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Sorghum (Sorghum bicolor L. Moench) Flour Pasting Properties Influenced by Free Fatty Acids and Protein¹

¹Paper No. 15861 from Purdue Agricultural Experiment Station. ²School of Food Science and Technology, Southern Yangtze University, 170 HuiHe Rd, Wuxi, Jiangsu Province 214036, Peoples Republic of China. ³Department of Food Science and the Whistler Center for Carbohydrate Research, Purdue University, Food Science Building, West Lafayette, IN 47907. ⁴Corresponding author. E-mail: hamakerb@foodsci.purdue.edu

A second unusually high viscosity peak appeared at the cooling stage (50°C) of a Rapid Visco-Analyser (RVA) profile of short-term stored (two months at room temperature) whole grain sorghum flour, while freshly ground flour had a typical pasting curve with one viscosity peak at the 95°C holding period. The formation of the second viscosity peak was caused by liberation of free fatty acids (FFA), mainly palmitic (15.6%), oleic (41.9%), and linoleic (37.9%) acids from stored flour. After the flour samples were pretreated with pepsin or the protease thermolysin, the second peak disappeared in the presence of FFA while the high viscosity was partially retained, indicating that flour protein was another essential component to the production of the actual peak. Effects of dithiothreitol (DTT), pH, and NaCl on RVA profiles of stored flour suggested that disulfide-linked protein and electrostatic interaction are required for the peak production. In the presence of sufficient FFA, similar cooling stage viscosity peaks appeared in the RVA profiles of flour samples from maize, rice, millet, and wheat; thus, the effect was not unique to sorghum flour. Coinciding with previously reported findings from our laboratory of a three-component interaction and discernable complex in a model system, a similar three-component (starch, protein, and FFA) interaction was revealed in natural flour systems resulting in formation of an unusual and notably high cooling stage viscosity peak. Practical applications and an interaction mechanism are discussed.