- Email: [email protected]
Flavour scalping by food packaging
Review The sorption of food constituents, especially aroma compounds, by polymeric packaging materials has received considerable attention during the past decade, and is generally considered to be a problem within the food packaging industry. Flavour scalping, as this phenomenon is termed, may cause decreased consumer acceptance of the food product due to loss of aroma intensity or the development o( an unbalanced flavour profile. This article presents an overview of research conducted on the subject.
The first and foremost function of a food package is to protect the product and to preserve its inherent quality. In order to do this the interactions between the foodstuff and the package have to be minimized. Some food packaging materials, such as glass and metal, are both perfect barriers and almost inert, and they do not interact with the packaged food to any considerable extent. In contrast, polymeric packaging materials are neither inert nor perfect barriers, and interactions with the product can cause problems [it should be noted, however, that during the past few years great efforts have been made in the development of so-called active packaging, in which controlled and desired interactions between the food and the package actually prolong the shelf life or improve the quality of the productl. Food-packaging interactions include not only mass transport and Factions between the product and the packaging, but also interactions with the environment as a result of the inability of the package to provide a perfect barrier. Figure ! summarizes possible interactions between foods, their packages and the atmosphere. The sorption of food constituents by the packaging material can have detrimental effects on the quality of the product. Most of the research has been focused on the sorption of aroma compounds, but other food components, such as fats, pigments and organic acids, can also be sorbed by polymeric packaging materials t-~. The adverse effects of sorption can result in either damage to the package or a direct loss of product quality. The sorption of fats or organic acids by the food-contact layer in a l:olymer laminate can cause separation (delamination) of di.fferent layers of the laminated'L Other sorbed compounds might swell the polymer, acting as plasticizers, resulting in increased diffusivity, and thereby a higher permeability (T.M. Hensley, MSc thesis, Michigan State University, MI, USA, 1991). In other words, the barrier properties of the package are reduced, causing a shorter product shelf life. The sorption of aroma compounds, known as flavour scalping, might result in loss of flavour intensity. Furthermore, individual compounds that make up an aroma can be sorbed by the polymer to different extents. This can lead to the product having an
Flavour scalpingby food packaging Tim Nielsen and Margaretha J~igerstad unbalanced flavour profile. Yet another problem arises with refillable containers, Sorbed aroma compounds that are not totally removed during the washing process can be transferred to the next product that is filled into the same package, and possibly c a u ~ off-flavour~.
Experimental design During the past decade, flavour scalping phenomena have been extensively investigated by several research groups throughout the world 6--~'~. It is a complex field, and .several factors have been proven to have important effects on the extent of sorption of different flavour compounds by various packaging materials, Many studies have dealt with determining the mass transport coefficients for organic vapoms in polymer films ~a-:°-~'-35, The transport of vapours is easier to evaluate mathematically than 1he transport of liquid sorbates, and the results can often be explained by sorption and diffusion theories, Studies of aroma vapour sorption are, of course, of ~eat value, not only to confirm existing theories but also to determine the influence of various factors on the degree of sorption. No foodstuffs, however, are present in the gaseous phase, and a more realistic approach is to study aroma sorption by polyrne~ from liquids, either model ,solutions or actual liquid fooct~. However, the addition of an aqueous phase to the experimental system makes evaluation of the results more complex. There are different ways of expressing the extent of sorption of flavour compounds by polymers. The solubility of vapours is usually described by the solubility coefficient of the permeant in the polymer. Results of studies of the sorption of aroma compounds present in the aqueous phase, on the other hand, are presented in various ways. Several investigators have used the loss of aroma compound as a percentage of the original amount present in the liquid to express the degree of sorptionS.]°-H'td-"L Others have calculated the partition coefficient of the analyte between the polymer and aqueous phases ]~t~-jo3-~. The partition coefficient is defined as the concentration of the substance in the polymer divided by the concentration of the substance in the aqueous phase, and is a unitless parameter. Another parameter used to describe the extent of sorption is the distribution ratio, defined as the ratio of the quantity of the compound sorbed by the polymer film to the quantity remaining in the sample solution. The lack of uniformity Tim Nielsen and Margarelha |ige~-tad are at the Department of A0~alied in the presentation of the results, together with the use Nutrition &FoodChemistry,ChemicalCenter, LundUni~,ersity,PO Box t24, of various techniques of preparing the samples, different storage conditions, etc., means that care must be taken 5-221 00 Lund,Sweden. Trends in Food Science & Technology November 1994 IVol. 51
,~,,~. El...... ~,t-,,,,L~,~-'~ .aa~,~SO~,~
353
Polymer Environment film Foodstuff
Migrating
substances _~ !_ (2)
"- PERMEATION Oxygen Water vapour Carbon dioxide Other gases
Adverse consequences (1) Oxidation Microbial growth Mould growth Off-flavour (2) Dehydration Decarbonation
----~
MIGRATION
Monomers Additives
Off+f layout Safety problems
SORPTION
Aroma compounds Fats Organic acids Pigments
Loss of aroma intensity Development of unbalanced ftavour profile Damage Io Ihe package •
Fig. 1 Possible interactions between foods, their packages and the environment, logether with the adverse consequences.
