Urea is hydrolyzed by urease (EC 126.96.36.199) to ammonia, which is one of the components of total volatile base (TVB). TVB nitrogen has been used as a quality index of seafood acceptability by various agencies (Johnson and Linsay 1986, Cadwallader 2000, Gopakumar 2000). A good example is shark, which contains fairly high amounts of urea in the live fish.
Under improper handling, urea is converted to ammonia by urease, giving shark meat an ammonia odor that is not well accepted by consumers. To overcome this problem, the current practice of bleeding the shark near its tail right after harvest is very promising.
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ADENOSINE TRIPHOSPHATE DEGRADATION
Adenosine triphosphate (ATP) is present in all biological systems. Its degradation in seafood has often been reported (Fig. 1.3) (Gill 2000, Gopakumar 2000). The degradation products, such as inosine and hypoxanthine, have been used individually or in combination as indices of freshness for many years.
Ethylene acts as one of the initiators in fruit ripening. Its concentration is very low in green fruits but can accumulate inside the fruit and subsequently activates its own production. Table 1.16 lists the enzymes in the production of ethylene starting from methionine.
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The effect of ethylene is commonly observed in the shipping of bananas. The banana is a climacteric fruit with a fast ripening process. During shipping of green bananas, ethylene is removed through absorption by potassium permanganate to render a longer shelf life.
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