Biomedical Engineering Reference
was not entirely blocked with the use of white and yellow pigmented polyethylene.
Fanelli et al. 29 found that visible and ultraviolet (UV) light screens incorporated in
high density polyethylene dairy resin to protect vitamins from photodegradation
provided only limited protection. Butter lipids, including cholesterol, are susceptible
to oxidation where butter surfaces are illuminated by fluorescent lights or sunlight.
Photooxidation can result in off-flavors, as well as cholesterol oxidation products
having weak carcinogenic activity. 30 Emmons et al. 31 studied the photooxidation of
butter as influenced by various wrapping materials and found that only laminates of
aluminum foil and paper provided the necessary light protection required for shelf
life stability. Aluminum foil was also shown by Luby et al. 32 to prevent cholesterol
oxidation in butter exposed to fluorescent light, while margarine wrap, parchment,
wax paper, and polyethylene films were not effective light barrier materials.
The color of red meat products, whether fresh, frozen, or cured, is one of the
most important indicators that consumers use to assess quality. In frozen meats,
however, discoloration by light is a significant shelf life problem because freezing
does not protect against pigment photooxidation during storage. Andersen and
Skibsted 33 achieved partial protection against discoloration of frozen pork patties by
using polyethylene packaging which incorporated a UV-light absorber; the packag-
ing material was also very effective in preventing light-dependent lipid oxidation in
the frozen pork product. Cured meats are also very susceptible to light-induced
discoloration. The pigment nitrosomyoglobin, which is responsible for the color of
cured meats, dissociates rapidly upon exposure to light in the presence of oxygen. 34
Vacuum packaging using high barrier polymer films is useful to reduce discoloration;
however, additional improvements in color stability have been achieved by Andersen
et al. 35 with packaging systems that reduce the residual oxygen in the product below
a critical limit required for photooxidation.
The permeability of gases such as oxygen, nitrogen, and carbon dioxide through
polymeric materials increases as temperature increases but the extent of these
changes varies for different polymers. 1 Knowledge of the quality kinetics associated
with specific food products has permitted development of mathematical models to
predict shelf life from data collected at elevated storage temperatures. However,
when studying foods in plastic containers or with plastic closures, it is necessary to
develop new models which account for changes in permeability due to temperature
for specific packages. For plastics subjected to a wide range of temperature, perme-
ability is influenced by the glass transition temperature of the polymer, and these
effects are known to vary with temperature differently for different polymers. Also,
interpretation of data from multi-layer structures becomes very complex.
Another aspect of temperature in relation to shelf life is the application of thermal
treatments which reduce microbial loads and retard the growth of microorganisms.
When using plastic packages, processing temperatures must be kept below the glass
transition temperature of the polymer in order to maintain dimensional stability of
the package. Newly developed polymers and blow molding technologies have
increased the maximum application temperature associated with these containers.