In vitro starch digestibility and nutritionally important starch fractions in cereals and their mixtures

Dietary carbohydrates are characterized with respect to both chemical composition and likely site, rate and extent of digestion. Starch is divided into rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS). The present investigation aimed at evaluating the effect of starch source on nutritionally important starch fractions and the extent of in vitro starch digestion in four processed cereals, namely rice, ragi (Eleusine coracona), wheat, jowar (Sorghum vulgare) and their mixtures in varying proportions, using controlled enzymatic hydrolysis with pancreatin and amyloglucosidase. Rapidly available glucose (RAG) was also measured to derive a starch digestion index (SDI). The total starch and amylose content in the raw cereals ranged from 21-26.6 and 12.6-20.7 g/100 g, respectively. Among the four basic cereals, roti (flat, unleavened pancake, diameter 12 cm) produced from ragi had highest total starch, RDS and SDI. However, in mixtures containing two or more cereals RDS and SDI were lower than those observed in single cereals. Significant correlations were observed between RAG and SDI, or RDS, respectively (r = 0.91, r = 0.96, P < 0.01). An inverse relation between amylose content and SDI (r = -0.78, p < 0.01) indicates the influence of amylose on the extent of starch digestion. The results highlight the importance of starch source in determining the starch fractions of processed cereals and their mixtures. The simple in vitro measurement of starch fractions could serve as a tool for characterizing dietary carbohydrates with respect to their digestion in the gut.