1Maruha Nichiro Holdings, Inc. Central Reserch Institute, Tsukuba, Ibaraki 300-4295, 2Laboratory of Food Technology Department of Marine Bioscience Fukui Prefectural University, Obama, Fukui 917-0003, Japan
Vannamei prawn Penaeus vannamei killed by chilling in ice were processed into headless shell-on and pre-treated at 20°C for 0-7.5 h before freezing. IMP in the muscle increased up to 50% during the pre-treatment. After storage at −20°C or −40°C for 3 or 14 months, specimens were thawed at room temperature or at 4°C, weighed to determine the yield, and the number of aerobic bacteria and the rate of black discoloration were analyzed. When specimens were thawed at room temperature after storage, no effect of pre-treatment was observed regardless of storage temperature. However, black discoloration occurred in prawns stored at −20°C for 3 months and thawed at 4°C although the rate of discoloration was not correlated with the length of the pre-treatment. The discoloration did not occur in the specimens stored at −40°C even after thawing at 4°C. The rate of AMP and IMP decreased during the storage and the values were higher in the samples stored at −40°C than those stored at −20°C but the pre-treatment did not influence these values. Thus, our results suggest that the pre-treatment at 20°C before freezing strengthens the umami taste without affecting the quality of the product.