Intact cellular structure in cereal endosperm limits starch digestion in vitro

Abstract

Limiting the rate and extent of starch digestion is a major target for increasing the nutritional value of cereal-based foods. One mechanism that could be exploited is the ability of intact cell walls to protect intracellular starches from enzyme hydrolysis, but the extent to which this mechanism is valid for cereal endosperm cells is not well understood. This study aimed to isolate individual intact cellular structures from cereals, viz. wheat and sorghum, in order to elucidate the effect of intactness of cell walls on enzymic hydrolysis of entrapped starch. Intact cells were isolated from dry milled flour obtained using three grinding rolls coupled with a wet sieving technique using selected sieves having varying apertures. The intact cellular structure in wheat and sorghum hindered the hydrolysis of entrapped starch as observed from the lower extent of digestion (9 and 7%) compared to deliberately broken cells (19 and 17% under the same conditions). The extent of digestion was markedly increased once the intact cells were cooked (33 and 26% for wheat and sorghum cooked cells), but this was less than half the digestion extent of non-encapsulated cooked starches (77 and 62% respectively). Microscopic observations coupled with fluorescence labelling of enzyme, cell walls and starch suggest a) wheat and sorghum cell walls are effective barriers for access of amylase; and b) both an extensive protein matrix (particularly in sorghum) and non-catalytic binding of amylase on cell wall surfaces can limit the amylolysis of starch within intact cells. Furthermore, the presence of incompletely gelatinised starch inside cooked intact cells, suggests limited swelling of granules trapped inside the cells. This study shows how preservation of cellular matrices in cereal-based foods could be beneficial for increasing the amount of enzyme resistant starch in cereals with added nutritional benefits.