated 258
that silicon oil reacted with the plastic trays in a manner similar to vegetable oil. Although commercially available oil was highly contaminated, it was purified by heating the oil to about 200°C and bubble nitrogen through it while keeping the system under vacuum for 8 h. In order to simulate the reheating of a frozen dinner in a conventional oven, an emulsion of silicon oil, water, and gum arabic was formulated, put on a tray, covered with foil, frozen, and then baked. The percentage of silicon oil can be varied to simulate higher or lower fat foods. The volatile compounds migrated during cooking was determined by using DH/GC.28 Kinetic studies on the effect of oxygen concentration, temperature, and additives such as antioxidants on the generation of volatiles under simulated processing conditions were conducted with resins.81 A total of 15 mg of resins were ground to about 40 mesh by a Freezer/mill (Spex Industries Inc., Metuchen, NJ) at liquid nitrogen temperature, mixed with 3 g of dimethyl chlorosilane (DMCS)-treated 60/80 mesh glass beads (Alltech Association, Deerfield, IL), and packed inside a 1/4 in ID by 6 in L stainless steel tube. The sample-containing tube was placed inside an oven and connected to heated oxygen and nitrogen and the reaction proceeded under a gas flow rate of 15 ml/min at an increased temperature for 1 h. Oxygen concentration in the carrier gas was varied by diluting the oxygen with nitrogen gas and monitored by injecting a 0.5 ml aliquot into a gas chromatograph equipped with a CTRI-8700 column (Alltech Association, Deerfield, IL) and a thermal conductivity detector (TCD). Degradation products were collected during the reaction from 160 to 220°C directly in the analytical column kept at a subambient temperature using dry ice. The reaction was terminated by switching to helium gas at room temperature and turning off the reaction oven. After removing the dry ice from the analytical oven, the collected compounds were analyzed for total peak area for kinetic studies. The rate of reaction (da/dt: area per minute) at different oxygen concentrations (percent) in the carrier gas provides the effect of oxygen concentration on the generation of volatile compounds. Likewise, the effect of temperature was obtained from the rate of reaction against the reaction temperature.
IV. VOLATILE COMPOUNDS INDENTIFIED IN SPECIFIC PACKAGING MATERIALS A. Solvents When the packaging material is a composite structure fabricated of layers of various materials, which are laminated together by adhesives or primers, and may have added solventbased coating, lacquers, and inks, these structures may contain appreciable amounts of reactive, low-molecular-weight moieties. It is poor practice to have significant contact between such materials and the food itself. For example, reverse-printed inks should not be in direct contact with food, even when approved food colors are used. Overlacquering of the inks does not usually provide an adequate barrier because of incomplete cov-
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Food Science and Nutrition erage in a lacquer application. Adhesives may contain highly reactive components that may yield toxic compounds when in contact with foods.82-83 A number of low-molecular-weight adjuvants have a characteristic odor or impart characteristic taste notes to a food. Examples are formaldehyde in anchor resins, poly functional amines, organic acids, or monomers in reactive coatings. Hydrocarbons, ketones, alcohols, and esters, as main components of printing ink systems, may appear, accompanied fortuitously by traces of odiferious contaminants like unsaturated hydrocarbons or aldehydes. Fortunately, the human senses of smell and taste often show very high sensitivity to the presence of such volatiles. In most cases, the threshold for sensory detection of solvents used in ink and adhesive formulations is considerably below the toxicologically significant level.1-77 Thus, toluene, ethyl acetate, various aldehydes, and ketones range in sensory threshold from the parts per million to the parts per billion level. Table 1A and IB present some detection threshold figures for various solvents, both in practical terms of content per unit area and in sensory terms of the impression produced.77 The regulatory problem here is mitigated by the potential for economic damage from off-taste and odor in packaged goods. The advent of sensitive, reproducible analytical methods of detection of these traces of solvent residues has so completely replaced sensory evaluations in control of such residues in commercial products, that this is no longer a major concern.84 Table 1A Tolerance Limits of Solvents Expressed in mg/m2" Ink solvent Toluene Trichloroethylene Ethyl acetate MEK Isopropyl alcohol Isopropyl acetate Heptane Tetrahydrofurane Ethanol
Limit 10 100 30 10 30 10 30 15
Adhesive solvent Methylethyl ketone Methylisobutylketone Ethyl acetate
Li 15 10 20
>100
Table 1B Intensity of Odor of Certain Substances in Terms of Concentration in Air, Expressed in mg /1 7 7 Solvent Ethyl acetate Amyl acetate Amyl alochol Ethyl ether Carbon tetrachloride Chloroform Pyridine Pyridine
Detectable
Odor pronounced
Very strong
0.686 0.039 0.225 5.833 4.533 3.300 0.032 0.032
2.219 0.067 0.442 14.944 10.024 12.733 0.301 0.301
6.733 1.326 2.167 60.600 38.444 46.666 5.710 5.710
Off-flavor was detected in a fruity soft drink packaged in a laminated pouch, and residual toluene was found to be a major cause of the odor problem. A comparison of headspace samples withdrawn from sealed pouches showed that faulty pouches contained 26 to 28 times higher toluene levels than that found in good ones.70 The ultimate cause of excessive residual toluene - was found to be inadequate drying during lamination. Dichloromethane is used as a solvent in the manufacture of polycarbonates, but small amounts of it remain in the resin. The physical properties of the polycarbonates are also improved, if the level of the chlorinated solvent in the resin is reduced.51 The automated headspace analyzer was used to determine the trace amount of dichloromethane in polycarbonate resins. Levels of residual dichloromethane detected ranged from 1 to 100 ppm (wt/wt). The first step in removing the solvents from a coating or adhesive is to place the film in a ventilated oven, or to regulate the temperature of the conveyer tunnel, the time the material spends in it, and the flow rate of the air that is taking off the vapor phase. Problems involved with the residual solvents in ink can also be reduced by using solvents that have very low retention in the plastic materials. For PVDC-coated cellulose films, for instance, alcohol and ethyl acetate are the common solvents with the