Nutritional Evaluation of the Protein in Oilseed Products Heated with Sugars

Defatted flours and protein isolates from glandless cottonseed, peanut, and soybean were complexed with glucose or sucrose in a ratio of 1:1 by weight and heated at 100°C for 0, 2, or 6 hr. Quality changes by the heat treatment were assessed by determining the extent of browning (browning index), in-vitro protein digestibility, available lysine total amino acids, and computed protein efficiency ratio (C-PER). Whereas the sucrose-containing complexes changed very little in all the quality parameters determined, the counterparts containing glucose decreased substantially in protein quality along with the increased intensity of browning. Before heating, protein digestibility of the complex containing defatted flour of an oilseed was lower than that of the isolate counterpart. When heated, however, the isolate-glucose complexes decreased more in protein digestibility than the flour-glucose complexes. The available lysine content decreased by 76-83% for the complexes with glucose in contrast with 2-10% for those with sucrose. As to total amino acids, marked losses in arginine, lysine, tryptophan, and histidine were found in the complexes containing glucose, with the arginine loss as high as the lysine loss. Lysine-rich soy proteins lost lysine in greater percentages than lysine-poor peanut and cottonseed proteins. C-PER decreased by 63-86% in the glucose-containing complexes, with the highest decrease shown for soy flour-glucose. In-vitro protein digestibility, available lysine, and C-PER were all highly correlated (P < 0.01) with browning index. Available lysine was also highly correlated with C-PER.