EVALUATION OF PLASTICS PACKAGING MATERIALS FOR FOOD PACKAGING APPLICATIONS: FOOD SAFETY CONSIDERATIONS

EVALUATION OF PLASTICS PACKAGING MATERIALS FOR FOOD PACKAGING APPLICATIONS:

FOOD SAFETY CONSIDERATIONS

JACK R. GIACIN

Associate Professor School of Packaging

Michigan State University East Lansing, Michigan 48824

Received for Publication February 28, 1980 Accepted for Publication May 16, 1980

ABSTRACT

In evaluating a packaging system for food packaging applications, con- sideration must be given to the physical properties, chemical composition and extractivity o f the packaging material. The last point, extractivity, or migration from the packaging material to a food contact phase, is o f major concern in the selection and use of plastics packaging materials for food packaging.

The present article deals specifically with migration of indirect food additives from plastics packaging materials and the scientific principles related to migrant transport or diffusion, as they apply to food safety. These principles and their experimental basis are discussed.

INTRODUCTION

There are a number of criteria t o be considered in the selection of a

1. The stability of the foodstuff, which may involve the reaction of food components such as lipids, protein and certain vitamins to oxygen, light and changes in the water activity of the product. The stability of the product will be a function of its chemical, biochemical and physical nature and will be markedly influenced by the permeability or barrier properties of the package. The prevailing environmental conditions to which the product is exposed during distribution and storage. Such environmental conditions as temperature and relative humidity will dictate the barrier properties required of the package. The compatibility of the package with the method of preservation selected.

packaging system for food packaging. These include:

2.

3.

Journal of Food Safety 4 (1980) 257-276. All Rights Reserved Qopyright 1980 by Food & Nutrition Press, Inc., Westport, Connecticut 257

JACK R. GIACIN

4. The specific packaging material and its potential effect on the intrinsic quality and safety of the packaged product, due to migration from the packaging material to the food contact phase.

The last point, namely, the migration of potentially toxic moieties from the packaging material to a contact phase, is of major concern in the selection and use of plastics packaging materials for food packaging. Because of this inherent problem with plastics packaging materials and their wide utilization in food packaging, the present paper deals specifically with (a) plastic packaging materials, and (b) the migration of indirect food additives from plastic packaging materials as it applies to food safety.

In evaluating plastics packaging materials for food packaging applications, there are three analytical areas that are of concern. These are summarized in Table 1.

Table 1. Analytical considerations for plastic packaging materials

1. Characterization of the polymer in terms of physical properties, to include:

A. Thermal properties B. Mechanical properties C. Processing properties

2. Chemical composition of the plastics packaging material 3. Polymer extractivity o r migration propensity

As a result of the present atmosphere of Federal regulation, the extractivity or migration propensity of the packaging material is probably the single most important characterization parameter for food packaging. Therefore, in selecting an appropriate plastic packaging material for end use application is food packaging and to insure compliance with legal regulations, it is necessary to consider the following questions with respect to product/package interaction (see Table 2). It is important to note that the considerations listed in Table 2 relate specifically to the packaging material and its potential effect on product safety due to migration.

migrants by ingestion, as a result of migration from packaging material to the contacting food phase, can be expressed by eq. 1, the basic hazard or risk equation.

The risk or health hazard resulting from exposure to potentially toxic

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EVALUATION OF PLASTICS PACKAGING MATERIALS

Table 2. Basic considerations for product/package interaction ~

1. Is the plastic packaging material coming in contact with the

2. Does the packaging material contain only approved plastics

3. To what extent d o trace amounts of solvents, reaction and

product FDA approved for food packaging use?

additives in the permitted concentrations?

degradation by-products, additives, monomers and oligomers migrate from the packaging material into the packaged product or contact phase?

grating from the packaging material into the product t o be packaged?

5. What is the risk or health hazard t o the general public as a result of migration of moieties from the packaging material to a contact food phase and their subsequent ingestion?

4. How great is the sum of the weight of all constituents mi-

where: H = Hazard or risk E = Effective total quantity of migrant ingested B = Biological effectiveness

As shown, the hazard or risk (H) will be a function of (E), the effective total quantity of migrant ingested and (B), the biological effectiveness of the migrant, in terms of its biochemical response at that concentration. I t is understood that a number of factors will influence (B) the biological effectiveness, among these being the time frame over which the migrant quantity (E) is ingested.

migration of indirect food additives can be found in the text by Briston and Katan (1974).

