Amino acid composition and rheological properties of doughs of wheat flour and cowpea protein isolate modified chemically and enzymatically

The aim of this study was to modify the proteins of cowpea protein isolate enzymatically (trypsin/chemotrypsin and pepsin/pancreatin) and chemically (acetic and succinic anhydrates). The effect of the addition of this isolate to wheat flour at different substitution levels (5, 10, and 15%) was also evaluated to find the suitable level needed to improve the nutritional value of wheat protein without affecting the rheological properties of the dough which are necessary for making bread an other baking products. The protein contents of deffated cowpea lour and copea protein isolate were 21.91 and 69.72% on fresh weight basis, respectively. The protein content of the wheat flour mixed with untreated and treated (chemically and enzymatically) isolates increased with increasing the level of isolates addition. Cowpea protein isolate contained most essential amino acids in amount adequate to daily intakes of preschool children except for methionine. Methionine is the limiting amino acid in cowpea protein isolate. The amino acid composition of cowpea protein isolate was not affected markedly by enzymatic and chemical treatments. The addition of untreated and treated cowpea protein isolates to wheat flour increased lysine content in all mixtures and the increase rate was proportional to the amount of isolate added. Addition of untreated cowpea protein isolate to wheat flour improved farinograph parameters of wheat flour dough at all levels of addition, whereas chemically treated isolate adversely affected these parameters except pepsin/pancreatin treatment at 5% level which paralleled farinogram parameters of the control. Extensigraph parameters reduced for all mixture samples compared to the control. However, cowpea protein isolate treated with pepsin/pancreatin and added to wheat flour at 5 and 10% level balanced extensibility values and improved these parameters. Addition of untreated cowpea protein isolate to wheat flour reduced maximum viscosity temperatures (amylogram parameters), whereas chemically and pepsin/pancreatin treated isolates increased maximum viscosity and maximum viscosity temperature of wheat flour mixtures samples compared to the control. On the other hand, addition of cowpea protein isolate treated with trypsin/chemotrypsin reduced maximum viscosity value and increased maximum viscosity temperature to the control