Packaging Considerations - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 132

of the package and the external environment during storage and distribution condi-
tions; for example, the mass transfer of moisture from a humid environment into a
dried product. 1 Minimizing or preventing these mass transfers, and thereby achieving
a desirable shelf life, depends on package integrity (including seals and closures)
and requires the use of packaging materials that provide appropriate barrier proper-
ties and physical protection.
Mass Transfer
The mass transfer of water vapor, oxygen and other gases, volatile aroma compounds,
and other molecules occurs through permeation, sorption, and migration in packag-
ing systems. Permeation refers to a transfer of molecules through the package, either
from the product to the environment, or from the environment to the product.
Sorption refers to the penetration and dispersal of molecules from the product into,
but not through, the packaging material. The sorption of flavor compounds inherent
to a food product by the packaging material is commonly referred to as “scalping”.
In contrast, migration specifically refers to the transfer of low molecular weight
packaging material components into the food product as a result of contact and/or
interaction between the food and the package.
Gases and vapors permeate through polymeric materials by two mechanisms:
capillary flow and activated diffusion. 13 Capillary flow involves the permeation of
gases and vapors through pinholes, cracks, and microscopic pores in the packaging
materials. Activated diffusion is a solubility-diffusion process whereby gases and
vapors dissolve in the polymer at one surface, diffuse through the polymer, and then
evaporate at the other surface of the polymer.
Activated diffusion is the major permeation mechanism for multi-layered or high
barrier polymeric materials. Mass transfer basically follows a three-step process:
(1) adsorption, (2) diffusion, and (3) desorption. Adsorption and desorption are
determined by the solubility of the permeant gas or vapor molecules in the polymer.
Diffusion is the transport of mass as a result of random molecular motion in the
presence of concentration or partial pressure gradients.
The basic terms used to characterize permeation are: P, the permeability coeffi-
cient, which is a measure of the steady-state transfer rate of the permeant; D, the
diffusion coefficient, which is a measure of how rapidly permeant molecules pass
through the barrier material in the direction of lower concentration or partial pressure;
and S, the solubility coefficient, which refers to the amount of permeant molecules
dissolved in the material at equilibrium conditions. The simplest relationship among
these terms is given by:
P = D · S
In general, the flavor scalping by a polymer is related to S, while the loss of
permeant from a total package is determined by P. Measurement of these coefficients
has been well described by other authors. 1,14 Demorest and Mayer 15 discussed the
advantages of newer testing techniques that have been developed to rapidly and
reliably measure P, D, and S for various polymer/permeant pairs used in typical food
packaging applications.