Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 8

30

5˚C

15˚C
35˚C

25

20

Temperature Increases

Constant Water Content

15

10

5

0

0 0.2 0.4 0.6 0.8 1.0

WATER ACTIVITY

FIGURE 1.3 Temperature dependence of water sorption. The isotherms shown are those
of amylopectin. 69

Glassy State Glassy State Super Super cooled Liquid State Supercooled Liquid State cooled Liquid State Melt Melt

Molecular Molecular

Rotations

Molecular
Rotations

Transtational Mobility Translational Mobility anstational Mobility

Rotations

Glass Glass Transition Glass Transition ansition

Dissoiution Dissolution

TEMPERATURE, WATER ACTIVITY OR WATER CONTENT

FIGURE 1.4 A schematic representation of changes in modulus and the physical state
occurring in amorphous food materials as a function of temperature, water activity, or water
content.

translational mobility of molecules is possible. 5,6 The significant change in molecular
mobility is observed as the material is transformed from the supercooled solid into
the liquid-like state over the T g temperature range ( Figure 1.4 ). Relationships
between the physical state, mobility, and food texture have been reported, but very
little and controversial data exist on molecular mobility, reaction rates, and microbial
growth in concentrated and frozen food systems.