THESIS

The recrystallization process depends on the glass temperature
(Tg) of amorphous gel because the mobility of the chains determines their amorphous
gel. As a plasticizer, water controls the Tg of the amorphous gel. At very low water
content, the Tg is above room temperature and the amorphous gel is in a highly viscos
glassy state that effectively binders molecular mobility. Recrystallization increases
with increasing water content (depress of Tg below room temperature) up to 45 - 50%,
because of progressively more effective plasticization (increase molecular mobility);
with further increase of water content up to 90%, it decreases, apparently due to
excess dilution (Slade and Levine, 1987).
One can visualize the gel as starch chains with layers of water
molecules attached by hydrogen bonding. As the starch paste is cooled, the starch
chains become less energetic and the hydrogen bones become stronger, giving a
firmer gel. As a gel ages or if it is frozen and thawed, the starch chains have a
tendency to interact strongly with each other and thereby force water out of the
system. The squeezing of water out of the gel is called ‘syneresis’. Longer storage
gives rise to more interaction between the starch chain and eventually to formation of
crystals. This process, called ‘retrogradation’ is the crystallization of starch chains in
the gel. Because the crystalline areas differ from the non crystalline areas in their
refractive index, the gel becomes more rigid or rubbery, perhaps partially as a result
of crystallization and partially just from the interaction of the starch chains (Hoseney,
1990).
3) Storage temperature
Retrogradation is greatly affected by storage temperature.
Compared to storage at room temperature, storage of starch gels containing 45-50 %
water at low temperature but still above the glass temperature (Tg ≈ 5.0 o C) increases
the retrogradation, especially during the first day of storage, compared to starch gels
stored at room temperature (Gudmundsson, 1994). Storage temperature below Tg
virtually inhibits recrystallization. Higher temperatures (above 32 – 40 o C) effectively
reduce retrogradation. Colwell et al. (1969) studied the effect of storage temperature
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