Abstract
Dual-modification, hydroxypropylation and cross-linking were conducted to overcome
the undesirable properties of native rice starch and to improve its functional properties
during processing. The first stage of modification was hydroxypropylation using 6% to 12%
of propylene oxide. This was followed by cross-linking using the mixture of 12% sodium
trimetaphosphate (STMP) and 0.1% sodium tripolyphosphate (STPP). The level of hydroxypropylation enhanced the subsequent cross-linking and this was indicated by a marked
increase in phosphorus content and degree of substitution (DS). This was accompanied by
a significant decrease in paste clarity, swelling power, peak viscosity, breakdown and
consistency coefficient. The effects of different cross-linking agents: 2% STMP, the mixture
of 2% STMP and 5% STPP and the mixture of 12% STMP and 0.1% STPP, on functional
properties of rice starch hydroxypropylated with 8% propylene oxide were also
investigated. It was found that dual-modified rice starch cross-linked by STMP alone gave
the highest phosphorus content (0.192%) and degree of substitution (0.01) resulting in the
lowest peak viscosity, breakdown, final viscosity, consistency coefficient, swelling power,
solubility index and paste clarity. In addition, the dual-modified rice starch cross-linked
with mixed phosphate salts containing a high content of STMP (12% STMP and 0.1% STPP)
gave significantly higher phosphorus content (0.161%) and degree of substitution (0.009)
compared to that of the mixed phosphate salts containing low STMP content (2% STMP
and 5% STPP). It suggests that the higher amount of STMP enhanced the high phosphorus
substitution on the starch chains resulting in an increasing rigidity of the rice starch
granules.
