Kinetics of in vitro protein digestion in an extruded model sorghum-barley blend: Processing and property relationships

Abstract

Material-processing-property relationships guide designing and engineering food systems. A 60%-sorghum-40%-barley cereal-cereal multigrain blend was processed at different moisture (20–40%), screw speed (150–300 rev/min), feed rate (2–5 kg/h), and temperature (110–160⁰C) twin-screw extrusion conditions. In vitro protein digestion was assayed with the time-course pH-drop measurements. Monophasic and biphasic (rapid-slow) protein digestograms were obtained, which were objectively analysed (Sopade Objective Procedure) and best described (r² > 0.9, p ≤ 0.05) by one- and two-term exponential models. Extrusion at moisture contents above 30% and temperature above 140⁰C increased the rate of protein digestion (K_PR1). The in vitro protein digestibility (IVPD_AVG) increased with the screw speed, and K_PR1 and IVPD_AVG correlated (K_PR1 = 27.28 K_ST, r² = 0.546, p < 0.001; IVPD_AVG = 0.57 eGI_AVG, r² = 0.999, p < 0.001) with the rate of in vitro starch digestion (K_ST) and estimated average glycaemic index (eGI_AVG) from a previous study (Koa et al. [2017]. J. Cereal Sci. 75, 314–323). While some in vitro protein digestion, extrusion, and extrudate properties significantly (p ≤ 0.05) correlated, Pearson correlations revealed non-significant (p > 0.05) relationships between others. These findings are important for relative nutrient releases in multigrain products, as proteins and starches can be differently susceptible to extrusion cooking.