In assessing public safety, both the quantity of migrant ingested (E) and its biological effectiveness (B) must be considered. In this paper, we deal specifically with the various factors affecting the quantity of migrant ingested. The quantity of migrant ingested, as a result of migration, will depend upon such factors as migrant diffusivity as well as the solubility and affinity of the migrant in both the polymer and the contact food phase. The total quantity of migrant transferred can, therefore, be expressed as a mathematical function of the following parameters.

An excellent discussion of safety evaluation principles related to the

where :

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D = s = Kp =

[IT] po =

T = t = G =

Migrant diffusivity or diffusion coefficient Solubility of migrant in both the polymer and contact phase Partition coefficient characterizing the equilibrium distribution of migrant between package and contacting phase, at specific initial migrant concentrations unless shown to be independent of concentration Initial concentration of migrant in the polymer Temperature Time of exposure of product to package Geometry of system

A convenient summary of this whole field has been given by Katan (1979) and by Haesen and Schwarze (1979).

RELATIONSHIP TO PUBLIC SAFETY

Presently, plastics packaging materials are widely used in the food packaging industry because of their cost and outstanding service properties. However, in addition to the high molecular weight components (macromolecules), plastics also contain various low molecular weight species such as monomers and oligomers, additives such as heat and light stabilizers, antioxidants, plasticizers and U.V. absorbers, as well as processing aids such as lubricants, slip agents and antistatic agents. The latter compounds (i.e. additives and processing aids) cannot be o- mitted because of functional reasons, while residual monomer and oligomers are present as the result of manufacturing technology and they also cannot be totally eliminated from the fabricated packaging article. In contrast to the high molecular weight components or macromolecules, these low molecular weight compounds can transfer from the plastic packaging material to a contact food phase with the potential for physio- logically objectionable results.

The use of low molecular weight additives and the presence of residual monomer in plastics packaging materials is therefore subject to strict gov- ernment regulation, as migration of these compounds can affect the quality of the contained product. Migrants can and do influence the quality of the contained product, as exhibitied by sensorily determinable changes (i.e. changes in organoleptic properties such as odor and/or taste) or by toxicological considerations.

For situations where toxicological manifestations are concerned, migration levels are of particular importance, even when initial migrant

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residues are at relatively low concentration. Examples of this are the migration of residual vinyl chloride monomer (VCM) from polyvinyl chloride (PVC) into products packaged or transported in PVC articles (Breder et al. 1975; Federal Register 1975; Gilbert 1976; Morano et al. 1977) and the migration of acrylonitrile monomer from acrylonitrile copolymers to a food contacting phase (McNeal et al. 1979; DiPasquale 1978; Gawell 1979). Vinyl chloride monomer, a known carcinogen (Maltoni and Lefemine 1975; Creech and Johnson 1974; Lee and Harry 1974) has been, and continues to be, the focus of considerable attention, as it can become an indirect food additive via migration from its polymer, polyvinyl chloride. Regulatory considerations involving the migration of residual acrylonitrile from acrylonitrile copolymers has lead to the proposed restriction and limitations of high barrier acrylonitrile copolymers from selected end use applications and specifically for beverage containers (Federal Register 1977).

On appeal of this decision (Monsanto vs. Kennedy, 77-2023, 1979), the circuit court upheld part of the FDA decision and remanded part for reconsideration by the FDA. The court upheld that portion relating to the specific bottles that were the subject of the administrative hearing, and remanded for reconsideration that part relating to other uses of acrylonitrile in newer plastic bottles, that have an even lower residual level of acrylonitrile monomer. Such bottles are expected to exhibit an even more remote possibility of migration to food. Of much greater importance, however, is that the court remanded the FDA’s decision that any use of acrylonitrile, at any level, and under any circumstances, could potentially result in migration.