which increased the solubility of the limonene in the aqueous phase. Halek and Meyers t~ reported a 30% loss of limonene from model solutions to LDPE within 20 days. The sorption exhibited a biphasic rate, believed to be due to combined absorption and adsorption: the rapid decrease in limonene concentration during the first day indicated adsorption onto the polymer surface, and the slower rate of limonene loss observed during the rest of the experiment suggested absorption into the plastic. From a similar study, a partition coefficient (the concentration of the substance in the polymer divided by the concentration in the aqueous phase) of 5000 for limonene in LDPE was obtained t~'. The time to reach equilibrium sorption increased as the surface area of the polymer decreased, while the proportion of limonene sorbed remained constant, indicating a mainly absorplive proc e s s 12
The solubility of limonene in different polymers varies a great deal. Charara et al. u found that 70% of limonene was lost from cold-pressed orange oil in contact with LDPE for four days, while only a 30% loss was observed from orange oil in contact with high-density polyethylene (HDPE) and polyLimonene sorption The most extensively studied aroma compound, [,ropylene (PP). In another report, limonene in model with respect to its sorption by polymers, is limonene. solutions showed higher affinity for LDPE than for Limonene is an unsaturated terpene hydrocarbon present polyamide, which in turn sorbed limonene to a greater in citrus flavour. It makes up - 9 5 % of the oil fraction in extent than did polystyrene ~+. The distribution ratio of orange peel. It is a highly nonpolar substance, and has limonene increased with increasing ethylene content in shown a high affinity for many polymeric packaging ethylene vinyl alcohol (EVOH) copolymer 's. Orange materials. Studies have assessed limonene sorplion into juice lost 40% of its total iimonene content t0 LDPE, various polymers from both the vapour phase and the but only 20% to EVOH *~. In the same study no detectable amounts of limonene were sorbed by poly(ethylaqueous phase. ene terephlhalate) (PET). Other studies have, however, Di]rr et al. ~ reported a 50% loss of limonene from orange juice to low-density polyethylene (LDPE" see Box I shown that small amount.,, of limonene were sorbed by lbr a lull list of polymer PET from an orange-flavoured soft drink s. Limonene name abbreviations used) vapour was shown to have a higher affinity for LDPE Box 1. Polymernameabbreviations during two weeks of stor- than for PET I; DeLassus I~ obtained a higher solubility used age. Other investigators, in coefficient for limonene vapour in poly(vinylidene similar experimems, have chloride} (PVdC) than in EVOH; this was in turn higher EVOH: Ethylenevinyl alcohol reported decreases in lim- than the solubility in PE or oriented polypropylene. HDPE: High-densitypolyethylene Limonene sorption from model solutions and orange onene content in orange juice in contact with LDPE juice increased with temperature".'-', while the sorption LDPE: Low-densitypolyethylene ranging from 25% to 60% of limonene vapour decreased with an increase in temMDPE: Medium-densitypolyethylene (Ref~ 2. 7-9). The sorption peratureU'"L The extent of limonene sorption tended to PE: Polyethylene of limonene by LDPE from be less when limonene was present in it mixture than model solutions was much when it acted as the sole permeantlLak this applies for PET: Poly(ethyleneterephthalate) greater than that from or- limonene in either the vapour or the aqueous phase, and PP: Polypropylene ange juice "+. This was at- could be a result of competition for sorption sites. PVC: Poly(vinylchloride} tributed to the presence of Increasing the thickness of the polymer film, thereby other constituents in the increasing the mass of the polymer available to act as a PVdC: Poly(vinylidenechloride) ........... juice, such as the pulp. sorbant, increased limonene sorption s.t~. Saturation of when comparing values reported from different experiments. A brief review of published results concerning aroma sorption by polymeric packaging materials l'otlows.
354
Trends in Food Science & Technology November 1994 IVol. 51
the polymer film at high limonene concentrations, resulting in a decreased partition coefficient, has been observed '~.n-'. By biaxially drawing the polymer film, thereby increasing the crystallinity oi" the polymer, the extent of limonene sorption can be reduced t-~. Several studies combining limonene sorption measurements with sensoric evaluation have shown loss of limonene to be of little importance for the taste sensation x'-'t. Other investigators have even indicated that limonene sorption can be beneficial for the product since limonene can act as a precursor for off-flavours, forming e~-terpineoi in orange juice-'2--'L However, adverse effects on the packaging material result from limonene sorption. Limonene may swell LDPE films, giving rise to higher oxygen permeability through the polymer'-L Furthermore, the modulus of elasticity and the tensile strength of the packaging film may diminish as a result of limonene sorption by LDPE ~.
