Physicochemical studies using amylose as an in vitro model for resistant starch

Abstract

The effects of hydrolysing amylose gels and films with porcine pancreatic alpha-amylase were studied in order to construct an in vitro model of resistant starch. Both gels and films proved to be partially resistant to hydrolysis, their susceptibility decreasing with increasing substrate concentration. Storage of gels for 1 day resulted in levels of crystallinity from 6 to 12%. Hydrolysis of these gels produced significant increases (from 272 to 557%) in crystallinity in all cases. The amount of resistant material remaining after hydrolysis increased (from 33% to 50%) with increasing gel concentration. Storage of amylose gels for 7 days resulted in levels of crystallinity of 28 to 35%. Hydrolysis of these gels led to increases in crystallinity, but to a lesser extent (from 148 to 175%) than before. The amount of resistant material remaining after hydrolysis increased (from 41 to 50%) with increasing gel concentration. Size-exclusion chromatography of resistant material from hydrolysed films indicated the presence of a high molecular weight component corresponding to undigested amylose, in addition to a low molecular weight component corresponding to partially digested amylose. Chromatograms of resistant material from both gels stored for 1 day and those stored for 7 days were almost identical and contained only a low molecular weight component with no sign of undigested amylose. X-ray line-broadening analysis showed the average sizes of the crystallites in the resistant material produced by hydrolysis of gels stored for both 1 day and 7 days to be approximately equal. The ability of the enzyme to diffuse into its substrate played a major part in the resistance to hydrolysis of amylose films, but had a much less important role in the resistance of gels. Not only crystalline, but also amorphous portions of amylose gels, were found to be resistant to enzymic hydrolysis. The resistant portion of amylose gels was modelled in vitro with a view to explaining the structure of resistant starch in vivo.