lowest retention figure.77 This figure is below the threshold value at which odors.are detectable. A musty off-odor was detected in a plastic packaging film and identified as 4,4,6-trimethyl-l ,3-dioxane by GC/MS using El, CI, GC/MS/MS, and GC/IR. The compound was considered to be a reaction product of 2-methyl-2,4-pentanediol, which is used as a solvent coating to help ink adhere to the film when the film is printed, with formaldehyde from an unknown source.20 Although the effect of migrants from packaging materials on the development of off-flavor in the food has received a lot of attention, there is only a small amount of literature available reporting experimental data on the concentration of common migrants from packaging materials, which can cause off-flavor in food. Wilks and Gilbert1 tried to determine organoleptic taste detection thresholds in aqueous solution for toluene and 2-nitropropane (NP) migrating from can coating by using a model system. They found that the equilibrium solvent concentration levels for the two solvents were directly proportional to their respective initial concentrations. However, the observed storage times necessary to reach these equilibrium concentrations were independent of the respective initial concentrations of the solvents. These observations suggested the influence of concentration-dependent diffusion coefficients for 2-NP and toluene in the vinyl resin system.1 The concentrations at which an objectionable off-flavor was, noted in a ready-to-eat (RTE) cereal were determined as styrene, 4 ppm; 2-butanone, 25 ppm; hexane, 200 ppm; pentanal, 5 ppm; and toluene, 100 ppm.5 Recently, absolute off-flavor thresholds in cookies were determined for a variety of solvents
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used in packaging materials.8587 It was found in the study that partitioning in the cookies was complicated by the complexity of the food ingredients, and did not appear to follow a specific pattern based on solubilities or chemical affinities alone. The affinity order of the partition coefficients in the simple polyolefin system, however, was found to agree with the order expected from established solubility factors in polymers.86 Among eleven solvents explored, the aromatic hydrocarbons such as toluene, ethyl benzene, styrene, and xylene occupied the lower end of the threshold range of 0.5 to 5.5 ppm (wt/ wt) in cookies.87 It also showed that groups of structurally and functionally homogeneous compounds exhibited very close threshold values within their groups. By contrast, structurally and functionally heterogeneous compounds gave a wide range of threshold values. B. Monomers If the material is volatile, generally little or no danger is present when the migrant produces a characteristic change in the odor and/or taste of the packaged food, at levels appreciably below the threshold for chronic toxicity. For most of the volatile residues in packaging materials the problem of sales acceptance is usually much more acute than that of safety. However, a significant amount of attention have been directed to the toxicity of residual VCM (vinyl chloride monomer) in poly (vinyl chloride) and its copolymers. The published toxicity data are limited to inhalation and at exposure levels considerably above those likely to be found from migration.88 The possible relation of VCM to carcinogenesis poses two problems. The first concerns chronic exposure of a potential carcinogen at any level and the second relates to the legal questions arising from the Delaney clause, which bans the use of any material having any amount of a known carcinogen.89 When migration rates and mechanisms were measured, it was found that the zero-effective transfer is related to the diffusion of small molecules in slabs of macroscopic dimensions, where strong interactions create diffusion shells involving an approach to infinite transfer.90 Gilbert39 presented a migration model that proposed that potential migrants such as VCM may become bound or immobilized at active binding sites within a glassy polymeric matrix, at finite but low migrant concentrations. Such migrant binding may be attributable to effects such as chemisorption at specific functional groups (i.e., unreacted vinyl groups in vinyl polymers)91-92 and physical entrapment at specific sites in a glassy polymeric matrix.93 An inverse phase gas-solid chromatographic method was used to study the equilibrium partition coefficient.94-95 It was found that simple dissolution was the predominant mode of VCM/PVC interaction in a plasticized PVC resin. Current resin technology has significantly reduced the active site binding potential of the resins. As a result, current PVC resins are likely to undergo more efficient residual monomer reduction during fabrication to plastic articles.96
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A generalized review has been presented by Gilbert and Shepherd52 covering the typical levels of residual monomers to be found in the packaging materials and in the foods themselves. Although it is difficult to generalize about the levels of monomers likely to be found in retail packaging and foods, they summarized previous data in broad terms as shown in Table 2.1,1,1-Trichloroethane was also identified in both vinyl chloride polymers and contained products.64 Melamine-formaldehyde polymer is used for tableware because its hardness, heat resistance, and stability allows for repeated use without apparent degradation. The amount of both residual melamine and formaldehyde was determined by Ishiwata,62 and melamine was found in the range of 0.08 ppm (wt/wt) in 4% acetic acid. Migration of both compounds was strongly affected by heating and acidity. Under the repeated use conditions, melamine concentration in 4% acetic acid ranged from 0.2 to 6.2 ppm and formaldehyde from 0.7 to 2.8 ppm (wt/wt). It has been reported that residual styrene is a frequent cause of an odor problem in foods,97 and it was known that it is difficult to remove residual styrene monomer trapped in formed plastics.98 Passey70 investigated an off-flavor problem involved in chocolate and lemon cream cookies that were packaged in polystyrene trays and wrapped with printed cellophane. Both organoleptic analysis and gas chromatographic analysis showed styrene monomer from the polystyrene trays was the culprit, and the levels of residual monomer in the trays were determined as 0.18 to 0.20% (wt %). The source of an off-odor in maple syrup packaged in a container made from a copolymer resin of methyl methacrylate, styrene, and butadiene (MSB) was traced to residual monomers in the plastic construction.50 Toluene was also identified as one of the contaminants that contributed to the off-flavor in the syrup and was thought to be derived from residual solvent used during polymerization of the copolymer resin. The concentrations of each contaminant in the tainted syrups were determined as between 180 to 275 ppb of methyl methacrylate, between 80 to 110 ppb of toluene, and 10 ppb of styrene (wt/ wt). The amount of residual monomer can be reduced by melting and masticating the polymer with a scavenger such as myrcene for acrylonitrile prior to forming the final product.99 The amount of VCM in PVC can be reduced to 5=2 ppm by heating to >120°C, but lower than the fusion temperature, subjecting the polymer to high-speed turbulent agitation, and reducing the system pressure.96 C. Polyethylene Terephthalate (PET) The major significant volatile compound in the PET was characterized as acetaldehyde, also known to be a major cause for the color change of PET during aging and of concern in odor quality. A particularly susceptible product is the cola-