Factors affecting flavour scalping Experiments performed with aroma compounds other than limonene have also shed some light on the factors that influence the extent of sorption. The nature of the pelymer and the permeant are, of course, important: in addition, external factors, such as storage conditions, affect the solubility of the aroma compound in the film. Konczai e t a l ? -~ reported that larger amounts of apple aromas were sorbed by LDPE than by EVOH with a high ethylene content, or by co-PET. In another study, orange flavour compounds were shown to have a higher affinity for PP than for LDPE-'*. Scalping of volatiles from toothpaste by a series of polyolefins and copolymers has been investigated-'7: polyethylenes with a high density sorbed larger amounts of volatiles than did those with a low density, Moreover, the volatiles had a higher affinity for the copolymers ethylene vinylacetate and ethylene acrylic acid than for the homopc!ymers HDPE, MDPE and linear LDPE, The extent of sorption was not related to the crystallinity of the polymer, but to the presence of oxygen-containing co-monomers and to ionomerization (the formation of ionic bonds, crosslinking adjacent polymer chains). In contrast, it has been found that decreased crystallinity of a PE film resulted in increased solubility of various aromas 2s. Other researchers came to the same conclusion when studying the sorption of citrus oils by polyolefins ~. Several investigators have pointed out the impact of the carbon chain length of volatiles on their solubility in polymer films, For homologous series of esters, aldehydes and benzoates, the distribution ratio of the volatiles in PE increased threefold with each additional methylene group in the compound '-8. However, for molecules composed of 11 or more carbon atoms, the increment in distribution ratio was less or, in some cases, negative, in another experiment, the sorption of esters, ketone.,, and aldehydes by PP increased as the number of carbon atoms in the compounds increased 2~. Compounds with eight or more carbon atoms were sorbed from yoghurt drinks by HDPE, while shorter molecules remained in the product ~°. In the same study it was ob~rved that Trendsin FoodScience& TechnologyNovember1994 IVol. 51
highly branched molecules were sorbed to a greater extent than linear molecules. The carbon chain length is closely related to 1he boiling point of a molecule. and several researchers have indicated ~ relationship between the solubility and the boiling points of sorbates. Strandburg e t a l . ~l obtained a linear relationship between the logarithm of the .solubility coefficient in vinylidene chloride copolymer and the boihng points of linear esters, alkanes and ketones, Similar results have been reported for the solubility of alkyl esters in polyvinyl alcohol .'-~. The type of functional group on the volatile also seems to affect the solubility of aroma substances in polymeric packaging materials, in a study of the sorption of citrus oil constituents by polyolefins, terpenes showed the highest affinity for the polymers, followed by sesquiterpenes: larger amounts of esters and aldehydes were sorbed than of alcohols ~. In the same experiment it was observed that saturated aldehydes were sorbed to a greater extent than those containing double bonds. Pieper e t a l . ~ investigated the sorption of orange juice aromas by various laminates, all with LDPE as the food-contact polymer. Hydrocarbon compounds were the most readily sorbed by the packages. Ketones were sorbed more than aldehydes, which in turn were sorbed more than alcohols. Ethyl butyrate, an ester very prominent in orange flavour, could not be detected at all in any of the packaging materials. Similarly, others have observed that esters have a higher affinity for LDPE than do aldehydes, and that only small amounts of alcohols are sorbed by LDPE -~8-a-*. Disulphide compounds representing the flavour components in ~aion/garlicflavoured sour cream were sorbed to a very i arge extent by polystyrene~. The solubility of ethyl propionate in poly ~inyl alcohol was reduced as the relative humidity increased "~z. This was attributed to competition between ethyl propionate and water molecules for available sorption sites. The solubility coefficients for aldehyde vapours in linear LDPE were maximal at 25°C, with sorption being lower at both 5°C and 75cC (Ref. 35). in the same experiment, however, the sorption of aldehyde vapours by poly,,inyl chloride (PVC) decreased as the temperature increased. Prediction m o ~ ] s Most of the results from these past studies can be explained by a combination of enthalpic (chemical structure) and entropic (molecular weight and polymer conformation) effects, Factors that affect flavour sorption include temperature, concentration, chemical structure, molecular weight, polymer conformation and competition between sorbates. Recently, effort~ have been made to develop methods for predicting the sorption of food constituents by various polymers. The predictive models aim to describe the quantitative contributions of different factors: comparison of calculated values from mathematical models with experimental sorption data is u ~ d to evaluate the validity of the model or the hypothesis. Simple models ba~d on the compari~n of the solubility parameters of the investigated polymer and analyte 3~5
species can be used to predicl the extent o f sorption qualitatively ~. In order to obtain a m o r e precise q u a n t i tative e s t i m a t i o n o f the solubility, more sophistica:ed methods, which take several aspects into c o n s i d e r a t i o n , have b e e n d e v e l o p e d ~7-~'>.At present, the m o d e l s lack an accurate description o f i n t e r m o l e c u l a r forces, p o l y m e r c o n f o r m a t i o n and c o m p e t i t i o n b e t w e e n the low m o l e c u lar weight species. Therefore, the m o d e l s need further r e f i n e m e n t in order to be able to provide a perfect match b e t w e e n e x p e r i m e n t a l l y o b t a i n e d values and calculated data for a n y g i v e n s o r b a t e - s o r b a n t system.
t3 14 15
16 17 18 19 20
21 22
Conclusions It b e c o m e s apparent w h e n s t u d y i n g the published literature, which often i n c l u d e s c o n t r a d i c t o r y findings, that all the m e c h a n i s m s b e h i n d aroma sorption by polymers are not yet fully understood. Efforts by different research groups have resulted in s i m i l a r c o n c l u s i o n s : h o w e v e r , there are still s o m e areas that spark s o m e controversy. More research is required in order to m i n i mize, a n d h o p e f u l l y eliminate, the p r o b l e m o l flavour s c a l p i n e wilhi, n thc h~ocl p a c k a g i n g industry.