The court further stated that the FDA must determine with a fair de- gree of confidence that a substance migrates into food in more than insignificant amounts, with the FDA having the discretion to determine what level of migration is significant. However, the FDA could not de- cide that all migration, even as low as one molecule, would by itself be sufficient to satisfy the statutory c‘componentyy test for a food additive (Federal Food, Drug and Cosmetic Act 1958).

In a broader sense, it is also possible that other food safety decisions will be significantly affected by the views expressed in the Monsanto decision. The court went out of its way to state that the FDA need not be concerned about “negligible”, “insignificant” or de minimus safety issues. The importance of this decision is that it provides, for the first time, a course of action for the FDA to follow in future regulatory decisions involving its authority under the Food, Drug and Cosmetic Act (Hutt 1979; Food and Drug Packaging, 1979).

In food packaging regulations, the term migration is used to describe

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the transfer of substances from the package (usually a plastic material) to the foodstuff. A distinction is usually made between global migration and specific migration. Global migration refers to the total transfer or quantity of all substances migrating from the package into the packaged food, whereas specific migration relates to one or more identifiable substances (i.e. monomer) that is a constituent of the packaging material. Global migration limits, therefore, restrict the transfer of all substances to the food contact phase whether they are toxic or not. Such limits have no direct toxicological basis and the possibility exists that certain materials might be prohibited from use, simply on the grounds that they transfer too much of a harmless chemical to the foodstuff. In principle, the measurement of total or global migration from a packaging material is quite simple. It consists of placing a sample of the material, of known surface area, in contact with an appropriate food simulant or solvent, under defined time-temperature conditions (FDA Guidelines for Chem- istry and Technology Requirements of Indirect Food Additives Petitions 1976) and determining total transfer gravimetrically.

With respect to specific migration, limits are established to restrict the transfer into foods of those components known or deemed potentially hazardous to human health, but do not take into account the total quantity of other moieties transferring into the contact phase. Volatile materials of considerable relevance in this regard are the two monomers, vinyl chloride and acrylonitrile. Of interest also are other monomers such as styrene (Withey 1976; Withey and Collins 1978; Thomas, Wolff and Derache 1977) and vinylidene chloride (Warren and Ricci 1978).

As previously stated, in characterizing packaging materials in terms of food packaging regulations, the migration or extractivity of the package is probably the single most important parameter. In addition, the scientific principles related to migrant transport or diffusion have become of significant importance in terms of food safety regulations involving the migration of potentially toxic moieties (i.e. vinyl chloride). An exposition of these principles and their experimental basis are discussed in detail in subsequent sections of this paper.

THE MIGRATION PROCESS: GENERAL CONSIDERATIONS

Migration is a complex process depending in part (if no chemical reaction takes place) on the diffusivity of the migrating species. Diffusivity, or the diffusion coefficient (D), is defined as the tendency of a substance to diffuse (see Crank and Park 1968, for a discussion of Fick’s laws) through the polymer bulk phase. Migration can, therefore, be considered

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a mass transport process under defined test conditions (i.e. time, temperature and the nature and volume of the contacting phase). The driving force for migration is the concentration gradient, where dissolved species diffuse from a region of higher concentration (i.e. polymer) to a region of lower initial concentration (i.e. contact phase). The rate of diffusion of the migrant is related to the resistance against the movement of migrant within the bulk phase of the polymer wall.

Thus, if migration from a package to a contact phase is to occur, the migrant has to undergo the following two processes in succession: (a) diffusion of the migrant through the polymer bulk phase to the polymer surface, and (b) subsequent dissolution or evaporation of the migrant accumulated at the surface to the contact phase.

Further, the phenomenon of desorption or migration from a polymer to a contact phase can be considered a function of migrant-polymer interaction affinity and diffusion. The affinity or interaction of migrant with both the polymer and contact phases will determine the equilibrium amount of migrant transferred to a contacting phase. Such migrant-polymer interaction affinity will become increasingly more important as the migrant concentration decreases (Gilbert 1976). Diffusion of the migrant through the polymer will affect the rate of attaining equilibrium.

It is possible to define three packaging material/contact phase systems in terms of migrant diffusivity. These are summarized in Table 3 (Briston and Katan 1974).