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Food Science and Nutrition Table 2 Typical Monomer Levels in Retail Packaging and Foods 52
Monomer
Application
Vinyl chloride Acrylonitrile VDC
PVC bottles ABS tubs PVDC/PVC copolymer PVDC/PPfilm ABS tubs Polystyrene tubs
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Styrene
Typical range in polymer (ppm)
0.1—0.6
1.5—10.0 0.02—1.0 0.2—1.0 100—300
350—1200
type beverage. The mechanism for the thermal degradation of PET is considered to be similar to that for simple ester pyrolysis proceeding via a cyclic six-member transition state.100101 PET decomposes by a molecular mechanism with random chain scission at the ester links.102 Thermal and/or oxidative degradation products derived from polyester have been studied using model compounds such as ethylene dibenzoate, 2-hydroxyethyl benzoate, and diethylene glycol dibenzoate.'03 The vinyl compounds formed during these reactions can undergo further reactions to give a complex mixture of end products.
Foodstuff
Typical range in food (ppm)
Orange drink Soft margarine Liver pate Biscuits
Critical Reviews in Food Science and Nutrition Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/bfsn20
Volatiles in packaging materials Heasook Kim‐Kang
a
a
Packaging Division, Libra Laboratories Inc., Room No. 330, 44 Shelton Road, Piscataway, NJ, 08854 Version of record first published: 29 Sep 2009
To cite this article: Heasook Kim‐Kang (1990): Volatiles in packaging materials, Critical Reviews in Food Science and Nutrition, 29:4, 255-271 To link to this article: http://dx.doi.org/10.1080/10408399009527527
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Food Science and Nutrition Volatiles in Packaging Materials Heasook Kim-Kang
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ABSTRACT The increasing application of complex natural and/or synthetic polymers to food packaging has required definitive information on the characteristics of the finished products. High temperature encountered during the manufacturing process may induce thermal decomposition products that can migrate into the packaged product and cause undesirable flavor. A general methodology for testing polymer odor and odor contributors is discussed in this article with examples representing the odor of a variety of packaging materials. The precursors and the mechanisms of the major volatile components of each packaging material are presented.
I. INTRODUCTION Food packaging has been dominated in the last few decades by innovations in plastic packages. These include the development of different types of polymers, copolymers, the laminates, and packaging materials suitable especially for microwave heating. Considerabe improvement in design effectiveness has been achieved with the introduction of new materials and processes, making this area one of the most active in food product development. This growth has been paralleled by an increasing attention to the interaction between plastic packaging materials and foods. One of the practical concerns related to a polymeric package is the presence of compounds in toxicologically insignificant amounts, but at levels affecting aroma and/or taste of the packaged food. 13 Although the pertinent Food and Drug Administration (FDA) policy mainly emphasizes the total nonvolatile extractives, there has been an extensive history of odor and taste problems experienced in the development of new packages.4"9 A package can directly disrupt the flavor balance of a food in three ways: (1) subtraction, (2) reaction, and (3) addition. Subtraction occurs when components contributing to the desired flavor of the product are absorbed by the package.10"15 Reaction takes place when package components chemically interact with the food product to produce flavor artifacts. Sometimes the components in a packaging material, such as metal components, can act as a catalyst to accelerate the decomposition of food ingredients, resulting in undesirable flavor.16-17 Addition occurs when the package releases compounds that alter the flavor balance of the food.4'1820 Addition is by far the most common. The manufacture of packaging materials is often conducted under conditions of high temperature. These conditions can
easily induce thermal degradation with the formation of volatile compounds in packaging materials.2126 For example, a burnt polyethylene odor has been experienced in the paper/foil/ polyethylene laminate field. The increasing demand for single-serving meals, often referred to as "TV dinners", prompted the food companies to upgrade the quality of these meals and created what they refer to as gourmet dinners. The higher-quality dinners also required a package that presented a new and upscale image, but also had to meet the criterion of being dual-ovenable. This new trend led flavor and packaging chemists to develop an appropriate test to determine the levels of specific compounds migrating under intended-use situations, so that the optimum design for the packaging materials can be achieved.3 In a conventional oven, as in a microwave oven, food heated on plastic trays occassionally develops an off-taste or odor. These undesirable odors are considered to be derived from the breakdown products of the plastic material and/or residual solvents used in the manufacturing of the plastic trays.27-28 Packaging materials are of such a versatile composition that the precursors for volatile compounds can be from many sources. These can include residual monomers and oligomers, residual solvents from printing inks, adhesives, coatings, breakdown products of polymers, and additives, etc. This article summarizes volatile compounds identified in a variety of packaging materials, as well as the precursors of the major volatile components.