23 24
25 26
27
References 1 2 3
4 5 6 7
8 9
lO
Koch,I-, Robinson,t.. and Figge,K. I1t176~FeUe, 5eiten, Anstrichm. 78, 371-377 Marshall,M.R. Adams, I.P, ,andWilliams, I.W. 119851in Aseptipak 85, pp. 299-312, Scholland13u_qnessResearchInc., Princeton,NI, USA Olafsson,G., lagerslad.M., Oste, R. anti Wessl6n,g. q1qq.l~i, .~.ood 5ci. 58, 215-219 Olalsson,G., lagerstad,M., Oste, R, Wessl6n, B. and Hjeqherg. T (19931 Food Chem.47. 227-233 Nielsen,T.l 11994~I, foodScL 5eL227-230 Durr,P. and Schohinger.U. l] 9811in Fla~ourfll (Schreier,I., t.d.!. p. 179, Waller de Gruyler and Co., Berlin, Germany Mannheim,C.H., Millz, I, and Pa,sy,N. 1198/11in food and Pack,~gin~ !oteractions (Holchkiss, I.H., ed.l, Pl)- 68-82, American Chemk ,~I Society,, Pieper,G, Borgudd.L., Ackermanil,P. and Fellers,P. II~)92) I- i~od 5eL 57, 1401"I-1411 Hnose.K., Harte, B.R., Giacin, I.R,, Miltz, J, and Stine, C. I198~ m Food and Packaging InteractionsIHolchkiss,I.H., e:l.L pp. 28~-fi AmericanChemical Sociely Mannheim,C.H, Mihz. J. and Lelzler,A. dqB7'~]. Focrrt5ci. 52.
28 29
31) 31
32
33 ]4 35
36
737-740
ll 12
Halek,G.W. and Meyers,M.A.1198q~Pac~,ag. TechnoL 5ci. 2. 141-14~ Kwapong,O.Y. and Holchki.~s,].H. IIq871 I. Food Sci. 52, 761 71;1, 785
17 :18 :]9
Charara,Z.N., Williams, l.W,, Schmidl, R.H. and Marshall,M.R. I19921 I, Food Sci. 57, 963-qfi6, 972 Sizer,C.E., Waugh, P.L., Edstam,S. and Ackermann,P. 119F18)Food TechnoL 43, 152-I 59 Ikegami,I-., Nagashima,K., ,Shimoda,M., Tanaka,Y. and Osajima, Y. {199111. FoodS-ci. 56, 500-503, 509 hnai, T.,Harle. B.R. andGiacin, I.R. 11%Otl. foodScL55.158-16! Paik.I.S. (1992) Ir Agric. Food Chem. 40, 1822-1825 Detassus,P.T. 119861Tappil. 69, 16H-16L Boner,A.L., Kalyankat V, and Shoun,L.H. (19911I. FoodSci. .56, I051-I054 Theodorou,E, and Paik, 13-Hq921Packag. Technal. 5ci. 5, 2t-2.5 Mannheim,C.H. and Passy,N. 119871FlOssigesObst. 54, 585-588 DOFF,P., Schobinger,U. and Watdvogel,R. l19811Alimenta 20, 91-93 KuBy.V., Braddock.R.I. and Sadler,G.D. 1199411.FoodScL 59, 402-405 5adter,G.D. and Braddock, R.I. 1199011.FoodSci. 55, 587-588 Konczal.J,8., Harte, B.R., Hooiiat.P. and Giacin, I.R. 119926L Food 5ci. 37, 967-q72 Halek,G.W. anti Lultmann,1.P.11991~inFoodand Packaging h~teractions II (Risch, S.I. and Holchki~s,I.H, edsL pp. 212-226, AmericanChemicalSociety Brant.P., Michiels,D., Gregory, B., Laird, K. and Day, R. 119911in Food and Packaging Interactions II IRisch,5,1. and Hotchkiss, I.H., edsl, pp. 227-25(1,AmericanChemic~lSociely Shimoda,M., Ikegami,T. and Osajima. Y. 119881J.5ci. foodAgric. 42,157-163 Arora,D.K., Hansen,A.P. and Armagost,M.S. (19911in food and PackagingInteractionsII (Risch, S.I. and Holchkiss, J,H., eds'J, pp. 203-21 I, American ChemicalSociely Lin~SenrJ.PH.. Verheul,A, Roozen,I.e. and Poslhumus,M.A. 11991 Int. Dairy I. I, 33-40 Slrandburfi,G., DeLassus,P.T.and Howell, B.& {19q01in Barrier Pof)'mers and Structures IKoros,W.l., edJ, pp. 333-350, American Chemical Sociely Landois-Garza,l~ and Holchkiss, I.H, 119881in Foodand Packaging Interactions IHotchkiss,I.H., ed.L pp. 42-58, American Chemical Society Nielsen,T.I., I,igerslad,I.M.,Osle, R.[. and WesslOn,B.O. 119921 I. food Sci. 57, 491)~tt)2 Tuebe,I.M., Hoojjat, H., Hernandez. R.I., Giacin, I.R,and Harle, B.R. t lgqO] Packag. TechnoL Sci. 3, 133-140 Leuiven,A. and Slotlman, U. Ilqq2] Z tebeosm. Unters. Forsch. 1'94, 355-359 Matsui,T, Nagashima,K., Fukamachi,M., Shimoda, M. and Osajima~Y. I19921I. Agric. Food Chem. 40, Iq02-1905 Paik,I,S.and Wriler, M.S.I. Agric. Food Chem. fin pressl Li, S. and Polk,J,S, I. Fo~d 5ci. Im pressl Baner,At. and Piringer,O.G. i19941 I. Chem. Eng.Data 3q, ~41-.~48
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Trends in Food Science & Technology November 1994 IVol. 51
The first and foremost function of a food package is to protect the product and to preserve its inherent quality. In order to do this the interactions between the foodstuff and the package have to be minimized. Some food packaging materials, such as glass and metal, are both perfect barriers and almost inert, and they do not interact with the packaged food to any considerable extent. In contrast, polymeric packaging materials are neither inert nor perfect barriers, and interactions with the product can cause problems [it should be noted, however, that during the past few years great efforts have been made in the development of so-called active packaging, in which controlled and desired interactions between the food and the package actually prolong the shelf life or improve the quality of the productl. Food-packaging interactions include not only mass transport and Factions between the product and the packaging, but also interactions with the environment as a result of the inability of the package to provide a perfect