Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 28

T g reaction rates at a constant water content follow the Arrhenius-type temperature
dependence.
Sorption isotherms of low-moisture foods provide important information that
can be used in predicting shelf life. The relationship between a w and water content
can be directly used in evaluating effects of water content on stability. It is obvious
that the relationships between mechanical properties, temperature, and water content
of amorphous foods provide improved criteria for shelf life predictions. The sorption
isotherm combined with water plasticization data allows the establishment of critical
values for water content that can be related to stability. Water contents that are higher
than the critical values decrease the T g of the material to below ambient temperature,
which results in stickiness, caking, and probable crystallization of amorphous com-
ponents. The critical values may be directly used in establishing criteria for maxi-
mum water vapor permeability values for packaging materials that can maintain
stability and reduce water plasticization during storage.
S TABILITY M APS
The effect of a w on the relative rates of deteriorative changes has often been described
using stability maps which show the relative rate of enzymatic changes, non-enzy-
matic browning, lipid oxidation, microbial growth, and overall stability as a function
of water activity. 4 The rate of the various reactions may also be related to the physical
state, molecular mobility, water plasticization, and glass transition of amorphous
food solids, as shown in Figure 1.13 . Structural transformations, as well as diffusion-
controlled deteriorative reactions and those affected by crystallization phenomena
occur at increasing rates with increasing a w above the critical a w . It is likely that
water contents lower than the critical water content are needed for maximum stability.
Stability maps are applicable for evaluating storage stability of low- and intermedi-
ate-moisture foods. Water activities of most fresh foods are high and their stability
is based on other hurdles such as low pH and low storage temperature. However,
water is the predominant component of most foods and the physicochemical prop-
erties of water are the main factors that control their behavior during processing and
storage by contributing to the amount of heat required for heating or refrigeration
and by being the main solvent and plasticizer of the nonfat components.
The assumption that stability is related to glass transition allows establishment
of the stability map that describes the effect of water on changes that depend on the
relaxation times of mechanical changes in foods . Figure 1.13 relates deteriorative
changes, which are governed by T g , to water activity. In addition state diagrams and
sorption isotherms are useful as stability maps. They may also be used to obtain
material-specific data for the glass transition temperature at various water contents
and relationships between water content and water activity.
FUTURE RESEARCH
Characterization of the physical state of food materials and application of the poly-
mer science theories to the description of food properties and various kinetic phe-
nomena have significantly contributed to the present understanding of food stability.