Table 3. Migration from various packaging material/contact phase systems

Nonmigrating. Transfer can take place only from the package surface. The diffusion coefficient of the migrant cannot be measured Independently migrating. The diffusion coefficient is measurable under the test conditions and during the specified contact time Leaching. Penetration of the polymer matrix by a contacting phase, resulting in migrant transfer. This can be considered a leaching process

System 1

System 2

System 3

For situations described by System 1, the diffusion coefficient for the transferred species is not measurable and any migration which occurs takes place from the surface of the packaging material or at the package- contact phase interface. System 2 is representative of a more generalized situation where the diffusion coefficient for the migrant is measurable under the time-temperature conditions of the study. This situation covers relatively volatile moieties such as monomer and some special cases of

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JACK R. GIACIN

liquid or solid additives such as plasticizers and antioxidants. Here, the rate of migration and the amount of migrant transferred will be dependent upon the contact phase volume (i.e. finite bath conditions), bound- ary layer resistances in the extracting phase and the time scale for desorption, especially for migrant molecules which partition strongly toward the polymer phase (Koros and Hopfenberg 1978).

For situations described by System 3, components of the contacting phase, such as a solvent, penetrate into the packaging material, swelling the polymer and disordering its structure. The penetrant effectively acts as a plasticizer. The consequence is an increase in the diffusivity of the considered migrant and a resultant increase in the rate of migration. As pointed out by Katan (1978), migration in a type 3 system involves a leaching mechanism and migration is negligible in the absence of a contact phase (i.e. food or food simulating solvent) but significant in its presence.

MIGRATION OF INDIRECT FOOD ADDITIVES IN SELECTED PRODUCT/PACKAGE COMBINATIONS:

PHYSICO-CHEMICAL CONSIDERATIONS

In terms of the diffusion or migration of a substance (i.e. monomer, oligomer, adjuvant) from a polymer matrix t o a contact phase, two models can be proposed, a linear model and a non-linear model. For the situations described by the diffusion models proposed, the polymer/surface combination is a film or slab where migration studies are conducted under immersion conditions. These test conditions involve maintenance of two-sided contact between the test film and the contact phase, until there is no further increase in the level of migrant transferring from the polymer to the contacting phase (i.e. polymer/contact phaselmigrant system has attained equilibrium). The film samples to be tested are mounted in a migration cell of a design which will allow a constant area of contact and contact phase volume. The partition-concentration relationship for the two models is presented graphically in Fig. 1, where migrant concentration in the contacting phase, at equilibrium, is plotted as a function of the initial migrant concentration in the polymer phase.

As shown in Fig. 1, for the linear model the quantity of migrant transferring to the contact phase is directly proportional to the initial concentration of migrant in the polymer. For this model, the probability of the rate of diffusion of any one migrant molecule being equal to that of a- nother is quite high. Thus, over the range of concentrations showing this relationship, valid predictions of the equilibrium distribution of migrant

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“ , I - P O

IN I T I AL ?lI GRANT COXCENTRATION

FIG. 1. DIFFUSION MODELS: EQUILI- BRIUM CONCENTRATION OF MIGRANT

GRANT CONCENTRATION IN CONTACTING PHASE VS INITIAL MI-

in a specific polymer/contacting phaselmigrant system can be made. How- ever, for very low migrant concentrations, where differences in molecular environment can result in differences in the rates of diffusion of migrant molecules within the polymer matrix, predictions made from transport studies at high migrant concentrations are not necessarily valid. At these low concentration levels, the partition-concentration relationship may become non-linear as depicted by the lower curve in Fig. 1 (Gilbert 1976; Gilbert et al. 1980).

As shown in Fig. 1 for the non-linear model, below a critical initial migrant level, the equilibrium concentration of migrant in the contact phase is no longer directly proportional to the initial migrant concentration, but increasingly favors migrant distribution toward the polymer phase with lower initial concentrations.

LINEAR MIGRATION MODEL

The schematic representation of a simple, linear migration model is

26 5

JACK R. GIACIN

%

COiITACI P h A S t %

I f \

1 PACKAGING ' 6 I I A T E R I A L /

* CXITACT PHASE

INTERFACE IrJTERFACE

FIG. 2. SCHEMATIC REPRESENTATION OF LIN- EAR MIGRATION MODEL

[C]pe, [ C l c , = Equilibrium concentration of migrant in polymer and contact phase, respectively;

w i p e KP = -'

[CIC, , IT]^, = total concentration of mi-

grant in polymer; and ID]^ = initial concentration of dissolve2 migrant.