barrier. Figure ! summarizes possible interactions between foods, their packages and the atmosphere. The sorption of food constituents by the packaging material can have detrimental effects on the quality of the product. Most of the research has been focused on the sorption of aroma compounds, but other food components, such as fats, pigments and organic acids, can also be sorbed by polymeric packaging materials t-~. The adverse effects of sorption can result in either damage to the package or a direct loss of product quality. The sorption of fats or organic acids by the food-contact layer in a l:olymer laminate can cause separation (delamination) of di.fferent layers of the laminated'L Other sorbed compounds might swell the polymer, acting as plasticizers, resulting in increased diffusivity, and thereby a higher permeability (T.M. Hensley, MSc thesis, Michigan State University, MI, USA, 1991). In other words, the barrier properties of the package are reduced, causing a shorter product shelf life. The sorption of aroma compounds, known as flavour scalping, might result in loss of flavour intensity. Furthermore, individual compounds that make up an aroma can be sorbed by the polymer to different extents. This can lead to the product having an
Flavour scalpingby food packaging Tim Nielsen and Margaretha J~igerstad unbalanced flavour profile. Yet another problem arises with refillable containers, Sorbed aroma compounds that are not totally removed during the washing process can be transferred to the next product that is filled into the same package, and possibly c a u ~ off-flavour~.
Experimental design During the past decade, flavour scalping phenomena have been extensively investigated by several research groups throughout the world 6--~'~. It is a complex field, and .several factors have been proven to have important effects on the extent of sorption of different flavour compounds by various packaging materials, Many studies have dealt with determining the mass transport coefficients for organic vapoms in polymer films ~a-:°-~'-35, The transport of vapours is easier to evaluate mathematically than 1he transport of liquid sorbates, and the results can often be explained by sorption and diffusion theories, Studies of aroma vapour sorption are, of course, of ~eat value, not only to confirm existing theories but also to determine the influence of various factors on the degree of sorption. No foodstuffs, however, are present in the gaseous phase, and a more realistic approach is to study aroma sorption by polyrne~ from liquids, either model ,solutions or actual liquid fooct~. However, the addition of an aqueous phase to the experimental system makes evaluation of the results more complex. There are different ways of expressing the extent of sorption of flavour compounds by polymers. The solubility of vapours is usually described by the solubility coefficient of the permeant in the polymer. Results of studies of the sorption of aroma compounds present in the aqueous phase, on the other hand, are presented in various ways. Several investigators have used the loss of aroma compound as a percentage of the original amount present in the liquid to express the degree of sorptionS.]°-H'td-"L Others have calculated the partition coefficient of the analyte between the polymer and aqueous phases ]~t~-jo3-~. The partition coefficient is defined as the concentration of the substance in the polymer divided by the concentration of the substance in the aqueous phase, and is a unitless parameter. Another parameter used to describe the extent of sorption is the distribution ratio, defined as the ratio of the quantity of the compound sorbed by the polymer film to the quantity remaining in the sample solution. The lack of uniformity Tim Nielsen and Margarelha |ige~-tad are at the Department of A0~alied in the presentation of the results, together with the use Nutrition &FoodChemistry,ChemicalCenter, LundUni~,ersity,PO Box t24, of various techniques of preparing the samples, different storage conditions, etc., means that care must be taken 5-221 00 Lund,Sweden. Trends in Food Science & Technology November 1994 IVol. 51
,~,,~. El...... ~,t-,,,,L~,~-'~ .aa~,~SO~,~
353
Polymer Environment film Foodstuff
Migrating
substances _~ !_ (2)
"- PERMEATION Oxygen Water vapour Carbon dioxide Other gases
Adverse consequences (1) Oxidation Microbial growth Mould growth Off-flavour (2) Dehydration Decarbonation
----~
MIGRATION
Monomers Additives
Off+f layout Safety problems
SORPTION
Aroma compounds Fats Organic acids Pigments
Loss of aroma intensity Development of unbalanced ftavour profile Damage Io Ihe package •
Fig. 1 Possible interactions between foods, their packages and the environment, logether with the adverse consequences.
