Protein content correlates with the in vitro starch digestibility of raw barley flour

Abstract

Protein content is one of the most important criteria used to evaluate the nutritional and functional qualities of raw barley, yet, its relation with starch digestibility remain elusive. The in vitro starch digestion kinetics of six barley varieties were characterized and correlated with the total protein content, starch chain-length distribution and the particle size distributions (PSD) of raw barley flour. Both the logarithm of slope plot and sequential first-order kinetics model were applied to fit the digestion curves. Results showed that only a single digestible starch fraction was observed for all flour samples under the simultaneous digestion of pancreatic amylase and amyloglucosidase. The barley protein content significantly and negatively correlated with the digestion rate constant k (r = 0.93, p = 0.007), whereas it significantly and positively correlated with the C∞ (r = 0.89, p = 0.017) corresponding to the amount of starch digested at infinite digestion time. No significant correlations between PSD and kinetic parameters were found, whereas of barley containing higher protein content, it contained a relatively higher amount of medium amylopectin chains with degree of polymerization (DP) raging from 34 to 67, and a smaller averaged DP of amylose molecules. This could rationalize its slower starch digestion rate. The current study provides important information for barley breeders and manufacturers to develop barley products with preferred starch functional properties except for only according to the barley protein content.