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Abstract:

Starch is the most abundant glycemic carbohydrate in a normal human diet. Lowering the rate of its digestion in the small intestine provides a unique opportunity to maintain optimal blood glucose and insulin levels. Wheat milling into white flour disrupts the endosperm cell walls. One can assume that these cell walls can act as a physical barrier to the amylolytic enzymes during digestion. Against this background, the aim of this study was to investigate the impact of the degree of starch enclosure in wheat endosperm cellular structures on its physico-chemical properties and digestibility before and after hydrothermal processing. Hard wheat (Triticum aestivum L) kernels were milled and separated into different size fractions: regular flour (average particle diameter: 85 μm), fine (330 μm) and coarse (710 μm) farina. Epifluorescence microscopy images showed that the coarse particles had a significantly higher level of intact cells than regular flour. The coarse farina particles exhibited substantially retarded and, at the same time, up to 25% lower starch swelling than flour and fine farina, clearly indicating that the surrounding matrices impose physical restraints to starch swelling. Rapid Visco Analyzer (RVA) peak viscosity values at a dry matter content of 12% w/w were 4,121 cP for fine farina and 3,316 cP for coarse farina as a result of the increased cell walls intactness. At the same time, coarse farina exhibited lower breakdown values (1,005 cP) than did fine farina (2042 cP), since intact cells are more resistant to the applied shear. To fully comprehend the impact of the cell walls on starch pasting, Bacillus subtilis xylanase was added during RVA analysis to in situ hydrolyze arabinoxylan, the main cell wall component of wheat endosperm tissue. The peak viscosity values of coarse farina increased from 3,316 to 3,597 cP upon cell wall hydrolysis indicating that removing the physical barriers allowed starch granules to swell to a larger extent. Moreover, the breakdown values increased from 1,005 to 1,436 cP since the granules were more susceptible for disintegration once the cell walls were (partly) broken down. Nevertheless, the starch gelatinization properties such as measured by Differential Scanning Calorimetry were not affected by the degree of cell wall intactness. Finally, the rate and extent of in vitro starch digestibility went hand in hand with the degree of cell wall disintegration of the study samples indicating that the cell walls have a substantial impact on starch digestibility by acting as physical barrier and/or changing starch physico-chemical properties. In conclusion, intact wheat endosperm cell walls drastically limit the swelling and viscosity development of starch and, at the same time, retard (and limit) in vitro digestibility, offering innovative opportunities to design novel cereal-based products with potential claims on sustained energy release.