Surface structural features control in vitro digestion kinetics of bean starches

Although many studies have investigated the enzyme susceptibility of bean starches, the factors that reduce the lower hydrolysis of bean starch are not well understood. In the present study, we isolated starches from eight commercial bean varieties and established a causal relationship between micro and macro-structure with alpha-amylase susceptibility. The in vitro starch digestion kinetic profiles were monitored at a fixed enzyme concentration, and the data were fitted into the Logarithm of Slope model to obtain first-order rate coefficients and extent of digestion. Among eight varieties, pinto bean starch showed the lowest digestion rate (0.0013 min(-1)) and extent (40.7%), whereas red kidney bean starch has the highest rate (0.0042min(-1)) and extent (69.2%). In order to probe the interdependability of digestion kinetic parameters with starch microstructure, the enzyme binding capacity, molecular size, crystalline structure as well as thermal parameters of eight bean starches were correlated as Pearson's matrix. Whilst, significant correlation was not observed between digestion rate/extent and the structural parameters, the residual enzyme activity after binding with starch granules at non-hydrolyzing condition had significant negative correlation with the digestion rate and extent. Similarly, positive correlation was observed for degree of starch damage and enzyme susceptibility. Thus, it is concluded that for bean starches with similar biological origins, the available sites for initial binding of enzymes predominates the succeeding catalysis rate.