Packaging Considerations - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 135

TABLE 4.2
Representative Water Vapor Transmission Rates of Various
Food Packaging Polymers at 38°C and 90% Relative Humidity

Water Vapor Transmission Rate

Polymer (g.mm.m -2 .day -1 )

Polyvinylidene chloride (PVDC)
Polypropylene (PP), biaxially oriented
High density polyethylene (HDPE)
Polyvinyl chloride (PVC), rigid
Low density polyethylene (LDPE)
Polyethylene terephthalate (PET)
Ethylene vinyl alcohol copolymer (EVOH)
27 mol% ethylene
44 mol% ethylene
Nylon 6, unoriented

0.036
0.10
0.12
0.36
0.39
0.7

2.24
0.56
5.9

Adapted from Brown. 14

testing methodologies that may be different from the application conditions, perme-
ability data from packaging handbooks should be used only to compare packaging
alternatives. Estimates of changes in the moisture content of a packaged food during
storage can be calculated for any packages with known WVTR properties and
geometry, and for various lengths of exposure time by knowing the initial and
tolerable moisture range of the product (minimum and maximum) and the ambient
conditions of temperature and humidity. 1,14

Oxygen and Other Gases

Oxygen is a critical mass transfer component in a number of deteriorative reactions
that can have an effect on the shelf life of many packaged foods. Of major concern
are oxidative reactions associated with food components, including lipid oxidation
and the subsequent development of rancid flavors, as well as color and nutritive
changes, non-enzymic browning, and vitamin C degradation. The presence of oxygen
during processing and storage also permits spoilage due to growth of aerobic micro-
organisms and promotes various enzyme catalyzed reactions. On the other hand, the
depletion of oxygen due to respiration can lead to other deteriorative effects, such
as anaerobic microbial spoilage in specific products like sealed plastic packages of
fresh fruits and vegetables. The shelf life of oxygen-sensitive packaged food products
depends on exposure to oxygen during processing; the level of oxygen incorporated
into the sealed package; and the amount of oxygen ingress into the package during
storage, either through permeable packaging materials or through seals and closures.
A number of packaging innovations have been developed to control oxygen
incorporation and ingress into a package based on oxygen-absorbing components
or scavengers. These are successfully used mainly in Japan and Europe. 21,22 The
absorbents consist of easily oxidizable substances which can be inorganic (e.g., iron