II. MIGRATION THEORY The fundamental processes by which trace amounts of solvents, reaction byproducts, additives, and monomers migrate from polymer barriers used in food and beverage packaging applications into the contents of a package have been discussed by a number of scientists.29"31 The classic theory of diffusion of gases is based on Fick's laws.32 Diffusion in homogeneous substances is based upon the assumption that the rate of transfer, R, of a gas passing perpendicularly through the unit area of a section is proportional to the concentration gradient through the section. This is expressed as: R = - D(C) ^ dX
(1)
where D(C) is the diffusion coefficient in cm2/s, in general, D can be a function of the local diffusant concentration, C is the
H. Kim-Kang, B. S., M. S., Ph.D., Packaging Division, Room No. 330, Libra Laboratories Inc., 44 Shelton Road, Piscataway, NJ 08854.
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Critical Reviews In concentration of diffusant in mol/cm3, and X is the thickness of the material in centimeters. Fickian diffusion can be visualized as a series of jumps in which Brownian motion of the polymer chain segments produces transient voids in the vicinity of the diffusant, enabling it to move through the polymeric matrix. The size of the void required to permit a jump is, of course, related to the size of the diffusant. The thermal energy that must be localized near the diffusant is related to the volume of the fluctuation, the cohesive energy density of the polymer, and the presence of nearby penetrants, which can facilitate the motion of the surrounding chains.32 Particularly at high concentrations of sorbed diffusant, the plasticizing effect of neighboring diffusants can cause pronounced concentration dependency of D. The temperature dependence of the diffusion coefficient is generally described by an Arrhenius form.33 The slope of such plots is related to the amount of energy required to form the transient voids required by the diffusion process. However, the activation energy is not always constant over the whole range of temperatures. For many organic vapor-polymer systems above the glass transition temperature of the polymer, Tg, the apparent activation energy for diffusion decreases with increasing temperature.29 Furthermore, for a series of homologous organic compounds in a given polymer, the activation energy is often nearly independent of the size of the penetrant molecule if the diffusant is greater than or equal in size to the mobile segment of the polymer.29 At the low concentrations typical of most indirect additives, diffusion coefficients approach limiting values that are only a function of temperature, and mathematical description of the transport processes becomes simple. Non-Fickian transport is known to occur at relatively high-diffusant activities for water in some polymers and can complicate the otherwise Fickian transport processes of indirect additives. The amount of package components that may migrate from a plastic packaging material into solid or liquid food or foodsimulating solvents depends on the physical and chemical properties of both the food and polymer.34-35 The controlling factors for the degree of migration would be the original concentration of the migrants, the solubility in the contacting phase, and/or the partition coefficient between the polymer and the contacting phase, the temperature, time, and the morphological structure of the polymer.31-36 Polymer properties affecting migration can include molecular weight, percent crystallinity, chain branching, density, and affinity for migrants.34 Most experimental observations on the migration of small molecules from polymeric packaging materials into food or food-simulating solvents showed some non-Fickian behavior. Solvent adsorption . and swelling of the polymer have often been observed when the behavior is non-Fickian. A model for a solute diffusing in a swelling polymer was used to explain this phenomenon.36 In another case, where the migrant is sparingly soluble in the solvent, a stagnant solvent layer at the polymer surface may give rise to an initial migration behavior that is linear in time instead of linear in square root of time. In certain cases where 256
the solvent is not stirred or is highly viscous, the quiescent migration is not found to depend on the diffusion coefficient of the migrant in the solvent.36 Either alone or in combination, these models can be applied to describe most migration behavior in rubbery or semicrystalline packaging material.36 Migration was divided into three classes, based on the limiting control mechanism30-34-37 Class 1— Nonmigrating materials, with or without the presence of food Class 2— Independently migrating not controlled by the food, although the presence of food may accelerate the migration Class 3— Leaching; migration controlled by the food; negligible in the absence of food, significant in its presence Even though there is no absolute cutoff between these various classes, Class 3 systems were described by Bristori and Katan37 as those that have diffusion coefficients less than 10" 12 cm2 s" 1 in the absence of food, but 10~9 cm2 s" 1 or more in its presence. Their definition of nonmigrating is diffusion coefficients of less than 10~12 cm2 s" 1 . Residual, unreacted monomers from most addition type polymers will have significant volatility at normal conditions of storage and use. This leads to the interesting but seemingly contradictory situation that the equilibrium concentration of such a substance in the food product is expected to be essentially zero by theoretical considerations. At the outer, noncontact surface of the film some of the monomer escapes from the film as a gas. It is reasonable to think of the atmosphere as an infinite sink with respect to this tiny amount of gas so that, over time, the concentration of the monomer in the atmosphere remains essentially zero. At some point, a dynamic equilibrium will be achieved based on the partition coefficient and the ratio of the solubility of the monomer in the food product and in the film. If there was an artificial barrier that prevented escape of the gas into the atmosphere, this would result in a maximum concentration of migrant in the food and no further migration would occur. No such barrier exists, however, and therefore the gas molecules that escape into the atmosphere after this dynamic equilibrium has been achieved reverse the concentration gradient driving force. The monomer will now begin to migrate from the food into the film and into the atmosphere. Vom Brack et al.38 designed a mathematical equation for migration of volatile additives in packaging materials, and the measured value was found to fit the theoretical value well. Gilbert39 proposed an "essential zero" theory, where any migrant on the active site is not available for migration. The theory supported the idea of minimum limit of VCM (vinyl chloride monomer). The diffusion mechanism also depends on the volatility of the migrants. The more volatile the components are, the higher the concentration in the headspace. The migration does not
