Effect of pre-cooking times on enzymes, properties, and melanosis of Pacific white shrimp during refrigerated storage

Pre-cooked shrimp have gained interest due to their attractive color and flavor. However, shrimp without sufficient pre-cooking have faced melanosis, particularly during the extended storage. This leads to the rejection by consumers and the market. Therefore, pre-cooking with appropriate time would be a means to tackle such a problem. The objective of this study was to investigate the effect of precooking times on enzyme activities, properties, microstructure, and melanosis of Pacific white shrimp (Litopenaeus vannamei). Pacific white shrimp were subjected to pre-cooking at 80°C for various times (0 to 120 s). The residual activities of polyphenoloxidase (PPO) and protease decreased with increasing pre-cooking times (p < 0.05). Nevertheless, the increasing pre-cooking time resulted in more cooking loss. With cooking time above 60 s, a cooking loss of 9.85% was obtained (p < 0.05). Marked decreases in the relative PPO and protease activities were observed within the first 30 s of pre-cooking, and negligible activities were detected after 120 s. The microstructure study revealed that the muscle fibers of pre-cooked shrimps were less attached with concomitantly higher shrinkage of the sarcomere, compared with those of the raw counterpart. Shrimps pre-cooked with longer time showed a lower development of black spots as evidenced by a lower melanosis score throughout storage of 7 days at 4°C. Therefore, a pre-cooking time of 30 s at 80°C was sufficient to lower PPO and protease activities with the minimized cooking loss and melanosis during refrigerated storage.