Effect of hardening process prior to harvest on taste of the Pacific oyster

Abstract

Generally, in Hiroshima Prefecture, Japan, the cultivation of the Pacific oyster Crassostrea gigas involves a process called “hardening” in which the oysters are intermittently left under anaerobic conditions by hanging them on racks in the intertidal zone for several months before the primary suspension of the oysters from offshore rafts. We investigated the effects of intermittent anaerobic loading on oysters, mainly for taste, using triploid Pacific oysters in summer and normal diploid oysters in winter. High performance liquid chromatography (HPLC) was used to quantify free amino acids and ATP-related compounds, and spectroscopic analysis was used to measure the amount of glycogen, which is an energy source for oysters and indicates their nutritional condition. In addition, sensory evaluations were conducted by the panels. In the summer experiment, the levels of several free amino acids, mainly related to sweetness, and the umami taste in the sensory evaluation were significantly higher, whereas the amount of glycogen did not change in the hardened group compared to the control group. In contrast, in the winter experiment, the levels of many free amino acids decreased in the hardened group compared to those in the control group. At the same time, glycogen levels and soft tissue weight also decreased. These results showed that the rearing environments of the hardened and control groups may have differed significantly between summer and winter. However, this study suggests that providing the oysters with intermittent anaerobic conditions just before harvest in summer may improve the taste.