3. Result and discussion3.1. Effect

3. Result and discussion
3.1. Effect of the dosage of CA on degree of substitution (DS) and the
RS content
Fig. 1 shows the clear relationships between the percent of CA
content and DS and RS content of the treated sweet potato starch.
With the CA content increased from 10% (of starch dry weight) to
60%, the corresponding DS values increased from 0.036 to 0.141,
and the interrelated RS content ranged from 23.47% to 54.32%. It is
worth mentioning that when the concentration of CA was lower
than 50%, the DS and the RS contents increased remarkably with
the increase in CA concentration. However, they leveled off when
CA concentration was higher than 50%. The reason for the leveling
off probably was the significant steric inhibition which hindered
the accessibility of CA to starch, and prevented the reaction between
CA and starch.
3.2. Effect of HMT on the DS and the RS content of CA-starch
The RS content and DS of native starch and treated starch are
shown in Table 1. Among the HMT starches, the HMT-20 got the
highest RS content among the samples. This result was consistent
with the report of Li et al. (2011). They claimed that the moisture
content was the key factor in RS formation during HMT. The reason
might be that water creates hydrogen bonds between molecular
chains within the starch granule (Kurakake, Noguchi, Fujioka, &
Komaki, 1997). After CA treatment, the RS content increased
dramatically in comparison with the controls. However, there was
no significant rise when the starch was treated with the 20%
moisture. The moisture content of the native SPS was 12.53%. The
DS increased from 0.135 to 0.207 with the water contents of starch
increasing.
Fig. 2 shows that the linear relation between DS and RS content.
Conway and Hood (1976) reported that chemical substitution of
starch reduced its enzyme digestibility, probably because the bulky
and derivatized groups sterically hinder the formation of the
enzymeesubstrate complex. Bj€orck et al. (1989) also proved that
the digestive enzymes may not be able to degrade them completely
since the substituents hindered the attack of enzymes.