MOLECULAR PROFILE, PURITY AND PRESENCE OF TRYPSIN INHIBITORS IN COWPEA PROTEIN ISOLATES

ABSTRACT The most-used preparation process of protein isolates (PI) involves the isoelectric precipitation of the protein. Heating shortens the preparation time but this procedure may affect the purity, yield, molecular profile of the protein, and the activity of the trypsin inhibitor. This study aimed to investigate the effect of heating in the production of cowpea protein isolates. Crude whole beans (WB) were defatted with hexane, and the protein isolates obtained by isoelectric precipitation with (HPI), and without (NHPI) heating. The protein content of the WB and the PI was determined by the micro-Kjeldahl method, and the extraction yield estimated from the protein content at the end of extraction in relation to this content in the raw material. Possible losses of protein fractions were followed by SDS-PAGE, and the trypsin inhibitor activity determined by an enzymatic assay (BAPNA: benzoyl-DL-arginine-p-nitroanilide). Protein content in HPI was 83.3%, less than in the NHPI (92.2%). The HPI yield was lower (40.0%) as compared to the NHPI (42.3%). Electrophoresis indicated bands ranging from 13 to 262 kDa in WB; and the NHPI presented a protein fraction’s profile closer to that of the WB than to the HPI. The WB had the trypsin inhibitor activity, expressed as Trypsin Inhibitory Units (TIU), of 32.5±0.5 TIU /mg-protein; HPI showed 12.7±0.5 TIU /mg-protein (39% of that observed in WB) and the NHPI, 8.3±0.2 TIU /mg-protein (25.5%). Heating reduces the yield and purity of proteins in the isolates. However, the inhibitory activity of trypsin cowpea is most affected by the isolation procedure.