Antioxidant Activity of Protein Isolate Obtained by the pH Shift Technique from Hydrolyzed and Unhydrolyzed Shrimp Processing Discards

Protein isolates from hydrolyzed and unhydrolyzed shrimp waste were prepared using the pH shift method. The conditions for protein isolation from unhydrolyzed shrimp waste by the pH shift method were standardized with respect to waste to distilled water ratio (1:2.5, 1:5, 1:7.5, and 1:10 w/v) and the alkaline pH (9.0, 10.0, and 11.0) to which the waste was to be adjusted before protein precipitation for higher yield of protein isolate. The yield of precipitate with waste to distilled water ratio of 1:2.5 (17.3 ± 0.9) was lower (p < 0.05) compared to the other three dilutions. The pH to which the waste is adjusted before precipitating the proteins at pH 4.5 had no effect (p > 0.05) on the yield. The antioxidant activity of the protein isolate obtained by the pH shift method from unhydrolyzed waste was compared with that obtained from the waste hydrolyzed with Alcalase®. The DPPH scavenging activity of protein isolate after enzyme hydrolysis was significantly higher than that of protein isolate from unhydrolyzed waste. With respect to recovery of total antioxidant activity, protein isolation from unhydrolyzed waste was superior to hydrolyzed waste due to higher yield.