which increased the solubility of the limonene in the aqueous phase. Halek and Meyers t~ reported a 30% loss of limonene from model solutions to LDPE within 20 days. The sorption exhibited a biphasic rate, believed to be due to combined absorption and adsorption: the rapid decrease in limonene concentration during the first day indicated adsorption onto the polymer surface, and the slower rate of limonene loss observed during the rest of the experiment suggested absorption into the plastic. From a similar study, a partition coefficient (the concentration of the substance in the polymer divided by the concentration in the aqueous phase) of 5000 for limonene in LDPE was obtained t~'. The time to reach equilibrium sorption increased as the surface area of the polymer decreased, while the proportion of limonene sorbed remained constant, indicating a mainly absorplive proc e s s 12
The solubility of limonene in different polymers varies a great deal. Charara et al. u found that 70% of limonene was lost from cold-pressed orange oil in contact with LDPE for four days, while only a 30% loss was observed from orange oil in contact with high-density polyethylene (HDPE) and polyLimonene sorption The most extensively studied aroma compound, [,ropylene (PP). In another report, limonene in model with respect to its sorption by polymers, is limonene. solutions showed higher affinity for LDPE than for Limonene is an unsaturated terpene hydrocarbon present polyamide, which in turn sorbed limonene to a greater in citrus flavour. It makes up - 9 5 % of the oil fraction in extent than did polystyrene ~+. The distribution ratio of orange peel. It is a highly nonpolar substance, and has limonene increased with increasing ethylene content in shown a high affinity for many polymeric packaging ethylene vinyl alcohol (EVOH) copolymer 's. Orange materials. Studies have assessed limonene sorplion into juice lost 40% of its total iimonene content t0 LDPE, various polymers from both the vapour phase and the but only 20% to EVOH *~. In the same study no detectable amounts of limonene were sorbed by poly(ethylaqueous phase. ene terephlhalate) (PET). Other studies have, however, Di]rr et al. ~ reported a 50% loss of limonene from orange juice to low-density polyethylene (LDPE" see Box I shown that small amount.,, of limonene were sorbed by lbr a lull list of polymer PET from an orange-flavoured soft drink s. Limonene name abbreviations used) vapour was shown to have a higher affinity for LDPE Box 1. Polymernameabbreviations during two weeks of stor- than for PET I; DeLassus I~ obtained a higher solubility used age. Other investigators, in coefficient for limonene vapour in poly(vinylidene similar experimems, have chloride} (PVdC) than in EVOH; this was in turn higher EVOH: Ethylenevinyl alcohol reported decreases in lim- than the solubility in PE or oriented polypropylene. HDPE: High-densitypolyethylene Limonene sorption from model solutions and orange onene content in orange juice in contact with LDPE juice increased with temperature".'-', while the sorption LDPE: Low-densitypolyethylene ranging from 25% to 60% of limonene vapour decreased with an increase in temMDPE: Medium-densitypolyethylene (Ref~ 2. 7-9). The sorption peratureU'"L The extent of limonene sorption tended to PE: Polyethylene of limonene by LDPE from be less when limonene was present in it mixture than model solutions was much when it acted as the sole permeantlLak this applies for PET: Poly(ethyleneterephthalate) greater than that from or- limonene in either the vapour or the aqueous phase, and PP: Polypropylene ange juice "+. This was at- could be a result of competition for sorption sites. PVC: Poly(vinylchloride} tributed to the presence of Increasing the thickness of the polymer film, thereby other constituents in the increasing the mass of the polymer available to act as a PVdC: Poly(vinylidenechloride) ........... juice, such as the pulp. sorbant, increased limonene sorption s.t~. Saturation of when comparing values reported from different experiments. A brief review of published results concerning aroma sorption by polymeric packaging materials l'otlows.
354
Trends in Food Science & Technology November 1994 IVol. 51
the polymer film at high limonene concentrations, resulting in a decreased partition coefficient, has been observed '~.n-'. By biaxially drawing the polymer film, thereby increasing the crystallinity oi" the polymer, the extent of limonene sorption can be reduced t-~. Several studies combining limonene sorption measurements with sensoric evaluation have shown loss of limonene to be of little importance for the taste sensation x'-'t. Other investigators have even indicated that limonene sorption can be beneficial for the product since limonene can act as a precursor for off-flavours, forming e~-terpineoi in orange juice-'2--'L However, adverse effects on the packaging material result from limonene sorption. Limonene may swell LDPE films, giving rise to higher oxygen permeability through the polymer'-L Furthermore, the modulus of elasticity and the tensile strength of the packaging film may diminish as a result of limonene sorption by LDPE ~.
