During
HMT, increase in gel hardness was attributed to the
increased cross-linking between starch chains in the
particular amylose portion. These allowed the formation
of more junction zone in the continuous phase of the gel,
resulting in the increased gel hardness (Liu et al., 2000).
The high moisture content and the elevated temperature of
treatment could allow polymer chain motion and consequently
the greater effects (Hoover & Vasanthan, 1994b).
The most pronounced effect was expected for starch at high
moisture (25 g/100 g), but the observed range of gel
hardness was low, 48.21–56.88 g. Trends observed for the
high moisture starches were gel hardness increased for
some extent and gradually decreased with increasing
temperature and time of treatment. This was probably
because of partial gelatinization in particular at high
temperature and long time of HMT, which indicated by an
apparently low gelatinization enthalpy measured by DSC
(data not showed). These caused some of starch structure
to collapse and resulted in a less rigid starch gel.
The highest gel hardness of 60.38 g was obtained from
starch annealed at 55 1C for 24 h. Whereas, HMT of starch
with 20 g/100 g moisture at 110 1C for 1.5 h gave the highest
gel hardness of 61.23 g. These were considered to be the
optimal conditions for annealing and HMT on improving
starch gel hardness.
3.2.