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Food Science and Nutrition require a direct contact between food and packaging material. However, if the migrant is a high-boiling compound such as parabutylphenol, it requires a direct contact to result in offflavor in the food product.4
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III. METHODOLOGY A. Sensory Evaluation When evaluation techniques were compared with each other, organoleptic analysis worked better than instrumental analysis, since some off-flavors could be tasted but did not show up in headspace or in extracts of products.40 In addition, the threshold of human sense is often lower than that of the instrument. The odor quality of test plastic materials such as resins, films, or laminates can be estimated directly by panelists using a rating scale evaluation of intensity, or by forced choice methods such as triangle test, as the test forms.4'4142 The extent of migration of residual volatile compounds in packaging materials to the contacting phase can be evaluated after exposing water, food simulant, or actual food to packaging materials at accelerated conditions, at ambient temperature, or at actual cooking condition. I-4-21-28 Food can be exposed to packaging materials by direct contact or by vapor phase transfer to determine if the adverse odor/flavor problem is caused by any of the more volatile package constituents or caused by constituents, which can only be transferred by direct contact solvation. Glass containers are generally used as control packages. Sensory evaluation can be paralleled with objective instrumental analyses and the correlation between the sensory and instrumental results can be obtained. 171843 B. Instrumental Analysis Conventional methods of determining flavor concentration in materials such as food and confectionary products require lengthy solvent extraction, followed by gas chromatographic analysis. Sugisawa44 catagorized the methods used for the isolation of flavor concentrates under three general headings: (1) distillation techniques, including vacuum distillation;1845 steam distillation and carbon dioxide distillation; (2) extraction techniques, including solvent extraction, simultaneous steam-distillation-solvent extraction,4*48 carbon dioxide, and supercritical fluid extraction; and (3) miscellaneous techniques, including gas entrainment in open and closed systems,20-4952 and adsorption on charcoal and porous polymers,53'57 etc. Both the chemical composition and physical state of the sample matrix can affect the headspace composition. The analysis of flavor in packaging materials generally involves headspace gas chromatographic techniques. It is simple and reproducible. However, sensitivity of the analyses is too low. Although concentration of headspace onto polymers can increase the sensitivity, it requires the use of correction factors because individual volatile compounds have different affinities for the adsorbent.
The analytical method of choice for monomers is undoubtedly headspace gas chromatography. The monomers of primary concern to date have been vinyl chloride (VC), vinylidene chloride (VDC), acrylonitrile (AN), and styrene. Headspace gas chromatography consists essentially of placing the sample for analysis in a closed vessel: allowing the compound of interest to equilibrate between the sample and the surrounding vapor, usually with heating in order to obtain rapid equilibration: and, finally, to inject a sample of this equilibrated "headspace" into a gas chromatograph. (The term "equilibration" has been emphasized because this is an essential condition for quantitative analysis.) It is possible to perform the necessary operations manually, but automated chromatographs are available dedicated to headspace chromatography, which combine the advantages of automated sample handling with enhanced reproducibility.58'59 The popular choice of detector is flame ionization detector. However, a specific detector such as an electron capture detector,46'47-49-51 or a thermal energy analyzer,60^2 can be used, either to eliminate interferences or to increase sensitivity. Styrene is not an ideal candidate for analysis by headspace chromatography, for with a boiling point of 145°C its volatility is low in comparison with VC, VDC, or even AN. However, the concentration of styrene in polymers can be achieved by a modified solution headspace approach.63 This modified technique involves dissolving the polymer in an organic solvent in the normal manner and then altering the composition of the solvent phase to decrease styrene solubility and thereby increase its equilibrium concentration in the headspace. For this purpose water was found to be the most effective solvent, since water is normally free of interfering organic contaminants and it will not be sensed by the GC/FID. The modified headspace approach has also been applied for the analysis of 2-ethylhexylacrylate (BP 214°C) in polymers, and to the determination of 1,1,1-trichloroethane (BP 74°C) in PVC resins and bottles.64 The technique of selected ion monitor (SIM) mode GC/MS offers both a powerful confirmatory test, and the added advantage of increasing the sensitivity of the analysis.52-65'66 The popular tools for the identification of unknown volatile compounds include gas chromatography/mass spectrometry (GC/ MS) — electron impact (El), chemical ionization (CI), mass spectrometry/mass spectrometry (MS/MS) — and gas chromatography/infrared spectroscopy (GC/IR). However, a rapid approach can be made for the tentative identification of foodpackaging-derived volatile residues in foods and containers, by use of headspace/gas chromatography employing dual capillary columns of different polarities, combined with a computer data search program.8 Direct headspace analysis has been generally used to analyze for residual solvents in packaging materials.1'5-6-8'67-70 In a typical method for the determination of residual contaminants, a sample is dissolved in a solvent such as dimethyl formamide, isopropanol, or diethyl ether, and the solution is sealed in a glass container, which is equipped with a septum. The con1990
