Initial Freshness of Pacific Oyster (<i>Crassostea gigas</i>) Affects Its Quality and Self-life during Freezing Storage

Abstract

We investigated the effect of initial freshness of raw oysters on the quality and storage period of oysters during freezing storage. The expressible drips of the oysters were more effused as the freezing storage period increased. Also, the lower the initial freshness of the oysters, the more drips were released after thawing. The pH values decreased slightly during freezing storage at -20 °C for 12 months, but there was no significant difference (p > 0.05) according to initial freshness of raw oysters. The initial glycogen contents of oysters before freezing was between 722 and 585 mg/100 g whereas the glycogen contents of the oysters after freezing for 12 months ranged from 667 to 522 mg/100 g. The initial TVB-N value of S-1 with freshness of “good quality” was 3.9 mg N/100 g, but gradually increased during the freezing storage period and its value was 7.9 mg N/100g at 12 months of storage. S-7 of with an initial TVB-N value of 15.2 mg N/100 g increased sharply during freezing storage, reaching a maximum of 36.2 mg N/100g at 8 months after storage and then slightly decreased to 32.2 mg N/100g at 12 months of storage. S-5 with an initial PV of 12.6 meq/kg showed the highest value of 33.7 meq/kg at the 10 months of storage and S-7 with an initial PV of 17.5 meq/kg showed the highest value of 44.6 meq/kg at 8 months of storage. PL of oysters decreased with increasing storage period regardless of their initial freshness in all samples, while FFA increased during freezing storage The scores of all sensory evaluation parameters of S-1, which is the freshest sample of raw oysters before freezing, showed little change until 4 months after storage, and their scores began to slowly decrease after 4 months of storage and still could be accepted (scores of more than 6.0) at the end of storage. Sensory evaluation scores on color, taste and odor of S-7 after 8 months of storage showed unacceptable score of 6 or less, and fishy and sour taste were slightly stronger at 12 months of storage. These results indicate that raw oysters with TVB-N and PV of 3.9 mg/100 g and 4.2 meq/kg can maintain oyster quality for more than one year at -20°C. On the other hand, it is suggested that raw oysters with TVB-N and PV of 15.2 mg N/100 g and 17.5 meq/kg may not be stored for more than 8 months at -20°C.