Effect of partial gelatinization and retrogradation on the enzymatic digestion of waxy rice starch

The effect of partial gelatinization and retrogradation on in vitro enzymatic digestibility of waxy rice starch was investigated, and the relationship between the residual crystallinity and digestibility measured. An aqueous dispersion of starch (5%, dry weight basis) was partially gelatinized by heating at different temperatures (60, 65, or 70 °C for 5 min). The relative melting enthalpy values of the starch samples, based on the melting enthalpy of native starch, were 69.0, 36.7, and 8.5%, respectively. Retrograded starch samples were also prepared by storing a fully gelatinized starch paste (5% starch) at 4 °C for 2, 4, or 7 d, and the relative melting enthalpy values for the starch samples were 36.7, 67.2, and 79.9%, respectively. The partial gelatinization and retrogradation changed the enzymatic digestion behavior of the waxy rice starch samples, and the changes were significant in the initial stage of digestion. The digestion rate was reduced as the melting enthalpy increased. The amounts of slowly digestible starch (SDS) and resistant starch (RS) correlated positively with the relative melting enthalpy of the partially gelatinized or retrograded starch samples. The glycemic index (GI) estimated using an in vitro digestion test correlated negatively with the relative melting enthalpy. At similar melting enthalpy levels, the partially gelatinized starch samples were more resistant to enzymatic digestion than the retrograded starch samples, indicating that the thermal history and the crystalline morphology affected the enzymatic digestion behavior of starch.