Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 9

The T g often refers to the onset or midpoint temperature of the glass transition
temperature range, as determined using differential scanning calorimetry (DSC). For
synthetic polymers it is the most important factor controlling mechanical proper-
ties. 33 Amorphous or partially amorphous structures in foods are formed in various
processes that allow a sufficiently short time for removal of water or cooling of
concentrated solids to produce the supercooled liquid or glassy state. These processes
include baking, concentration, drum-drying, freeze-drying, spray-drying, and extru-
sion. 5-7 The amorphous food materials exhibit time-dependent changes as they
approach an equilibrium state. These transformations are characterized by changes
in mechanical properties, i.e., various collapse phenomena observed from a change
in structure or viscous flow resulting in stickiness, caking, and loss of poros-
ity, 6,7,10,34-36,38 and changes in diffusion, i.e., crystallization of amorphous sugars,
flavor retention and release, and, possibly, reaction kinetics. 5-7,10,15,16,37-39

E FFECTSOF W ATERONTHE P HYSICAL S TATE

The T g values of food materials vary from that of pure water at about -135°C 40-43
to those of polysaccharides, such as starch. Important T g values of food components
are those of sugars, 6,44 oligosaccharides, 45,46 and proteins. 47,48 Unfortunately, T g val-
ues for a number of biopolymers, such as anhydrous polysaccharides and proteins,
cannot be measured, as they undergo thermal decomposition at temperatures below
T g . These materials and other nonfat food solids generally become plasticized by
water. 6,9,45,46,48-50 Water plasticization can be observed from the depression of the
glass transition temperature with increasing water content, which also improves
detectability of the transition. Therefore, measured T g values for starch at various
water contents have been reported, but not for, e.g., anhydrous starch or gluten. 49,50
Prediction of the T g depression as a result of water plasticization is useful in
evaluation of effects of food composition on T g , as glass transition-related changes
often affect shelf life and quality. The Gordon-Taylor equation 51 has proved to be
particularly useful in fitting experimental data on T g and composition of amorphous
carbohydrates, proteins, and foods. 5,46,48,52 The Gordon-Taylor equation [Eq. (1.7)]
uses component T g values, T g1 and T g2 , and weight fractions, w 1 and w 2 , for solids
and water, respectively, and a constant k to obtain the T g of the mixture. The T g2 =
-135°C is often used for amorphous water. 5,6,46,53

wT kw T
wkw

+
+

11 g 2 2 g

T = (1.7)

g

1 2

W ATER A CTIVITYAND P HYSICAL S TATE

The effect of water and a w on the physical state of food solids is often observed
from structural changes that occur above a critical a w or water content, 5,32,54 as was
described in Figure 1.1 . Roos 55 established a linear relationship between a w and T g .
The linearity often applies over the a w range of 0.1 to 0.8, but the true relationship
over the whole a w range seems to be sigmoid. 56 The relationship between T g and a w