Extruded whole buckwheat noodles: effects of processing variables on the degree of starch gelatinization, changes of nutritional components, cooking characteristics and in vitro starch digestibility.

The effects of processing variables on the degree of gelatinization (DG), changes of nutritional components, cooking characteristics and in vitro starch digestibility of extruded whole buckwheat noodles were investigated and Pearson's correlations were explored. Results showed that buckwheat noodles with different DG values were obtained by changing the extrusion temperature (T = 100, 120, 140 and 160 °C) and moisture content (M = 32%, 40% and 48%). Both the total flavonoid content (TFC) and total phenolic content (TPC) were decreased with the increase of extrusion temperature, while they had the highest retention value when extruded at a moisture content of 40%. Severe extrusion conditions (high temperature and high shear) significantly increased the soluble dietary fiber (SDF) content from 3.13% to 4.73%. The cooking qualities of buckwheat noodles such as the cooking loss, broken rate and texture characteristics were positively related to the DG, while noodles became over-cooked when the DG was higher than 87.96%, resulting in the deterioration of cooking quality. In vitro starch digestibility results showed that severe processing conditions promoted the susceptibility of the starches to enzyme hydrolysis and the predicted glycemic index (pGI) increased from 78.04 to 86.92. These findings indicated that appropriate extrusion processing variables are critical to produce whole buckwheat noodles from the nutritional and quality perspective.