The particle sizes of milled wheat fractions affect the in vitro starch digestibility and quality parameters of wire-cut cookies made thereof

Abstract

Slow digestion of starch is linked to various health benefits. The impact of wheat particle size on in vitro starch digestibility and quality of wire-cut cookies was here evaluated by including four soft wheat fractions [i.e. flour (average diameter, 83 μm), fine farina (643 μm), coarse farina (999 μm) and bran (1036 μm)] in the recipe. The susceptibility of starch in these fractions to in vitro digestion decreased with increasing particle size, resulting in a 76% lower digestion rate for coarse farina than for flour as found with the single first-order kinetic model. Starch was protected from hydrolysis likely due to delayed diffusion of pancreatic α-amylase through the intact farina cell walls. When 20–65% starch in flour for the control cookie recipe was substituted with the same percentages in fine and coarse farina, the starch digestion rate decreased when substitution levels increased. A 62% lower digestion rate was found at 65% substitution with coarse farina. Cell wall intactness was largely preserved in the cookies and most of the starch appeared as ungelatinised granules. Further, the cookie spread ratio during baking was 48% and 33% higher and the cookies were 63% and 57% less hard than control cookies when made with 65% fine farina and 65% coarse farina, respectively. The relatively low specific surface area of large wheat particles resulted in low water absorption and less dense packing. In conclusion, encapsulation of starch by intact cell walls in coarse wheat fractions makes them promising ingredients when developing starchy food products for controlled energy release.