Factors affecting flavour scalping Experiments performed with aroma compounds other than limonene have also shed some light on the factors that influence the extent of sorption. The nature of the pelymer and the permeant are, of course, important: in addition, external factors, such as storage conditions, affect the solubility of the aroma compound in the film. Konczai e t a l ? -~ reported that larger amounts of apple aromas were sorbed by LDPE than by EVOH with a high ethylene content, or by co-PET. In another study, orange flavour compounds were shown to have a higher affinity for PP than for LDPE-'*. Scalping of volatiles from toothpaste by a series of polyolefins and copolymers has been investigated-'7: polyethylenes with a high density sorbed larger amounts of volatiles than did those with a low density, Moreover, the volatiles had a higher affinity for the copolymers ethylene vinylacetate and ethylene acrylic acid than for the homopc!ymers HDPE, MDPE and linear LDPE, The extent of sorption was not related to the crystallinity of the polymer, but to the presence of oxygen-containing co-monomers and to ionomerization (the formation of ionic bonds, crosslinking adjacent polymer chains). In contrast, it has been found that decreased crystallinity of a PE film resulted in increased solubility of various aromas 2s. Other researchers came to the same conclusion when studying the sorption of citrus oils by polyolefins ~. Several investigators have pointed out the impact of the carbon chain length of volatiles on their solubility in polymer films, For homologous series of esters, aldehydes and benzoates, the distribution ratio of the volatiles in PE increased threefold with each additional methylene group in the compound '-8. However, for molecules composed of 11 or more carbon atoms, the increment in distribution ratio was less or, in some cases, negative, in another experiment, the sorption of esters, ketone.,, and aldehydes by PP increased as the number of carbon atoms in the compounds increased 2~. Compounds with eight or more carbon atoms were sorbed from yoghurt drinks by HDPE, while shorter molecules remained in the product ~°. In the same study it was ob~rved that Trendsin FoodScience& TechnologyNovember1994 IVol. 51
highly branched molecules were sorbed to a greater extent than linear molecules. The carbon chain length is closely related to 1he boiling point of a molecule. and several researchers have indicated ~ relationship between the solubility and the boiling points of sorbates. Strandburg e t a l . ~l obtained a linear relationship between the logarithm of the .solubility coefficient in vinylidene chloride copolymer and the boihng points of linear esters, alkanes and ketones, Similar results have been reported for the solubility of alkyl esters in polyvinyl alcohol .'-~. The type of functional group on the volatile also seems to affect the solubility of aroma substances in polymeric packaging materials, in a study of the sorption of citrus oil constituents by polyolefins, terpenes showed the highest affinity for the polymers, followed by sesquiterpenes: larger amounts of esters and aldehydes were sorbed than of alcohols ~. In the same experiment it was observed that saturated aldehydes were sorbed to a greater extent than those containing double bonds. Pieper e t a l . ~ investigated the sorption of orange juice aromas by various laminates, all with LDPE as the food-contact polymer. Hydrocarbon compounds were the most readily sorbed by the packages. Ketones were sorbed more than aldehydes, which in turn were sorbed more than alcohols. Ethyl butyrate, an ester very prominent in orange flavour, could not be detected at all in any of the packaging materials. Similarly, others have observed that esters have a higher affinity for LDPE than do aldehydes, and that only small amounts of alcohols are sorbed by LDPE -~8-a-*. Disulphide compounds representing the flavour components in ~aion/garlicflavoured sour cream were sorbed to a very i arge extent by polystyrene~. The solubility of ethyl propionate in poly ~inyl alcohol was reduced as the relative humidity increased "~z. This was attributed to competition between ethyl propionate and water molecules for available sorption sites. The solubility coefficients for aldehyde vapours in linear LDPE were maximal at 25°C, with sorption being lower at both 5°C and 75cC (Ref. 35). in the same experiment, however, the sorption of aldehyde vapours by poly,,inyl chloride (PVC) decreased as the temperature increased. Prediction m o ~ ] s Most of the results from these past studies can be explained by a combination of enthalpic (chemical structure) and entropic (molecular weight and polymer conformation) effects, Factors that affect flavour sorption include temperature, concentration, chemical structure, molecular weight, polymer conformation and competition between sorbates. Recently, effort~ have been made to develop methods for predicting the sorption of food constituents by various polymers. The predictive models aim to describe the quantitative contributions of different factors: comparison of calculated values from mathematical models with experimental sorption data is u ~ d to evaluate the validity of the model or the hypothesis. Simple models ba~d on the compari~n of the solubility parameters of the investigated polymer and analyte 3~5
species can be used to predicl the extent o f sorption qualitatively ~. In order to obtain a m o r e precise q u a n t i tative e s t i m a t i o n o f the solubility, more sophistica:ed methods, which take several aspects into c o n s i d e r a t i o n , have b e e n d e v e l o p e d ~7-~'>.At present, the m o d e l s lack an accurate description o f i n t e r m o l e c u l a r forces, p o l y m e r c o n f o r m a t i o n and c o m p e t i t i o n b e t w e e n the low m o l e c u lar weight species. Therefore, the m o d e l s need further r e f i n e m e n t in order to be able to provide a perfect match b e t w e e n e x p e r i m e n t a l l y o b t a i n e d values and calculated data for a n y g i v e n s o r b a t e - s o r b a n t system.
t3 14 15
16 17 18 19 20
21 22
Conclusions It b e c o m e s apparent w h e n s t u d y i n g the published literature, which often i n c l u d e s c o n t r a d i c t o r y findings, that all the m e c h a n i s m s b e h i n d aroma sorption by polymers are not yet fully understood. Efforts by different research groups have resulted in s i m i l a r c o n c l u s i o n s : h o w e v e r , there are still s o m e areas that spark s o m e controversy. More research is required in order to m i n i mize, a n d h o p e f u l l y eliminate, the p r o b l e m o l flavour s c a l p i n e wilhi, n thc h~ocl p a c k a g i n g industry.
