RS and starch degradation and free glucose release
The dramatic decrease in starch in RS4 and R7954 during germination showed a similar trend to that reported earlier and the even digestion of amylopectin and amylose in R7954 was consistent with results obtained by in vitro starch digestion. In contrast, the digestion model of starch in RS4 was not even and the relative rapid hydrolysis rate of starch in RS4 in this study reversed the result of the in vitro hydrolysis using cooked samples and other high amylose material such as barley. Due to the similar level of amylase activity between the two rice lines , the more rapid decrease in total starch and nonsynchronous degradation of amylose and amylopectin during the germination of
RS4 might be explained by differences in starch structure.
The major RS type in RS4 is the RSIII fraction, which is retrograded
starch consisting mainly of amylose . The
amylose double helices packed into RSIII lead to a crystal formation
that hinders the accessibility of a-amylase to the glycosidic bonds.
The slower relative digestion rate of amylose in RS4 in vitro might
be due to its high content of the RSIII type. However, amylopectin
accounts for approximately 80e85% of total starch and also adds to RS, in particular by short-chain amylopectin.
Amylopectin in mutant RS4 has an increased fraction of
short chains with 8 DP 12 and reduced fractions of DP > 12 , indicating that a higher branch density could lead to
incomplete digestion and hence to a higher ratio of amylopectin to
total starch residues . This could account for RS left
after germination. It might be possible to increase the resistant
ability of common cooked starch not only by debranching amylopectin
and dual-retrogradation treatments , but also by increasing the branch density of
amylopectin.