Review

The sorption of food constituents, especially aroma com-

pounds, by polymeric packaging materials has received con-

siderable attention during the past decade, and is generally considered to be a problem within the food packaging indus-

try. 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 pack- aging materials, such as glass and metal, are both per- fect 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 inter- actions 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 com- ponents, 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 sorp- tion 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 com- pounds 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

Tim Nielsen and Margarelha |ige~-tad are at the Department of A0~alied Nutrition & Food Chemistry, Chemical Center, Lund Uni~,ersity, PO Box t24, 5-221 00 Lund, Sweden.

Flavour scalping by 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 cau~ 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 mathemat- ically 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 the- ories but also to determine the influence of various fac- tors 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 exper- imental system makes evaluation of the results more complex.

There are different ways of expressing the extent of sorption of flavour compounds by polymers. The solu- bility 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 de- fined as the concentration of the substance in the poly- mer divided by the concentration of the substance in the aqueous phase, and is a unitless parameter. Another par- ameter used to describe the extent of sorption is the dis- tribution 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 in the presentation of the results, together with the use of various techniques of preparing the samples, different storage conditions, etc., means that care must be taken

Trends in Food Science & Technology November 1994 IVol. 51 ,~,,~. El ...... ~,t-,,,,L~,~-'~ .aa~,~SO~ ,~ 353

Polymer Environment film Foodstuff Migrating

substances

_~ !_ "- PERMEATION Oxygen (2) Water vapour

Carbon dioxide Other gases

- - - - ~ MIGRATION Monomers Additives

SORPTION

Adverse consequences

(1) Oxidation Microbial growth Mould growth Off-flavour

(2) Dehydration Decarbonation

Off+f layout Safety problems

Aroma compounds Loss of aroma Fats intensity Organic acids Development of Pigments 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 dur- ing the rest of the experiment suggested absorption into the plastic. From a similar study, a partition co- efficient (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 pro- c e s s 12

The solubility of limonene in dif- ferent 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 con- tact with high-density polyethylene (HDPE) and poly- [,ropylene (PP). In another report, limonene in model solutions showed higher affinity for LDPE than for polyamide, which in turn sorbed limonene to a greater extent than did polystyrene ~+. The distribution ratio of limonene increased with increasing ethylene content in ethylene vinyl alcohol (EVOH) copolymer 's. Orange juice lost 40% of its total iimonene content t0 LDPE, but only 20% to EVOH *~. In the same study no de- tectable amounts of limonene were sorbed by poly(ethyl- ene terephlhalate) (PET). Other studies have, however, shown that small amount.,, of limonene were sorbed by PET from an orange-flavoured soft drink s. Limonene vapour was shown to have a higher affinity for LDPE than for PET I; DeLassus I~ obtained a higher solubility coefficient for limonene vapour in poly(vinylidene chloride} (PVdC) than in EVOH; this was in turn higher than the solubility in PE or oriented polypropylene.

Limonene sorption from model solutions and orange juice increased with temperature".'-', while the sorption of limonene vapour decreased with an increase in tem- peratureU'"L The extent of limonene sorption tended to be less when limonene was present in it mixture than when it acted as the sole permeantlLak this applies for limonene in either the vapour or the aqueous phase, and could be a result of competition for sorption sites. Increasing the thickness of the polymer film, thereby increasing the mass of the polymer available to act as a sorbant, increased limonene sorption s.t~. Saturation of

when comparing values reported from different exper- iments. A brief review of published results concerning aroma sorption by polymeric packaging materials l'otlows.

Limonene sorption The most extensively studied aroma compound,

with respect to its sorption by polymers, is limonene. Limonene is an unsaturated terpene hydrocarbon present in citrus flavour. It makes up -95% of the oil fraction in orange peel. It is a highly nonpolar substance, and has shown a high affinity for many polymeric packaging materials. Studies have assessed limonene sorplion into various polymers from both the vapour phase and the aqueous phase.

Di]rr et al. ~ reported a 50% loss of limonene from or- ange juice to low-density polyethylene (LDPE" see Box I

lbr a lull list of polymer

Box 1. Polymer name abbreviations used

EVOH: Ethylene vinyl alcohol

HDPE: High-density polyethylene

LDPE: Low-density polyethylene

MDPE: Medium-density polyethylene

PE: Polyethylene

PET: Poly(ethylene terephthalate)

PP: Polypropylene

PVC: Poly(vinyl chloride}

PVdC: Poly(vinylidene chloride) . . . . . . . . . . .

name abbreviations used) during two weeks of stor- age. Other investigators, in similar experimems, have reported decreases in lim- onene content in orange juice in contact with LDPE ranging from 25% to 60% (Ref~ 2. 7-9). The sorption of limonene by LDPE from model solutions was much greater than that from or- ange juice "+ . This was at- tributed to the presence of other constituents in the juice, such as the pulp.

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 sen- sation 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 copoly- mers 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, crosslink- ing 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, alde- hydes and benzoates, the distribution ratio of the volatiles in PE increased threefold with each additional methyl- ene 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

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 be- tween 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 sorp- tion of citrus oil constituents by polyolefins, terpenes showed the highest affinity for the polymers, followed by sesquiterpenes: larger amounts of esters and alde- hydes were sorbed than of alcohols ~. In the same exper- iment 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 promi- nent 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 alco- hols are sorbed by LDPE -~8-a-*. Disulphide compounds representing the flavour components in ~aion/garlic- flavoured 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 mo~]s Most of the results from these past studies can be ex-

plained by a combination of enthalpic (chemical struc- ture) and entropic (molecular weight and polymer con- formation) effects, Factors that affect flavour sorption include temperature, concentration, chemical structure, molecular weight, polymer conformation and compe- tition 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 differ- ent factors: comparison of calculated values from math- ematical 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

Trends in Food Science & Technology November 1994 IVol. 51 3~5

species can be used to predicl the extent o f sorpt ion qual i ta t ive ly ~. In order to obta in a more precise quant i - tat ive es t imat ion o f the solubi l i ty , more sophist ica:ed methods , which take several aspects into cons idera t ion , have been deve loped ~7-~'>. At present, the models lack an accurate descr ipt ion of in te rmolecu la r forces, po lymer confo rma t ion and compet i t ion be tween the low molecu- lar weight species. Therefore, the models need further re f inement in order to be able to provide a perfect match be tween exper imenta l ly ob ta ined values and calcula ted data for any g iven sorba te - sorban t system.

Conclusions It becomes apparent when s tudying the publ ished

literature, which of ten inc ludes cont rad ic tory f indings, that all the mechan i sms behind aroma sorpt ion by poly- mers are not yet ful ly unders tood. Efforts by different research groups have resulted in s imi lar conc lus ions : however , there are still some areas that spark some controversy . More research is required in order to min i - mize, and hopeful ly e l iminate , the p rob lem o l f lavour sca lp ine wilhi, n thc h~ocl packaging industry.

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~41 -. ~48

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356 Trends in Food Science & Technology November 1994 IVol. 51