A mechanistic approach to studies of the possible digestion of retrograded starch by alpha-amylase revealed using a log of slope (LOS) plot

The rate and extent of digestibility of starch were analysed using the logarithm of the slope (LOS) method. Digestibility curves with a-amylase were obtained for starches in their native, gelatinised and 24 h retrograded form. A LOS plot of the digestibility curves was then constructed, which allowed the rate constant (k) and the concentration of the product at the end of the reaction (C infinity) to be calculated. It also allowed the identification of rapid and slow phases in starch digestion. Upon gelatinisation, both k and C infinity increased with dramatic changes notably in C infinity; however after starch samples had been stored for 24 h at room temperature, k was not affected but C infinity decreased. This suggests that retrograded starch is virtually inert to amylase action. Both k and Coo were strongly related to the increase in degree of order of the alpha-glucan chains, monitored by FTIR-ATR spectroscopy, in retrograded starch.