266


shown in Fig. 2. In this model, i t is presumed that the total concentration of migration species [IT] is dissolved within the amorphous region of the polymer and is freely diffusible. The quantity of migrant distributing to the contact phase will depend upon migrant diffusivity as well as on the solubility and affinity of the migrant in both the polymer and contact phases. For the linear model, the equilibrium concentration of migrant in the contact phase, resulting from contact with the packaging material, is directly proportional to the initial migrant concentration and can be estimated by solution of eq. 3 (Gilbert et al. 1975).

where: [C] ce = Concentration of migrant in contact phase at equi-

[C] pe = Concentration of migrant in polymer phase at equi-

[IT] po = Initial concentration of migrant in the polymer Wp and Wc

Kp

librium

librium

= Weight of polymer and contact phase, respectively = Partition coefficient defined as the concentration

of migrant in polymer at equilibrium divided by the equilibrium concentration of migrant in the contact phase :

For the migration of VCM from polyvinyl chloride packaging material, we assume: (a) the initial monomer level [IT] ppb (wt/wt), which is representative of residual monomer levels achieved in PVC film by present technology and (b) Wp, Wc, and Kp are 1 g, lOOg and 100, respectively. From Eq. 3, [C] ce would be equal t o 10 parts per trillion (ppt), assuming VCM migration followed a linear migration model at these levels of residual monomer concentration.

In all probability, at sufficiently low levels of migrant or penetrant, the equilibrium concentration of migrant in the contact phase will be below that estimated for [ C J ce by eq. 3, because of non-ideal sorption and diffusion of the migrant within the polymer bulk phase.

to be approximately 2

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JACK R. GIACIN

Non-linear Model

Investigators studying transport or diffusion of penetrants in glassy polymers have observed non-ideal sorption and diffusion of fixed gases and organic vapors, as a function of initial penetrant concentration (Barrer e t al. 1957; Michaels e t al. 1963; Vieth and Sladek 1965; Vieth and Amini 1976). These results were attributed to a two-mode concur- rent sorption mechanism for glassy polymers, namely, ordinary dissolution and “hole” filling. The latter mechanism being the result of physico- chemical binding to a finite number of active binding sites (“holes”) within the glassy polymer matrix (polymer-penetrant interaction affinity). For the dual-mode sorption mechanism, it is assumed that the sorbed species is in equilibrium with the dissolution mode and exhibits a reduced level of mobility.

A t concentrations of a migrant in the polymer matrix significantly greater than that required to saturate active binding sites, the bulk of the potential migrant is simply “dissolved” within the amorphous region of the polymer matrix. The dissolved species is freely diffusible and readily migrates into a contacting phase (i.e. foodstuff). The rate of transport for the dissolved species is diffusion controlled and will follow a linear migration model. However, below the saturation level or some critical level of migrant, migrant diffusivity becomes concentration dependent.

of a migrant, obeying a multi-mode sorption mechanism, is presented graphically in Fig. 3. An account of, and an extension of this model is incorporated in a review by Gilbert e t al. (1980) and by Koros and Hop- fenberg (1978) and references cited therein.

In this model, it is presumed that the total initial migrant concentration [IT] is comprised of three thermodynamically distinct molecular populations. The total concentration of migrant in the polymer is therefore the sum of the following concentrations: [ IT] p0 = [ ID] p0 + [Is] + [II] po, where ID]^^ is the concentration of a freely diffusible species which is “dissolved” in the amorphous region of the polymer matrix and follows ordinary solution chemistry, obeying Henry’s law. [Is] concentration of migrant which is sorbed to active sites, or bound by physical entrapment within the glassy polymer matrix. The rate of diffusion of the sorbed species is assumed to be markedly different than that of the dissolved, or Henry’s law, species. An additional sorption mode is also proposed, corresponding to an irreversibly bound or “immobilized” species, of concentration [ 111 (Gilbert 1976; Gilbert et al. 1980).

