In vitro starch digestibility and estimated glycemic index of chemically modified corn starches

Normal corn starch was chemically modified by oxidation, acetylation, hydroxypropylation, and cross-linking, and the digestibility and glycemic indices (GI) of these modified starches were examined by in vitro hydrolysis using pancreatic α-amylase. During the early stage of hydrolysis (up to 60 min) with prime (ungelatinized) starches, the modified starches were hydrolyzed at greater extents than the unmodified starches. However, at the late stage of hydrolysis (after 3 h), the modified starches showed lower degrees of hydrolysis than the unmodified starch. The amount of resistant starch (RS) in prime starches were 23.4%, 35.1%, 34.2%, and 13.9% in the acetylated, oxidized, hydroxypropylated, and cross-linked starches, respectively, whereas the unmodified starch contained 11.9%. The amount of slowly digestible starch (SDS) was decreased as the RS content was increased by the modification, indicating that some of the SDS transformed to RS. By gelatinization, all the starches, regardless of type of modification, were more quickly hydrolyzed and reached maximum levels within 20 min. Like the prime starches, the modified starches contained higher contents of undigested starches after gelatinization. The modifications except cross-linking reduced rapidly digestible starch (RDS) content but increased the RS content when the starches were gelatinized. The hydroxypropylated starch had the lowest GI values, which were estimated from the hydrolysis profiles (70.6 and 86.2 in prime and gelatinized states, respectively), whereas the GI values of cross-linked starch were similar to those of the unmodified starch. Therefore, chemical substitution such as hydroxypropylation and acetylation, and oxidation can be used to reduce the starch digestibility and raise the RS content.