Chemistry Reference
In-Depth Information
other hand, the phenomenon is strongly dependent on the water content of
food products, it offers a rational basis for understanding the role played by
water in food processing operations such as drying, freezing, extrusion,
baking or during storage. The basic knowledge may be found in many
topics and reviews (Ferry, 1980; Sperling, 1986; Perez, 1994). Recently, the
relevance of the glass transition to food technology has been emphasized
strongly (Levine and Slade, 1988; Simatos and Karel, 1988; Slade and Levine,
1985, 1993, 1994; Noel et al., 1990; Roos and Karel, 1991a; Karel et al., 1993;
Champion et al., 2000b; LeMeste et al., 2002).
Glass transition denotes all phenomena observed when a glass is trans-
formed in a supercooled material when heated, or the reverse when cooled
(Figure 11.7). A glassy material is a rigid solid, but its microstructure is non-
crystallized (amorphous), with only short-distance order, like a liquid. The
supercooled state is the state of the material between the glass transition
Figure 11.7.
DSC thermogram of amorphous lactose, to show the successive changes in
physical state in the course of rewarming. The amorphous lactose was prepared by freeze-
drying and dried over P
2
O
5
for at least 7 days. Scanning rate for DSC was 58C/min. (Redrawn
from Roos and Karel, 1991b.).