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Critical Reviews In tainer is then placed in a heated bath at 70 to 80°C for 1 to 2 h, and a portion of the headspace is removed and analyzed using GC. Examples of this method include methylene chloride in polycarbonates31 and plasticizer in 2,4-diacetate cellulose.71 Headspace analysis using solid samples has also been applied to synthetic rubbers,72 vinyl chloride in polyvinyl chloride,73 and residual hydrocarbon solvent in polyacrylic acid.74 Kolb58 has used a "discontinuous gas extraction'' procedure and found that this procedure was useful in the determination of residual solvents in printed foil, but was too time-consuming for routine analyses. Just as the computer has become a valuable asset in mathematical modeling of migration studies, it has also become indispensable in multiresidue methods for the measurement of food-packaging-derived volatiles in food and containers. The procedure combines the sensitivity and repeatability of automated headspace sampling, the resolving power of capillary GC, and the speed of automated data reduction to handle the many replicate measurements required. A typical analysis of a given sample may require up to 10 chromatograms, including controls and duplicates. This procedure is most useful in screening large numbers of laboratory samples for several components at once.75 Vapor samples can be prepared by drawing the headspace of sealed packages through tubes containing sorbents such as TenaxR GC (Alltech Association, Deerfield, IL). Both filled and empty packages can be used in these analyses. A gentle vacuum is applied to one end of the trap using an aspirator with a needle attached at the other end of the trap to puncture the bag.76 A plastic material can be subjected directly to gas chromatographic analysis by introducing a sample of about 1 cm2 into an injection block on the chromatograph, using an insert designed to inject solid substances. However, the surface area of sample is so small that the variation in area can result in a false reading.77 Dynamic headspace/gas chromatography (DH/GQ has gained popularity as an effective and sensitive technique for the analysis of volatile compounds.78 Residual volatile compounds in plastic materials were investigated using similar procedures.21-24-25-41-79 Although there is a guideline for determination of nonvolatile migrants described in parts 170 to 189 in the Code of Federal Regulations (CFR),80 using heptane, 3% acetic acid, 8% alcohol, and water as testing medium, none of the test conditions are suitable to determine the degree of migration of volatile compounds from a packaging material during storage and/or cooking condition. Risch3-28 carried out an extensive study to select a fat simulant that was pure enough not to interfere with the identification of volatile compounds derived from packaging material as low as the ppb level and stable at temperatures equivalent to cooking in an oven, such as 350T for 30 min. Purified silicon oil was found adequate for both catagories. Oil absorption test and surface hardness tests also demonstrated 258
that silicon oil reacted with the plastic trays in a manner similar to vegetable oil. Although commercially available oil was highly contaminated, it was purified by heating the oil to about 200°C and bubble nitrogen through it while keeping the system under vacuum for 8 h. In order to simulate the reheating of a frozen dinner in a conventional oven, an emulsion of silicon oil, water, and gum arabic was formulated, put on a tray, covered with foil, frozen, and then baked. The percentage of silicon oil can be varied to simulate higher or lower fat foods. The volatile compounds migrated during cooking was determined by using DH/GC.28 Kinetic studies on the effect of oxygen concentration, temperature, and additives such as antioxidants on the generation of volatiles under simulated processing conditions were conducted with resins.81 A total of 15 mg of resins were ground to about 40 mesh by a Freezer/mill (Spex Industries Inc., Metuchen, NJ) at liquid nitrogen temperature, mixed with 3 g of dimethyl chlorosilane (DMCS)-treated 60/80 mesh glass beads (Alltech Association, Deerfield, IL), and packed inside a 1/4 in ID by 6 in L stainless steel tube. The sample-containing tube was placed inside an oven and connected to heated oxygen and nitrogen and the reaction proceeded under a gas flow rate of 15 ml/min at an increased temperature for 1 h. Oxygen concentration in the carrier gas was varied by diluting the oxygen with nitrogen gas and monitored by injecting a 0.5 ml aliquot into a gas chromatograph equipped with a CTRI-8700 column (Alltech Association, Deerfield, IL) and a thermal conductivity detector (TCD). Degradation products were collected during the reaction from 160 to 220°C directly in the analytical column kept at a subambient temperature using dry ice. The reaction was terminated by switching to helium gas at room temperature and turning off the reaction oven. After removing the dry ice from the analytical oven, the collected compounds were analyzed for total peak area for kinetic studies. The rate of reaction (da/dt: area per minute) at different oxygen concentrations (percent) in the carrier gas provides the effect of oxygen concentration on the generation of volatile compounds. Likewise, the effect of temperature was obtained from the rate of reaction against the reaction temperature.