A proposed non-linear or active site migration model for the transport

is the

26 8


f I

3

CONTACT PHASE

PACKAG I ,dG MATERIAL

[ C k ,

[kip, + “‘Ip0 COilTACT PHASE

I I ~ T E R F A C E I i ITERFACE

FIG. 3. SCHEMATIC REPRESENTATION OF NON- LINEAR MODEL

IT]^^ = total concentration of migrant in polymer; [ID]p and [IS], = concentration of dissolved and sorbed species in polymer respectively; [ I 1 l P = concentration of totally immobilized species. [ C ] c = concentration of migrant in contact phase at equifibrium and

[A+] = concentration of active sites

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JACK R. GIACIN

It is further assumed that an equilibrium exists between the dissolved and sorbed species within the polymer bulk phase and that the equilibrium distribution of migrant between the polymer and a contacting phase involves only that portion of the migrant population which is diffusible, namely [IDlp.

From this model it can be seen that the mechanism which controls diffusion of a migrating species through the polymer bulk phase involves two distinctly different sorption processes. A portion of the total migrant concentration in the polymer is dissolved within the amorphous region of the polymer and obeys Henry’s law. A species of concentration [Is] po follows an active site sorption mechanism. The total concentration of migrant in the polymer, therefore, can be considered as consist- ing of a continuum of thermodynamically distinct molecular populations.

In terms of migration, such a model predicts that transport of a migrant from a polymer to a contacting phase would exhibit a concentration-dependent distribution of migrant between the polymer and contact phase and migration would approach zero at some finite, but low initial migrant concentration (Gilbert 1976).

Gilbert’s 1976 hypothesis of an “effective zero” migration of a migrant from a packaging material (polymer) to a contacting phase has been confirmed by the results of Morano et al. (1977) and Kashtock et al. (1980), who studied the sorption-desorption of vinyl chloride in selected polyvinyl chloride-food simulating solvent systems. Such an observed concentration dependent partition distribution would have significant implications in terms of the Food and Drug Administration regulations.

Under the assumptions that: (a) there is a finite number of active sites within the polymer matrix and (b) the equilibrium between the dissolved and sorbed species will favor the sorbed species as the concentration of active sites becomes significant with respect to [IT] three generalized cases can be envisaged. These are summarized here:

Case 1 - Transport at high migrant levels [ID1po [Is]po+ [IIlp,

Case 2 - Transport at low migrant levels ([IT] in ppm range) [ID] PO 2 [IS] pOs [III PO

Case 3 - Transport at migrant levels where

As shown for situations described by Case 1, the concentration of

For transport a t high

[IT 1 po = [I11 po

freely diffusible species [ID] of sorbed species; therefore, [IT]

is much greater than the concentration = [ID]

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migrant concentrations (i.e. Case l), it follows that migrant diffusivity will be independent of the initial migrant concentration and the equilibrium concentration of migrant distributing to a contact phase will be directly proportional to [IT] the initial concentration of migrant.

The example described by Case 2 represents a more generalized situation, involving the transport of trace levels of migrant (i.e. monomer, oligomer, residual solvent, etc.), where the concentration of species sorbed to active sites [IS] po is of the same order of magnitude as the dissolved species [ID] For this case, the rate of migrant transport or flux through the polymer bulk phase will be a function of the diffusivities of the migrant populations dissolved in the matrix and sorbed to active sites. Further, the partition coefficient (Kp) will be a function of:

and the distribution of the migrant from the polymer to a contacting. phase will approach zero as [ID] approaches zero. This case would result in a concentration-dependent diffusion coefficient (D) as well as a concentration-dependent partition coefficient (Kp) (Morano et af. 1977).

Alternatively, if the sorbed species [IS] po are not in local equilibrium with the Henry law species [ID] an additional retardation to the diffusion rate will result due to the kinetic limitation preventing equilibrium between Henry’s law and sorbed molecules (equilibrium between sorbed and dissolved species) (Koros and Hopfenberg 1978).

In the extreme situation detailed by Case 3, the total concentration of migrant is approximately equal to that of the completely immobilized species. For this case, [IT] within the time scale for typical sorption-desorption studies.

