Packaging Considerations - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 136

powder) or organic (e.g., ascorbic acid) and are typically contained in an oxygen-
permeable sachet which is placed inside the food package. Different absorbent
complexes have been designed to work most effectively at particular water activity
ranges, while others can absorb both oxygen and carbon dioxide for specific use in
roasted and ground coffee packages. Other packaging considerations involving oxy-
gen ingress relate to increased oxygen permeability of some polymers exposed to
moisture. Polymers such as PVDC can provide an excellent barrier to oxygen
permeation under both wet and dry conditions. However, hydrophilic polymers such
as ethylene vinyl alcohol copolymers (EVOH) and cellophane containing hydroxyl
groups, and nylon polymers containing amide groups exhibit large increases in
permeability as the polymers absorb water. EVOH has superior gas barrier properties
under dry conditions; therefore, coextrusion of water-sensitive EVOH as an internal
layer in structures consisting of water vapor barrier polymers such as polypropylene
(PP), polyethylene (PE), or polycarbonate (PC) can help protect its oxygen barrier
properties. 23 Multi-layer plastic packages containing EVOH are being used increas-
ingly in a variety of applications, including retortable containers for thermally
processed foods which may be reheated prior to consumption using a microwave
oven.
Other gases such as carbon dioxide are of concern in specific packaging appli-
cations, for example, in modified atmosphere packages where maintaining a desir-
able gas composition within the package requires high barrier packaging materials.
Permeability of packaging materials to carbon dioxide is also critical in preventing
the loss of carbonation in soft drinks packaged in polyethylene terephthalate (PET),
particularly in small volume containers which have a large surface-to-volume ratio.
Innovative visual leak indicators based on the detection of oxygen and carbon dioxide
have been reviewed recently. 24 Indicators have been developed for the detection of
oxygen, for example, to confirm the effectiveness of vacuum packaging or an oxygen
scavenging function, or to indicate an anaerobic state within a package. Carbon
dioxide indicators have been developed to detect an increase in concentration as a
sign of microbial growth.
The analysis of the transfer of oxygen and other gases through packaging
materials can be carried out in a manner similar to water vapor transfer using known
values for permeability of the packaging material to the particular gas, the package
dimensions, and the partial pressure of the gas inside and outside the package: 14

PAt
L

Q = ∆ p

where Q is the total amount of permeant, P is the permeability, A is the area of
permeation, t is the elapsed time, L is the thickness, and p is the pressure difference

of the permeant across the diffusion path. This simple treatment of permeation is,
however, based on a number of assumptions including that diffusion is in a steady
state condition, and that both diffusion and solubility of the permeant are independent
of concentration. Examples and calculations for other permeation situations, including
multilayer materials, are well described elsewhere. 1,14 Table 4.3 provides representative
oxygen and carbon dioxide permeabilities of some typical food packaging polymers.

∆