Effect of thermal treatments on in vitro starch digestibility of sorghum dried noodles

Abstract

In this paper, sorghum grains were pretreated by roasting (RT), microwave (MW), stir-frying (SF) and heat-moisture treatment (HMT). The effects of pretreated sorghum grains on in vitro starch digestibility of sorghum dried noodles made from sorghum and wheat flour were investigated. The results showed that HMT treated noodles contained the highest amount of resistant starch (RS) and the lowest amount of rapidly digestible starch (RDS). The hydrolysis kinetic parameters and estimated glycemic index (eGI) decreased in all of the treated samples. The treated starches had lower molecular weight and less proportion of short chains of amylopectin than those of the untreated sample. X-ray diffraction demonstrated that the relative crystallinity of starch in noodles was increased by HMT and RT treatments while it was decreased by MW and SF treatments compared to untreated noodles. Fourier transform infrared (FTIR) analysis revealed that the short-range ordered degree and intra-molecular hydrogen bond intensity were both enhanced by thermal treatments. A tighter and smoother microstructure with fewer pores and cracks in the treated noodles was observed by scanning electron microscopy (SEM). These structural changes could provide a better understanding of the lower starch digestion rate.