IV. VOLATILE COMPOUNDS INDENTIFIED IN SPECIFIC PACKAGING MATERIALS A. Solvents When the packaging material is a composite structure fabricated of layers of various materials, which are laminated together by adhesives or primers, and may have added solventbased coating, lacquers, and inks, these structures may contain appreciable amounts of reactive, low-molecular-weight moieties. It is poor practice to have significant contact between such materials and the food itself. For example, reverse-printed inks should not be in direct contact with food, even when approved food colors are used. Overlacquering of the inks does not usually provide an adequate barrier because of incomplete cov-
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Food Science and Nutrition erage in a lacquer application. Adhesives may contain highly reactive components that may yield toxic compounds when in contact with foods.82-83 A number of low-molecular-weight adjuvants have a characteristic odor or impart characteristic taste notes to a food. Examples are formaldehyde in anchor resins, poly functional amines, organic acids, or monomers in reactive coatings. Hydrocarbons, ketones, alcohols, and esters, as main components of printing ink systems, may appear, accompanied fortuitously by traces of odiferious contaminants like unsaturated hydrocarbons or aldehydes. Fortunately, the human senses of smell and taste often show very high sensitivity to the presence of such volatiles. In most cases, the threshold for sensory detection of solvents used in ink and adhesive formulations is considerably below the toxicologically significant level.1-77 Thus, toluene, ethyl acetate, various aldehydes, and ketones range in sensory threshold from the parts per million to the parts per billion level. Table 1A and IB present some detection threshold figures for various solvents, both in practical terms of content per unit area and in sensory terms of the impression produced.77 The regulatory problem here is mitigated by the potential for economic damage from off-taste and odor in packaged goods. The advent of sensitive, reproducible analytical methods of detection of these traces of solvent residues has so completely replaced sensory evaluations in control of such residues in commercial products, that this is no longer a major concern.84 Table 1A Tolerance Limits of Solvents Expressed in mg/m2" Ink solvent Toluene Trichloroethylene Ethyl acetate MEK Isopropyl alcohol Isopropyl acetate Heptane Tetrahydrofurane Ethanol
Limit 10 100 30 10 30 10 30 15
Adhesive solvent Methylethyl ketone Methylisobutylketone Ethyl acetate
Li 15 10 20
>100
Table 1B Intensity of Odor of Certain Substances in Terms of Concentration in Air, Expressed in mg /1 7 7 Solvent Ethyl acetate Amyl acetate Amyl alochol Ethyl ether Carbon tetrachloride Chloroform Pyridine Pyridine
Detectable
Odor pronounced
Very strong
0.686 0.039 0.225 5.833 4.533 3.300 0.032 0.032
2.219 0.067 0.442 14.944 10.024 12.733 0.301 0.301
6.733 1.326 2.167 60.600 38.444 46.666 5.710 5.710
Off-flavor was detected in a fruity soft drink packaged in a laminated pouch, and residual toluene was found to be a major cause of the odor problem. A comparison of headspace samples withdrawn from sealed pouches showed that faulty pouches contained 26 to 28 times higher toluene levels than that found in good ones.70 The ultimate cause of excessive residual toluene - was found to be inadequate drying during lamination. Dichloromethane is used as a solvent in the manufacture of polycarbonates, but small amounts of it remain in the resin. The physical properties of the polycarbonates are also improved, if the level of the chlorinated solvent in the resin is reduced.51 The automated headspace analyzer was used to determine the trace amount of dichloromethane in polycarbonate resins. Levels of residual dichloromethane detected ranged from 1 to 100 ppm (wt/wt). The first step in removing the solvents from a coating or adhesive is to place the film in a ventilated oven, or to regulate the temperature of the conveyer tunnel, the time the material spends in it, and the flow rate of the air that is taking off the vapor phase. Problems involved with the residual solvents in ink can also be reduced by using solvents that have very low retention in the plastic materials. For PVDC-coated cellulose films, for instance, alcohol and ethyl acetate are the common solvents with the lowest retention figure.77 This figure is below the threshold value at which odors.are detectable. A musty off-odor was detected in a plastic packaging film and identified as 4,4,6-trimethyl-l ,3-dioxane by GC/MS using El, CI, GC/MS/MS, and GC/IR. The compound was considered to be a reaction product of 2-methyl-2,4-pentanediol, which is used as a solvent coating to help ink adhere to the film when the film is printed, with formaldehyde from an unknown source.20 Although the effect of migrants from packaging materials on the development of off-flavor in the food has received a lot of attention, there is only a small amount of literature available reporting experimental data on the concentration of common migrants from packaging materials, which can cause off-flavor in food. Wilks and Gilbert1 tried to determine organoleptic taste detection thresholds in aqueous solution for toluene and 2-nitropropane (NP) migrating from can coating by using a model system. They found that the equilibrium solvent concentration levels for the two solvents were directly proportional to their respective initial concentrations. However, the observed storage times necessary to reach these equilibrium concentrations were independent of the respective initial concentrations of the solvents. These observations suggested the influence of concentration-dependent diffusion coefficients for 2-NP and toluene in the vinyl resin system.1 The concentrations at which an objectionable off-flavor was, noted in a ready-to-eat (RTE) cereal were determined as styrene, 4 ppm; 2-butanone, 25 ppm; hexane, 200 ppm; pentanal, 5 ppm; and toluene, 100 ppm.5 Recently, absolute off-flavor thresholds in cookies were determined for a variety of solvents
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Critical Reviews In
