Susceptibility of starch to in vitro enzyme hydrolysis in rice, rice flakes and intermediary products

Abstract

The susceptibility of starch to in vitro hydrolysis by different enzymes was studied in raw rice, flaked rice and intermediary products obtained at various stages during processing of rice to flakes. Starch hydrolysis was lowest, namely 100 to 370 g/kg, in raw rice but increased to 410 to 870 g/kg after processing and up to 100% after cooking, depending on the enzymes used and the assay conditions selected. Starch degradation was maximal after treatment with pancreatin followed by amyloglucosidase and least by α-amylase. However, addition of pepsin increased the susceptibility of starch hydrolysis indicating that a fraction of starch might be complexed with protein. This appeared to be the case with flakes obtained by single pressing in a heavy duty cereal roller flaker rather than those produced by repeated gradual pressing in an edge-runner. Conditions favourable for starch retrogradation and also deep-fat frying of flakes reduced the susceptibility of starch to enzyme hydrolysis. Heat treatment of moist paddy (rice) and mechanical damage during flaking increased the availability of starch in rice to enzymic hydrolysis. A part of starch got complexed with protein, thereby partly decreasing its susceptibility to hydrolysis.