1 [ 111 pp and no transfer will be recorded

Relationship to Package/Product Combinations

was a film or slab, where migration studies are conducted under immersion conditions involving two-sided contact between the test film and the contact phase. However, for package end use application, the poly- merlsurface combination is a film or slab, where there is only one-sided contact between the container wall and the contact phase. This case becomes more difficult to analyze if the migrating species is a relatively volatile moiety such as residual monomer, as movement or diffusion of the migrant (i.e. monomer) through a film or slab will involve simulta-

For the diffusion models described, the polymer/surface combination

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JACK R. GIACIN

FIG. 4. DESORPTION MODEL: GENERALIZED CON- CENTRATION PROFILE

= film thinkness; [c]ci, [C]E. = initial concentration of migrant in contact phase and external environment, respectively. to, t l , t2, t3 . . . tn = concentration gradient

at any time ( t )

neous transfer of the migrant to both the contact phase and the external environment .

A generalized concentration profile for this case is presented in Fig. 4, showing the concentration gradient of the migrant, within a cross-section- al area of the polymer film or slab, with time. This profile is constructed based on the following assumptions: (a) diffusion is multi-directional, involving simultaneous transfer of migrant to both the contact phase and external environment; (b) the external environment provides an infinite sink for the migrant; and (c) the contact phase provides finite bath conditions. This case would result in an initial increase in the level of migrant transferring to the contacting phase, followed by the concentration de- creasing and ultimately approaching zero, within some academically long time scale for desorption.

This is shown graphically in Fig. 5, where migrant concentration in the contact phase is plotted as a function of time. The time scale of predicted total migrant desorption, however, in many realistic cases may be con- siderably greater than the useful shelf life of the package. For compari- son, the general relationship of migrant concentration in the contact phase with time, following two-sided contact under immersion condi-

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LOG TINE

FIG. 5. GENERALIZED CONCENTRATION PROFILE WITH TIME (T) FOR MIGRATION INVOLVING:

( A ) one-sided contact between test film and contact phase; and ( B ) two-sided contact between test film and contact phase

tions, is shown by the upper curve in Fig. 5.

will therefore depend upon such factors as migrant diffusivity, as well as the solubility and affinity of the migrant in the polymer and contact phases and in the external package environment, respectively. Thus, it is difficult to separate experimentally true equilibrium levels of migrant in the contact phase from situations corresponding to transient levels for a polymer/surface combination involving one-sided contact, if the migrant is relatively volatile and can diffuse to both the contact phase and the external environment concurrently.

measurement of true equilibrium levels of migrant in the contact phase, corresponding to finite bath conditions (i.e. finite volume of contact phase). Such studies can be used to estimate extraction or migration levels for package/product combinations involving one-sided contact. The corresponding estimated equilibrium concentration in the contact phase would represent an upper limit or maximum value, as shown in Fig. 5.

The effective quantity of migrant in the contact phase a t any time (t),

The situations described for the immersion conditions, however, allow

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JACK R. GIACIN

Table 4. Variables to be considered in the migration of indirect food additives

1. Polymer film thickness: Considers effective film thickness for predicting

2. Migrant concentration: Considers multi-mode sorption and the concentra-

3. Contact phase: Considers the influence of environment on migrant diffu-

4. Polymer morphology: Considers the effect of processing (thermal mechan-

migration.

tion dependency of migrant diffusion

sivity and partition distribution

ical history) on partition distribution of migrant between polymer and contacting phase

5. Temperature: Considers activation energy of diffusion 6. Time: Considers the time scale for desorption

For further discussion on theoretical considerations of diffusion in the plastics package/contents system, the reader is referred to Figge and Ru- dolph (1979) and Smith et al. (1979A, B), and references cited therein.

SUMMARY AND CONCLUSION

In conclusion, it can be seen that the migration of indirect food additives is a very complex process subject to a number of variables which are summarized in Table 4.

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GILBERT, S. B. 1976. Migration of minor constituents from food packaging materials. J. Food. Sci. 41 (4), 955.

GILBERT, S. G., MILTZ, J. and GIACIN, J. R. 1980. Transport considerations of potential migrants from food packaging materials. Food Process. Preservation,

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EVALUATION OF PLASTICS PACKAGING MATERIALS FOR FOOD PACKAGING APPLICATIONS: FOOD SAFETY CONSIDERATIONS