Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 15

a result of plasticization of the physical structure by temperature or water. 54,65 A
critical a w at which crispness is lost has been found to be specific for each product,
but a change often occurs over the a w range of 0.35 and 0.50. 54,66 Loss of crispness
is obviously a result of extensive water plasticization above the critical water content
or a w that is sufficient to depress the T g of the material to below ambient temperature,
as described in Figures 1.4 and 1.9 .

C RYSTALLIZATION P HENOMENA

Crystallization of amorphous food components, e.g., sugar crystallization and starch
retrogradation are probably the most dramatic time-dependent changes that affect
structural properties and quality of low-moisture and cereal foods. 5,6,9,10,15,16,38,67
Makower and Dye 68 found that amorphous glucose and sucrose were stable at 25°C
when relative humidities were lower than 5 and 12% corresponding to a w of 0.05
and 0.12, respectively. At higher storage humidities, water sorption resulted in
crystallization and release of sorbed water. Water plasticization and depression of
T g to below ambient temperature are responsible for crystallization of amorphous
sugars in foods as a result of increased free volume and molecular mobility, decreased
viscosity, and enhanced diffusion as shown in Figure 1.10 . 5,15,16 Crystallization seems
to initiate at T g or corresponding a w and proceed with a rate determined by the
temperature difference T - T g to a maximum extent also defined by the T - T g .
Crystallization in gelatinized starch, which is typical of starch-containing foods and
at least partially responsible for bread staling, is also governed by T g . 69,70
The kinetics of crystallization of sugars and starch components at a constant
temperature above T g can be related to water content and a w , which define T - T g . 16,70
Foods that contain mixtures of sugars have a more complicated crystallization

Sorption isotherms: Crystalline sugar

Amorphous sugar

Extent of Crystallization

Time to crystallization:

Seconds

Hours

Weeks

Years

RELATIVE HUMIDITY

FIGURE 1.10 The difference in water sorption between amorphous and crystalline sugars.
Water sorption above a critical relative humidity depresses the glass transition of the material
to below ambient temperature and causes time-dependent crystallization and loss of adsorbed
water. The extent of crystallization has a maximum at an intermediate relative humidity.