Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 13

T- T g (˚C) Viscosity (Pa s)

60 4.2x10 2
50 2.6x10 3
40 2.4x10 4
30 3.9x10 5
20 1.3x10 7
10 1.5x10 9
0 1x10 12

Water Plasticization

WEIGHT FRACTION OF SOLIDS

FIGURE 1.8 A state diagram showing the decrease in viscosity occurring in amorphous
food materials as they are plasticized by temperature or water above the glass transition
temperature or corresponding water content.

perature range. It should be noticed that these changes have been known to occur
above a critical water content or a w during food storage. According to Levine and
Slade, 10 collapse phenomena may include or have an effect on stickiness and caking
of food powders, plating of particles on amorphous granulas, crystallization of
component compounds, structural collapse of dehydrated structures, release and
oxidation of encapsulated lipids and flavors, enzymatic activity, nonenzymatic
browning, graining of boiled sweets, sugar bloom in chocolate, ice recrystallization,
and solute crystallization during frozen storage.
Peleg 60 used stiffness as a general term referring to the response of food materials
to an external stress. A single model [Eq. (1.9)] based on Fermi's distribution model,
where X is a w , T, or water content, m, could be used for modeling their effects on
stiffness. The stiffness parameter, Y, as a function of a w , T, or m can also be related
to its value at a reference state, Y s , and a constant, a X , which is a measure of the
broadness of the transition. The reference value, X s , obtained from b = -X s /a X ,
indicates the value for a w , T, or m, that decreases Y to 50% below Y s . Peleg 20,57,60
emphasized that Eq. (1.9) predicts a change in Y, which may be any property that
is related to stiffness, including instrumental and sensory measures of mechanical
properties, e.g., crispness, within the T g range. The stiffness parameters when plotted
with critical a w , m, or T values provide valuable information on the extent of changes
occurring at and above the glass transition.

Y
Y

1

=+

ln

s

− 1 b X (1.9)

a

X

Peleg 57 has also shown that the “stiffness equation” can be combined with the
Gordon-Taylor equation to establish three-dimensional relationships between water
content, temperature, and a change in the stiffness parameter.