Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 11

Glassy State Glass Glass Transition Glass Transition ransition Flow

years

months
days
hours
minutes
seconds

Stability Zone C Cr itical Zone Critical Zone itical Zone Mobility Zone

TEMPERATURE, WATER ACTIVITY OR WATER CONTENT

FIGURE 1.6 A schematic representation of relaxation times resulting in time-dependent
changes in mechanical properties and diffusion, described by the extent of change in property,
in amorphous food materials as a function of temperature, water activity, or water content.

T g of solids

Supersaturated
liquid state

T m

Solution

T m

Partial freeze-concentration

Maximum ice formation

T g

Glass

Ice and glass

T g of water

Glass

C g

WEIGHT FRACTION OF SOLIDS

FIGURE 1.7 A typical state diagram of food materials showing the composition depen-
dence of the glass transition temperature, T g , equilibrium ice melting temperature, T m , the
effect of maximum freeze-concentration on the observed glass transition, T ′ g , corresponding

solute concentration C ′ g , and the onset of ice melting in the maximally freeze-concentrated

material, T ′ m .

PHYSICAL STABILITY

R ELAXATION T IMES AND M ECHANICAL P ROPERTIES

Mechanical properties of amorphous food materials are affected by glass transition
which may change textural characteristics. Rates of changes in mechanical properties
can be analyzed in terms of relaxation times. Williams et al. 58 reported that a single