23 24
25 26
27
References 1 2 3
4 5 6 7
8 9
lO
Koch,I-, Robinson,t.. and Figge,K. I1t176~FeUe, 5eiten, Anstrichm. 78, 371-377 Marshall,M.R. Adams, I.P, ,andWilliams, I.W. 119851in Aseptipak 85, pp. 299-312, Scholland13u_qnessResearchInc., Princeton,NI, USA Olafsson,G., lagerslad.M., Oste, R. anti Wessl6n,g. q1qq.l~i, .~.ood 5ci. 58, 215-219 Olalsson,G., lagerstad,M., Oste, R, Wessl6n, B. and Hjeqherg. T (19931 Food Chem.47. 227-233 Nielsen,T.l 11994~I, foodScL 5eL227-230 Durr,P. and Schohinger.U. l] 9811in Fla~ourfll (Schreier,I., t.d.!. p. 179, Waller de Gruyler and Co., Berlin, Germany Mannheim,C.H., Millz, I, and Pa,sy,N. 1198/11in food and Pack,~gin~ !oteractions (Holchkiss, I.H., ed.l, Pl)- 68-82, American Chemk ,~I Society,, Pieper,G, Borgudd.L., Ackermanil,P. and Fellers,P. II~)92) I- i~od 5eL 57, 1401"I-1411 Hnose.K., Harte, B.R., Giacin, I.R,, Miltz, J, and Stine, C. I198~ m Food and Packaging InteractionsIHolchkiss,I.H., e:l.L pp. 28~-fi AmericanChemical Sociely Mannheim,C.H, Mihz. J. and Lelzler,A. dqB7'~]. Focrrt5ci. 52.
28 29
31) 31
32
33 ]4 35
36
737-740
ll 12
Halek,G.W. and Meyers,M.A.1198q~Pac~,ag. TechnoL 5ci. 2. 141-14~ Kwapong,O.Y. and Holchki.~s,].H. IIq871 I. Food Sci. 52, 761 71;1, 785
17 :18 :]9
Charara,Z.N., Williams, l.W,, Schmidl, R.H. and Marshall,M.R. I19921 I, Food Sci. 57, 963-qfi6, 972 Sizer,C.E., Waugh, P.L., Edstam,S. and Ackermann,P. 119F18)Food TechnoL 43, 152-I 59 Ikegami,I-., Nagashima,K., ,Shimoda,M., Tanaka,Y. and Osajima, Y. {199111. FoodS-ci. 56, 500-503, 509 hnai, T.,Harle. B.R. andGiacin, I.R. 11%Otl. foodScL55.158-16! Paik.I.S. (1992) Ir Agric. Food Chem. 40, 1822-1825 Detassus,P.T. 119861Tappil. 69, 16H-16L Boner,A.L., Kalyankat V, and Shoun,L.H. (19911I. FoodSci. .56, I051-I054 Theodorou,E, and Paik, 13-Hq921Packag. Technal. 5ci. 5, 2t-2.5 Mannheim,C.H. and Passy,N. 119871FlOssigesObst. 54, 585-588 DOFF,P., Schobinger,U. and Watdvogel,R. l19811Alimenta 20, 91-93 KuBy.V., Braddock.R.I. and Sadler,G.D. 1199411.FoodScL 59, 402-405 5adter,G.D. and Braddock, R.I. 1199011.FoodSci. 55, 587-588 Konczal.J,8., Harte, B.R., Hooiiat.P. and Giacin, I.R. 119926L Food 5ci. 37, 967-q72 Halek,G.W. anti Lultmann,1.P.11991~inFoodand Packaging h~teractions II (Risch, S.I. and Holchki~s,I.H, edsL pp. 212-226, AmericanChemicalSociety Brant.P., Michiels,D., Gregory, B., Laird, K. and Day, R. 119911in Food and Packaging Interactions II IRisch,5,1. and Hotchkiss, I.H., edsl, pp. 227-25(1,AmericanChemic~lSociely Shimoda,M., Ikegami,T. and Osajima. Y. 119881J.5ci. foodAgric. 42,157-163 Arora,D.K., Hansen,A.P. and Armagost,M.S. (19911in food and PackagingInteractionsII (Risch, S.I. and Holchkiss, J,H., eds'J, pp. 203-21 I, American ChemicalSociely Lin~SenrJ.PH.. Verheul,A, Roozen,I.e. and Poslhumus,M.A. 11991 Int. Dairy I. I, 33-40 Slrandburfi,G., DeLassus,P.T.and Howell, B.& {19q01in Barrier Pof)'mers and Structures IKoros,W.l., edJ, pp. 333-350, American Chemical Sociely Landois-Garza,l~ and Holchkiss, I.H, 119881in Foodand Packaging Interactions IHotchkiss,I.H., ed.L pp. 42-58, American Chemical Society Nielsen,T.I., I,igerslad,I.M.,Osle, R.[. and WesslOn,B.O. 119921 I. food Sci. 57, 491)~tt)2 Tuebe,I.M., Hoojjat, H., Hernandez. R.I., Giacin, I.R,and Harle, B.R. t lgqO] Packag. TechnoL Sci. 3, 133-140 Leuiven,A. and Slotlman, U. Ilqq2] Z tebeosm. Unters. Forsch. 1'94, 355-359 Matsui,T, Nagashima,K., Fukamachi,M., Shimoda, M. and Osajima~Y. I19921I. Agric. Food Chem. 40, Iq02-1905 Paik,I,S.and Wriler, M.S.I. Agric. Food Chem. fin pressl Li, S. and Polk,J,S, I. Fo~d 5ci. Im pressl Baner,At. and Piringer,O.G. i19941 I. Chem. Eng.Data 3q, ~41-.~48
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Trends in Food Science & Technology November 1994 IVol. 51