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used in packaging materials.8587 It was found in the study that partitioning in the cookies was complicated by the complexity of the food ingredients, and did not appear to follow a specific pattern based on solubilities or chemical affinities alone. The affinity order of the partition coefficients in the simple polyolefin system, however, was found to agree with the order expected from established solubility factors in polymers.86 Among eleven solvents explored, the aromatic hydrocarbons such as toluene, ethyl benzene, styrene, and xylene occupied the lower end of the threshold range of 0.5 to 5.5 ppm (wt/ wt) in cookies.87 It also showed that groups of structurally and functionally homogeneous compounds exhibited very close threshold values within their groups. By contrast, structurally and functionally heterogeneous compounds gave a wide range of threshold values. B. Monomers If the material is volatile, generally little or no danger is present when the migrant produces a characteristic change in the odor and/or taste of the packaged food, at levels appreciably below the threshold for chronic toxicity. For most of the volatile residues in packaging materials the problem of sales acceptance is usually much more acute than that of safety. However, a significant amount of attention have been directed to the toxicity of residual VCM (vinyl chloride monomer) in poly (vinyl chloride) and its copolymers. The published toxicity data are limited to inhalation and at exposure levels considerably above those likely to be found from migration.88 The possible relation of VCM to carcinogenesis poses two problems. The first concerns chronic exposure of a potential carcinogen at any level and the second relates to the legal questions arising from the Delaney clause, which bans the use of any material having any amount of a known carcinogen.89 When migration rates and mechanisms were measured, it was found that the zero-effective transfer is related to the diffusion of small molecules in slabs of macroscopic dimensions, where strong interactions create diffusion shells involving an approach to infinite transfer.90 Gilbert39 presented a migration model that proposed that potential migrants such as VCM may become bound or immobilized at active binding sites within a glassy polymeric matrix, at finite but low migrant concentrations. Such migrant binding may be attributable to effects such as chemisorption at specific functional groups (i.e., unreacted vinyl groups in vinyl polymers)91-92 and physical entrapment at specific sites in a glassy polymeric matrix.93 An inverse phase gas-solid chromatographic method was used to study the equilibrium partition coefficient.94-95 It was found that simple dissolution was the predominant mode of VCM/PVC interaction in a plasticized PVC resin. Current resin technology has significantly reduced the active site binding potential of the resins. As a result, current PVC resins are likely to undergo more efficient residual monomer reduction during fabrication to plastic articles.96
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A generalized review has been presented by Gilbert and Shepherd52 covering the typical levels of residual monomers to be found in the packaging materials and in the foods themselves. Although it is difficult to generalize about the levels of monomers likely to be found in retail packaging and foods, they summarized previous data in broad terms as shown in Table 2.1,1,1-Trichloroethane was also identified in both vinyl chloride polymers and contained products.64 Melamine-formaldehyde polymer is used for tableware because its hardness, heat resistance, and stability allows for repeated use without apparent degradation. The amount of both residual melamine and formaldehyde was determined by Ishiwata,62 and melamine was found in the range of 0.08 ppm (wt/wt) in 4% acetic acid. Migration of both compounds was strongly affected by heating and acidity. Under the repeated use conditions, melamine concentration in 4% acetic acid ranged from 0.2 to 6.2 ppm and formaldehyde from 0.7 to 2.8 ppm (wt/wt). It has been reported that residual styrene is a frequent cause of an odor problem in foods,97 and it was known that it is difficult to remove residual styrene monomer trapped in formed plastics.98 Passey70 investigated an off-flavor problem involved in chocolate and lemon cream cookies that were packaged in polystyrene trays and wrapped with printed cellophane. Both organoleptic analysis and gas chromatographic analysis showed styrene monomer from the polystyrene trays was the culprit, and the levels of residual monomer in the trays were determined as 0.18 to 0.20% (wt %). The source of an off-odor in maple syrup packaged in a container made from a copolymer resin of methyl methacrylate, styrene, and butadiene (MSB) was traced to residual monomers in the plastic construction.50 Toluene was also identified as one of the contaminants that contributed to the off-flavor in the syrup and was thought to be derived from residual solvent used during polymerization of the copolymer resin. The concentrations of each contaminant in the tainted syrups were determined as between 180 to 275 ppb of methyl methacrylate, between 80 to 110 ppb of toluene, and 10 ppb of styrene (wt/ wt). The amount of residual monomer can be reduced by melting and masticating the polymer with a scavenger such as myrcene for acrylonitrile prior to forming the final product.99 The amount of VCM in PVC can be reduced to 5=2 ppm by heating to >120°C, but lower than the fusion temperature, subjecting the polymer to high-speed turbulent agitation, and reducing the system pressure.96 C. Polyethylene Terephthalate (PET) The major significant volatile compound in the PET was characterized as acetaldehyde, also known to be a major cause for the color change of PET during aging and of concern in odor quality. A particularly susceptible product is the cola-
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Food Science and Nutrition Table 2 Typical Monomer Levels in Retail Packaging and Foods 52
Monomer
Application
Vinyl chloride Acrylonitrile VDC
PVC bottles ABS tubs PVDC/PVC copolymer PVDC/PPfilm ABS tubs Polystyrene tubs
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Styrene
Typical range in polymer (ppm)
0.1—0.6
1.5—10.0 0.02—1.0 0.2—1.0 100—300
350—1200
type beverage. The mechanism for the thermal degradation of PET is considered to be similar to that for simple ester pyrolysis proceeding via a cyclic six-member transition state.100101 PET decomposes by a molecular mechanism with random chain scission at the ester links.102 Thermal and/or oxidative degradation products derived from polyester have been studied using model compounds such as ethylene dibenzoate, 2-hydroxyethyl benzoate, and diethylene glycol dibenzoate.'03 The vinyl compounds formed during these reactions can undergo further reactions to give a complex mixture of end products.
Foodstuff
Typical range in food (ppm)
Orange drink Soft margarine Liver pate Biscuits
