Changes in starch physical characteristics following digestion of foods in the human small intestine

Abstract

Factors controlling the concentration of resistant starch (RS) in foods are of considerable interest on account of the potential for this type of fibre to deliver health benefits to consumers. The present study was aimed at establishing changes in starch granule morphology as a result of human small-intestinal digestion. Volunteers with ileostomy consumed six selected foods: breakfast cereal (muesli), white bread, oven-baked French fries, canned mixed beans and a custard containing either a low-amylose maize starch (LAMS) or a high-amylose maize starch (HAMS). Analysis showed that digesta total RS (as a fraction of ingested starch) was: muesli, 8·9 %; bread, 4·8 %; fries, 4·2 %; bean mix, 35·9 %; LAMS custard, 4·0 %; HAMS custard, 29·1 %. Chromatographic analysis showed that undigested food contained three major starch fractions. These had average molecular weights (MW) of 43 500 kDa, 420 kDa and 8·5 kDa and were rich in amylopectin, higher-MW amylose and low-MW amylose, respectively. The low-MW amylose fraction became enriched preferentially in the stomal effluent while the medium-MW starch fraction showed the greatest loss. Fourier transform IR spectroscopy showed that absorbance at 1022 per cm decreased after digestion while the absorbance band at 1047 per cm became greater. Such changes have been suggested to indicate shifts from less ordered to more ordered granule structures. Further analysis of amylose composition by scanning iodine spectra indicated that the MW of amylose in ileal digesta was lower than that of undigested amylose. It appears that high-MW amylose is preferentially digested and that MW, rather than amylose content alone, is associated with resistance of starch to digestion in the upper gut of humans.