Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 14

12

50

Viscosity
Calorimetric T g at 25˚C

Water
Sorption

10

40

8

30

6

20

Critical
Water
Content

4

10

2

Critical a w

0

1.0 0 0 0.2 0.4 0.6 0.8

WATER ACTIVITY

FIGURE 1.9 A hypothetical representation of changes in viscosity and water content in
amorphous food materials as a function of water activity, a w . The critical a w depresses the
glass transition temperature, T g , to ambient temperature (25°C).

S TICKINESS , C AKING , AND C OLLAPSE

Stickiness and caking of food powders and collapse of structure have been shown
to be related to glass transition with rates governed by the temperature difference,
T- T g . 6,9,10,15,16,61 Downton et al. 62 proposed that particles of amorphous powders
stick together if sufficient liquid can flow to build strong enough bridges between
the particles. Sufficientflow occurred when the particle surface viscosity was
decreased to a critical value of about 10 7 Pa s. The main cause of stickiness is
plasticization of particle surfaces by water which allows interparticle binding and
formation of clusters. 63 Stickiness may be considered to be a time-dependent property
of amorphous food solids. The viscosity in the glassy state is extremely high and
contact time between particles must be very long to result in bridging. The dramatic
decrease in viscosity over the glass transition temperature range, as shown in
Figure 1.9 , obviously reduces the contact time and causes interparticle fusion result-
ing in stickiness and caking. 5,62
Collapse of structure refers to viscous flow occurring as the material cannot
support its own weight. Structural collapse is a phenomenon that causes loss of
quality of dehydrated, especially freeze-dried, foods. 34,38 Tsourouflis et al. 34 found
that the decrease in viscosity and flow were dependent on water content and tem-
perature. Studies of Tsourouflis et al. 34 and To and Flink 35,36 suggested that there is
a critical viscosity above which time to collapse may substantially increase. It was
later shown that collapse was observed above T g when the relaxation time for collapse
became sufficiently short and of practical importance. 64

C RISPNESS

Crispness is essential to the quality of various low-moisture cereal and snack foods.
Crispness of low-moisture foods is affected by water content and it may be lost as