Introduction:
In an article entitled Studies on Black Spotted Shrimp (1951), E. A. Fieger defines the formation of black spot on shrimp as melanosis.[1] This harmless but objectionable discoloration occurs primarily along the swimmerets, head, tail, and nearby shell areas.[2] Melanosis is an enzymatic browning process induced by polyphenoloxidase (PPO) which induces the oxidation of phenolic substrates to quinones. It is a natural mechanism caused by enzyme reactions that start as soon as crustaceans are taken from water, coming into contact with the oxygen of the atmosphere. After harvest and death, PPO systems are still active and can promote the development of black pigments. PPO catalyzes two basic reactions: hydroxylation to the o-position adjacent to an existing hydroxyl group of the phenolic substrate (monophenol oxidase activity), and oxidation of diphenol to o-benzoquinones (diphenol oxidase activity). Monophenol oxidase catalyzes the hydroxylation of monophenols to o-diphenols. This enzyme is referred to as tyrosinase in shrimp.[3]
Discussion:
Preventing the breakdown of tyrosine by the enzyme tyrosinase is vital to the seafood industry due in large part to unfavorable consumer response to melanotic shrimp. The use of sulfiting agents such as sodium bisulfite and sodium metabisulfite or formulations with sulfiting agents has been the traditional treatment method for retarding this breakdown in fresh, frozen, or thawed crustaceans since the early 1950’s.[4] E. A. Fieger’s 1951 article provides historical reasoning, “In the past decade shrimp production has increased considerably through the use of larger boats which were able to extend the off-shore fishing areas. This fact has resulted in longer storage on the trawlers and a longer period of time between catching the shrimp and their ultimate consumption.” Thus, sulfiting agents, particularly sodium bisulfite, were introduced in an effort to yield a more valuable harvest. This practice continues today, and companies such as WorldwideJanitor.com have made sodium bisulfite widely available to shrimpers. Continue reading →