Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 3

INTRODUCTION
Water is the most important diluent of water soluble food components and plasticizer
(softener) of various water miscible polymeric compounds as well as often the main
food component. Chemical reactions, enzymatic changes, and microbial growth may
occur readily in foods with high water contents when their occurrence is not restricted
by environmental factors such as pH or temperature. Water has several effects on
food stability, palatability, and overall quality. The physicochemical state of water
is related to water activity, a w , which is a measure of water availability for the growth
of various microorganisms 1-3 and physicochemical stability 4,5 of high-moisture foods.
Water as a plasticizer has an additional effect on the shelf life of low- and interme-
diate-moisture foods. 6
Rates of deteriorative changes and microbial growth at normal food storage
conditions often depend on water content and a w . Food deterioration due to microbial
growth is not likely to occur at a w < 0.6. 2 However, chemical reactions and enzymatic
changes may occur at considerably lower water activities. Typical deteriorative
changes of low-moisture foods include enzyme-catalyzed changes, nonenzymatic
browning, and oxidation. Enzymatic changes and nonenzymatic browning have been
found to occur above a critical water content and to show a rate maximum at an
intermediate-moisture level which is followed by a decrease at higher water activities. 4
Oxidation may have a high rate at low water contents, the rate may go through a
minimum with an increase in a w , and then it may decrease at higher water activities. 4
Low-moisture nonfat food solids are often a mixture of disordered molecules;
i.e., they exist in an amorphous, metastable, non-equilibrium state. The amorphous
solids may exist in a highly viscous, solid, glassy state or in a supercooled, viscous
liquid state. The change between these states occurs over a transition temperature
range that is referred to as glass transition . The glass transition can be observed
from changes in heat capacity, dielectric properties, various mechanical properties,
volume, and molecular mobility. 6-8 Introduction of the polymer science principles
to the characterization of food materials, 6,9-14 and especially water plasticization, has
improved understanding of the physicochemical principles that affect relaxation
times and rates of mechanical changes in low- and intermediate-moisture and frozen
foods. 5,6,9,14-16 Unfortunately, combined effects of glass transition, water content, and
temperature on the kinetics of various chemical reactions and deterioration are not
well established. 3,6,7,17-19 However, attempts to establish methods for the prediction
of the physical state and rates of deteriorative changes of amorphous foods, based
on the T g concept, have been made. 6,9,14,19 A major assumption related to shelf life
and quality is that stability is maintained in the glassy state and various changes
may occur above T g with rates determined by the temperature difference, T - T g . 5,6,9
Unfortunately, it cannot be over-emphasized that the transition occurs over a tem-
perature range and a single transition temperature is not always well defined. 20
This chapter defines water activity and plasticization and describes their effects
on physicochemical properties of food materials and effects of water on the physical
state, and physical, chemical, biochemical, and microbial changes. The shelf life
stability of high-moisture foods is discussed, but the emphasis is on various effects
of a w , glass transition, and water plasticization on temperature-, water content-, and