Packaging Considerations - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 133

Moisture
One of the most common shelf life stability concerns in relation to packaging and mass
transfer arises from the exchange of water vapor between the food and the surrounding
atmosphere. Moisture transfer can result in a number of undesirable changes depending
on the specific product and whether moisture is taken up or lost. These changes could
include: physical processes such as caking of powders and loss of crispness resulting
from moisture sorption, or hardening resulting from desiccation; microbial spoilage
resulting from increased availability of water due to moisture sorption; and chemical
processes which can be enzymic (e.g., proteolytic hydrolysis of fish products) or non-
enzymic (e.g., Maillard browning reactions in dry products). 16,17
Foods packaged in non-hermetically sealed containers are subject to moisture
gain or loss depending on a number of factors: 18 the storage relative humidity; the
sorption properties of the product; the water activity gradients relative to the storage
atmosphere; and the water vapor permeability of packaging materials. The barrier
performance of the packaging system employed can influence the relative importance
of each of these factors.
The relative humidity (RH) of a storage environment is generally of little concern
for food products packed in water vapor impermeable packaging such as glass and
metal. However, it should be noted that processing and environmental conditions
may promote the external corrosion of metal packaging materials, especially tin-
plate. The presence of oxygen in retorts, corrosive vapors and chemicals in the
storage atmosphere, and contact with soluble chlorides, sulfates, and nitrates or other
salts should be minimized to avoid failure of package integrity through corrosion. 1
Storage RH can have a significantly greater impact on the mass transfer of moisture
for paper and plastic-based packaging, primarily due to the permeability of these
materials. Paper-based packaging materials are hygroscopic and considered rela-
tively permeable to water vapor, but coating with paraffin wax or polyethylene, and
laminating between surface layers of various plastics can provide additional barrier
properties and suitability for specific applications in moist environments. Plastic-
based packaging materials provide a broad range of resistance to the permeation of
water vapor, which is dependent on the chemistry of the polymer(s). The polarity
of a polymer influences its resistance to gases and water vapor permeation. Highly
polar polymers such as those containing hydroxyl groups (polyvinyl alcohol, cellu-
lose) are the poorest water barriers, whereas non-polar hydrocarbon polymers such
as polyethylene (PE) and polypropylene (PP) are excellent water barriers. The
hydrocarbon content which is responsible for low water vapor sensitivity, however,
is also responsible for relatively high gas permeabilities of many plastics. Selection
of a polymeric material that can provide suitably balanced protection against mois-
ture and/or oxygen transmission will depend on the requirements of each food
product. Hydrophilic polymers exposed to moisture can also be affected significantly
in terms of reduced oxygen barrier properties as discussed in the next section.
The concept of water activity in terms of deteriorative reactions and shelf life
stability was addressed in Chapter 1 of this topic. Water activity (a w ), defined as the
ratio of the vapor pressure of water in equilibrium with the food in comparison with
the vapor pressure of pure water at the same temperature, is essentially a measurement