Influence of protein in low paste viscosities of Bambara groundnut flours from heat-treated Bambara groundnut seeds

Abstract

Heat treatment of Bambara groundnut seeds has been reported to cause low paste viscosities in resulting flours. Structural changes in Bambara groundnut protein, due to heat treatment, causes the protein to encapsulate starch, making it unavailable to paste causing low paste viscosities. In this study, trypsin was used to hydrolyze proteins in the flour and the microstructure analysis confirmed the disappearance of aggregates. Flour microstructure analysis confirmed hydrolysis of protein from previously aggregated status and showed liberated individual starch granules. Following the treatment of flours by trypsin, Confocal laser scanning microscopy did not show a protein signal. Hydrolysing Bambara groundnut proteins significantly increased the flour paste viscosities (P < 0.05). The final viscosities for flours from 20 % moisture-conditioned and infrared heat-treated seeds for 5 min were 733.9 mPa s before protein hydrolysis and 2081.71 mPa s after protein hydrolysis. The gelatinization temperature (81 ◦C) did not show a significant change following protein hydrolysis. Sodium dodecyl-sulphate polyacrylamide gel electrophoresis band intensity increased indicative of disulfide bonding and protein polymerisation when microwave and infrared heat treatment were combined. There were changes in Bambara groundnut protein secondary structures such as an increase of 57 % in β-sheet along with a 60 % reduction in the α-helix as shown by the Fourier-transform infrared spectroscopy. The changes in secondary structure of Bambara groundnut protein were caused by microwave and infrared heating. Heat treatment of Bambara groundnut seeds is partly responsible for the reduction in paste viscosities of their flours.