Effects of on-board handling and processing techniques on the quality of sharks from the Gulf of Mexico

Abstract

A shark fishery in the Gulf of Mexico utilizing the existing shrimp fleet can potentially become a reality. A significant increase in product quality, marketing and consumer education must be developed and maintained. During trial harvesting, seven longline sets yielded ten different species of sharks with Silky (Carcharhinus falciformis), representing 89% of the total sharks landed. An enzymatic procedure for the simultaneous determination of urea and ammonia was developed to serve as a quality control tool. The method is based on the transamination of α-ketoglutarate in the presence of glutamate dehydrogenase and reduced nicotinamide adenine dinucleotide (NADH). The consumption of NADH at 340 nm is stiochiometrically equivalent to the ammonia in the test sample. Urea is determined by the same reaction scheme after the addition of urease. To test the applicability of the method, the quality of refrigerated shrimp and crab meat was monitored; ammonia and urea increased during storage and there was a good correlation between these compounds and the traditional spoilage indicators. During iced storage, urea is hydrolyzed to ammonia by urease and trimethylamine oxide (TMAO) is reduced to trimethylamine by TMAO-reductase. When using improved on-board handling practices, the rates of these reactions were substantially reduced and the shelf-life of sharks was markedly increased. The natural flora isolated were Moraxella-Acinetobacter, Micrococcus and Pseudomonas species. The distribution of urease positive organisms, responsible for urea conversion to ammonia, increased from 4.2% at the time of harvesting to almost 80% after 7 days of iced storage. When using a modified atmosphere containing 70% CO2 and 30% O2, the on-board shelf-life of eviscerated sharks could be increased by extending the lag phase of microbial growth. On-board chemical treatments like salt brines and lactic acid were also effective in reducing urea, ammonia and TMAO. During nine months of frozen storage, the hydrolysis of urea to ammonia and the reduction of TMAO to trimethylamine (TMA) were completely eliminated. However, a slow breakdown of TMAO to formaldehyde and dimethylamine (DMA) by TMAOase was evident. The formation of formaldehyde and its subsequent interaction with structural proteins of the shark muscle could in part be responsible for the 20% loss in protein extractability.