Important Factors for Selecting Food Packaging Materials Based on Permeability

One of the critical factors in selection of food packaging materials is permeability and/or transmission rate to help maintain food quality. Conditions of product/material use such as temperature and relative humidity are critical for determining optimal testing conditions to determine permeability rate. Basics of permeability, interpretation of data and relationship to selection of materials for specific applications will be discussed. In addition, the relationship of permeability to aroma and flavor retention will also be covered. Introduction The type of food, chemical composition, size, storage conditions, expected shelf life, moisture content, aroma/flavor and appearance are just a few of the characteristics that must be taken into consideration when selecting the right material for a food product. A continuing trend in food packaging is the design of packages to extend the shelf life of foods while maintaining fresh-like quality. This places a high demand on selecting materials that not only provide the needed properties to maintain the quality of the food but it must be done at a cost effective price. The permeability of the packaging material is one of the most critical features of the package for affecting the quality of the food product. Materials can be selected to provide a very long shelf life, but one must ask whether there is a need for the best barrier. Furthermore, does the extension of the shelf life justify the cost of the material and the quality of the food? Therefore, knowing the important factors for material selection based on permeability is an essential part of the package design process. Discussion Terminology A discussion of permeability involves many terms that are commonly misunderstood or confused. Therefore, a definition of terms is necessary to better understand the factors that affect permeability (Robertson, 1993). Permeability is defined as transfer of molecules from the product to the external environment, through package or from the external environment through the package, to the product. See figure 1. Sorption – movement of molecules contained by the product into but not through the package. See figure 2. Migration – movement of molecules originally contained by the package, into the product. See figure 3. These three components make up the majority of interactions that take place. Although permeability doesn’t directly measure each of these interactions, it does have an influence on each. For example, a material such as low density polyethylene is a good water barrier but will scalp certain flavor and aroma compounds from foods. Plasticizers may be added to polymers such as polyvinyl chloride to affect their flexibility and permeability but migration of the additives is also affected. Flexible Packaging Conference 2004 2 The equation used to express permeability is expressed as follows: P = (D)(S) P = Permeability coefficient D = Diffusion coefficient which is a measure of how rapidly penetrant molecules are moving through the barrier, in the direction of lower concentration or partial pressure. D is a kinetic term that describes how fast molecules move in a polymer matrix. S = Solubility coefficient which is the amount of transferring molecules retained or dissolved in the film at equilibrium conditions. S is a thermodynamic term that relates to how many molecules dissolve in a polymer matrix. When an equal concentration of molecules are moving through a polymer at a constant rate, a steady state is reached and is also called Fickian Behavior. When measuring water and oxygen permeability, this is common behavior for most packaging polymers such as low density polyethylene and polyester. However, some do not have steady state behavior. Polymers such as nylon and ethyl vinyl alcohol diffuse at different rates depending upon penetrant concentration and time. Polymers with this behavior are referred to as non-steady state or non-Fickian polymers. The chemical nature of the polymers and the chemical nature of the permeants are what determines whether the polymer will behave in a Fickian or non-Fickian manner. It is also important to understand the difference between the terms permeability rate and transmission rate. According to ASTM, (1994) transmission rate (TR) is defined as the movement of a permeant in unit time through a unit area under specified conditions of temperature and relative humidity (Table 1). The thickness of the material is not incorporated into the definition but is implied to be the thickness of the test film sample. Permeability (P) is defined as the movement of a permeant through a unit area of unit thickness induced by a unit vapor pressure difference between two specific surfaces under specific conditions of temperature and humidity at each surface (Table 1). In other words, permeability is the arithmetic product of permeance and thickness. The confusion of the proper or improper use of these terms was the topic of a paper which compared a variety of equations for predicting permeability in multilayer films cited in different packaging textbooks (Cooksey et al., 1999). In general, it was found that most of the equations were the same basic equation but stated in slightly different ways. Other equations either accurately or inaccurately dealt with permeability (P) and transmission rates (TR) to achieve the final answer. The confusion over P and TR is not new. In fact the confusion is so serious that there have been law suits that essentially center on this basic issue. The key fact to keep in mind is that permeability data accounts for thickness of the material and transmission rate assumes the thickness of the test sample. Both are useful depending on how you use the data. The following equations were recommended for the applications described (Cooksey et al, 1999). • Calculating transmission rate for a multilayer laminate (TRT) from transmission rate data is useful if you’re using data instrumentally measured or comparing actual films for performance. Flexible Packaging Conference 2004 3       +       +       =