Studies on Methods and Standards for Evaluating Quality of Fresh Post-harvest Oyster (Crassostrea Virginica)
Type of Degreethesis
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Oysters are one of the most popular seashell foods in the world due to high nutritive value and delicious and unique taste. The consumption of oysters has become an important part of the diet for consumers in the U.S.A. Oysters are usually consumed fresh, but deterioration of quality occurs over the time of storage, which may bring a risk of safety as well as costumers’ rejection to the product. This study was aimed at developing a set of scientific methods to identify and evaluate the quality and safety of raw/live oyster, C. virginica. A method of solid phase microextraction (SPME) coupled with GC/MS was optimized to study the volatile profile characteristics, and chemical analyses including moisture, ash content, pH value, total volatile basic nitrogen (TVBN) and textural analyses were conducted to evaluate changes of quality during oyster storage. Thirty-two volatile compounds including alkenes, aldehydes, alcohols, ketones, organic sulfides and others were identified in the oyster volatile profile. Nine out of them were responsible for the aroma of fresh oysters. Intensity and odor characteristics of some volatiles such as 2, 4-heptadienal, 2-nonenal, 2-decenal, 2-octen-1-ol, 7, 10, 13-hexdecatrienal, (E, E)-2, 4-decadienal, and indole, were closely related to the deterioration of oyster. The decomposition of polyunsaturated fatty acids, especially C18:3n-3, C20:5n-3 and C22:6n-3 was related to the presence of specific aldehydes.Linear regression analyses of the storage period of oysters vs. various quality indicators showed that the strongest correlation was between storage period and the ash contents (r = -0.976) followed by the total volatile basic nitrogen (TVBN) (r = 0.953) and cutting force (r = -0.893). TVBN level of 11 mg /100 g could be used as the limit of acceptability for cold stored Eastern oyster. Parameters in texture analyses (cutting force and chewiness on adductor muscle) had strong correlation with storage period before the death of oysters. Further studies are required to confirm the suitability of ash content as a quality indicator of oysters. The textural and chemical results indicated a shelf-life of 20 days for Eastern oyster, C. virginica stored at 4°C.