Weighing up whey fortification of foods: Implications for kinetics of starch digestion and estimated glycemic index of model high-protein-low-carbohydrate food systems

Mixtures of whey protein isolate (20 and 50%) and wheat starch, as a model high-protein-low-carbohydrate (HPLC) food, were extruded in a twin-screw extruder by changing moisture, temperature and screw speed. Longitudinal expansion increased with whey, and screw speed enhanced transverse expansion. In vitro starch digestion revealed monophasic digestograms for extrudates and non-extrudates. A modified first-order kinetic model suitably predicted the starch digestograms. Whey concentration of the non-extrudates did not significantly affect the digestion parameters, but extrusion increased these parameters. In the extrudates, whey increased (very) rapidly digested starch and glycemic index (GI), while rate of digestion and glycemic load (GL) reduced. Whey-fortification can increase bioactives in HPLC foods, but there is a need to increase resistant starch in such foods for maximum benefits. Being possibly the first study on the kinetics of starch digestion in whey-starch formulations, the present study highlights ways of increasing resistant starch in HPLC foods.