Chemistry Reference
In-Depth Information
0.7 a
w
0.50
0.55
0.60
0.65
0.75
0.80
0.85
0.90
0.70
WATER ACTIVITY
Figure 2.6.
Relative nucleation (
) and crystallization (
*
) rates for lactose at various water
activities at room temperature. The glass transition of lactose is defined by water activity, and
crystallization occurs above the critical water activity. The rate of nucleation at a low water
activity is high but crystal growth occurs slowly which results in a low overall rate of crystal-
lization. The maximum rate and extent of crystallinity (---) is achieved around 0.7 a
w
(Jouppila
et al., 1997).
storage environment (e.g. Saltmarch and Labuza, 1980; Vuataz, 1988; Joup-
pila et al., 1997). Increasing relative humidity increases water sorption and
water activity, which causes water plasticization and increases the tempera-
ture difference, T-T
g
. The T-T
g
of lactose defines the rate of crystallization,
as shown in Figure 2.6.
Jouppila and Roos (1994b) determined glass transition temperatures
for freeze-dried milk powders, which contained various amounts of fat. The
T
g
of non-fat solids at various water contents was almost the same as that of
lactose (Figure 2.2). The water sorption properties of the non-fat solids were
not affected by the fat component. Jouppila and Roos (1994b) developed
state diagrams for milk powders, which defined critical values for water
content and water activity for stability. Combined T
g
and water sorption
data suggested that a water content of 7.6 g/100 g of non-fat solids depressed
T
g
to 248C. The corresponding water content for pure lactose was 6.8 g/100 g
of solids. The critical a
w
was 0.37. These values, being similar to those shown
in Figure 2.2, are in good agreement with several studies which have found
critical water contents and storage relative humidities for milk powders based
on water sorption properties (e.g. Warburton and Pixton, 1978).
