Characteristics of native and enzymatically hydrolyzed ragi ( Eleusine coracana) and rice ( Oryza sativa) starches

Abstract

The compositional, structural and enzymatic digestibility profiles of starches isolated from ragi and rice were determined. Although, the gelatinization temperature (73±2 °C) and GPC elution profiles of both starches on Sepharose CL-2B gel were comparable, the peak (PV: 273 RVU) and set back (SB: 196 RVU) viscosities and also the thermic energy (TE: 34.78 cal/g) values for ragi starch were considerably higher than for rice starch (PV: 200 RVU; SB: 150 RVU and TE: 26.54 cal/g). Upon digestion with α-amylase (AA) ( human salivary), β-amylase (BA) ( barley malt), pullulanase (PN) ( Klebsiella pneumoniae) and amyloglucosidase (AG) ( Aspergillus niger), the hydrolysates from AA contained glucose, maltose and oligosaccharides of 15–20 DP prominently, whereas, those from BA, PN and AG contained only maltose, maltotriose and glucose, respectively. The molecular weight ( M W) and degree of crystallinity of the starch residues from the enzymatic digests of ragi starch were significantly higher than of rice starch. The SEM examination of the residues indicated high degree of fragmentation in the case of rice but only a few bigger chunks in case of ragi. These parameters may help to explain subtle differences in the digestibility and physiological properties that exist between ragi and rice starches.