Scalping—an intriguing word with bizarre connotations—is used in the plastics, semi-rigid, and flexible package converting industries to describe loss of desirable content constituents into plastic package materials in proximity to the contents. Use of the word is limited in industry because of a general, but hardly universal, appreciation of the negative contribution made by plastics in removing volatile flavor components—when it is not supposed to occur.
Not a core topic of research among plastics and plastic package developers, flavor scalping has perhaps percolated to or near the top of academic research on package materials under various nomenclatures, sorption, mass transport, migration, or a sub-classification under permeability. Technically, sorption describes the take-up of molecules contained in the product contents by the package material. Regardless of the semantics, flavor scalping has emerged during the past 15 years as a major, usually unrecognized challenge to food packaging technologists.
In the good old days, glass and metal package materials dominated and were almost inert to interaction with packaged food. During the post–World War II era that witnessed the debut and golden era of plastic packaging, the limitations on plastic materials largely confined food contents in plastic to dry and very-short-shelf-life liquids in refrigerated distribution. The advent of aseptic packaging with its polyethylene interior and long ambient-temperature shelf life demonstrated that lipophilic plastic materials such as low-density polyethylene could and did interact with liquid contents, adding plastic processing additive constituents and/or removing desirable compounds from the food itself. Although this was recognized, because most aseptic packaging was of fluid milk until the 1980s little was done except to measure and quietly report on the phenomenon.
With the application of aseptic packaging for fruit beverages and their more aromatic constituents, and the development of serious interest in retort pouches and plastic cans and, more recently, hot-filled pouches, the issue has become of no small concern.
Obviously, the removal of desirable constituents such as aroma compounds, acids, lipids, and pigments from a food by plastic package materials has detrimental effects, such as a decrease in flavor intensity or alteration of the flavor profile. Furthermore, sorption of aromatics can damage the package material by causing delamination or polymer swelling, thus leading to further sorption.
The longer the time of contact between food contents and interior plastic package materials and the higher the temperature, the greater is the sorption from the food into the adjacent plastic. Because the permeant from the food into the plastic can be a vapor, it is generally a major constituent of the desirable flavor quality. Thus, one result is measurable loss of desirable flavor from the product, a quality loss perceived by consumers. This phenomenon explains a significant portion of the loss of quality of so-called shelf-stable foods in plastic package structures.
Although much of the reported research on flavor sorption has been on sorption of limonene in orange juice into polyethylene (the interior coating on paperboard cartons), many other food compounds are also involved: hexanal, ethyl acetate and its analogs in apple juice aroma, tomato volatiles, aldehydes, methyl ketones, sulfur compounds, methyl esters, alkyl pyrazines, neral, geraniol, octanol, decanol, and others. These compounds are represented in a wide variety of food products, including fruit beverages, dairy products, and vegetable and meat products. The removal or reduction of one or more of these compounds from food results in flavor loss or change—a condition frequently reported during sensory evaluation of long-term, ambient-temperature-distributed foods.
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The plastic materials implicated in sorption are generally but not exclusively polyolefins such as polyethylene and polypropylene. Polyethylene is of particular concern because it is so widely employed as the interior material in plastic package structures. Polyethylene and its analogs are probably the most used heat sealants and water and water-vapor barriers for flexible packages because they are readily machined, have forgiving sealing characteristics, and are inexpensive. Used as heat sealants, polyethylenes are almost always on the package interior in direct contact with the food contents.
One interesting research finding is that oxygen permeability is not necessarily related to flavor barrier. Some good oxygen barriers, e.g., polyamides, are not good flavor barriers, probably because of their hydrophilic properties.
The mechanisms for flavor sorption into plastics have been described as dependent on permeant transmission rate F = q/At, where q is quantity of compound, A is area of contact, and t is time of contact. F is also sometimes referred to as the gas transmission rate. The higher the value, the greater the scalping, since the compound is being removed from the interior surface, exposing more plastic to new compound. When the vapor pressure difference between the volatile on the interior and that in the plastic is incorporated, permeance R becomes the result. By then adding the thickness of the package material, the permeability constant P is the measure of steady-state transfer through the plastic of the permeant or sorbed compound. P is based on diffusion and solubility coefficients. Diffusion is the rate at which the compound passes through the plastic, and the rate is quite different for simple gases such as oxygen than for more complex aroma molecules. Obviously, the greater the diffusion, the more will be sorbed from the external food in contact with the plastic.
Variables affecting permeation and diffusion, besides chemical composition of the permeant (the compound being sorbed into the plastic) include the following:
• Chemical Composition of the Plastic Polymer. The most important variables affecting the permeability and diffusion process include cohesive energy density that results in intermolecular, van der Waals, or hydrogen bonds and regular periodic arrangement of such bonding groups; and the polymer glass transition temperature (Tg) above which free vibrational and rotational motion of polymer chains occur. Glassy polymers such as polyvinylidene chloride have low diffusion coefficients for aroma molecules at low concentrations and therefore display high flavor barrier. Polyolefins are non-glassy polymers and have high diffusion coefficients for aromas and therefore tend to sorb rapidly.
• Polymer Morphology. This refers to the physical state by which crystalline and amorphous regions exist in the same polymer package structure. Morphological properties that influence permeability and diffusivity include structural regularity or chain symmetry leading to three-dimensional order or crystallinity; and chain alignment or orientation to allow laterally bonding groups to approach each other to the optimum interaction distance and thus tend to form crystalline materials. Many plastic package structures such as polypropylene film and polyester bottles are oriented for use. The more ordered the polymer molecular structure, the lower the rate of sorption of the food aroma constituent.
• Penetrant Concentration. The higher the concentration of the sorbed material, the higher the rate of transport into the polymer structure. Not incidentally, the presence of co-permeants affects the rate of transport of the molecules through the polymer, usually by increasing the rate.
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• Relative Humidity. Increases in moisture content of moisture-sensitive plastics such as polyamides or ethylene vinyl alcohol copolymer increases the rate of permeation of the aromatic compounds. The water appears to function as a plasticizing agent in opening the polymer structure to molecular transport.
• Temperature. Permeability, diffusion, and solubility coefficients follow van’t Hoff–Arrenhius relationships, resulting in increases in permeant values with increasing temperature. Because the effect is accentuated at ambient temperatures compared to refrigerated temperatures, the adverse results have appeared more frequently in foods contained in retort pouches, aseptic packages, and hot-filled flexible packages. Thus, although flavor scalping may be retarded by reduced distribution temperature, in commercial practice this alternative is usually unacceptable.
Flavor scalping has been identified as a cause of quality deterioration in packaged food products in ambient long-term distribution. Perhaps, by inference, some of the low-quality consumer perceptions of food products retorted or hot filled in plastic cans, trays, or pouches can be attributed to flavor scalping. Because of the direct temperature relationship, the contact of food at elevated temperature (during thermal processing and hot filling) with interior plastic accelerates flavor scalping.
Polyethylene is perhaps the major flavor scalping family of polymers today and should be avoided if the problem is controlling. In descending order, polyvinylidene chloride, polyester, and even, to a degree, oriented polypropylene tend to display less flavor scalping. The best flavor barriers are silica coating, which is rarely used or available, and metallization, which is not employed as a contact surface with foods.
Food and food packaging technologists face a major challenge in developing plastic packaging that can resist the ravages of flavor scalping. Despite the array of data published on the topic, no single answer that can be applied commercially has yet emerged, meaning that more valid information and imagination are required.
PRODUCTS & LITERATURE
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by AARON L. BRODY
President and CEO, Packaging/Brody, Inc.