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The blackening of sea shrimp is not pollution, but is actually caused by enzymes. The blackening of the head of the sea shrimp is due to an enzymatic reaction that occurs in the body of the sea shrimp, and this reaction has nothing to do with external pollution. Whether the shrimp is polluted or not, and whether the heavy metals are excessive or not, these can not be judged with the naked eye.
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A substance called tyrosinase is found in both animals and plants, and this substance has different structures in different organisms, but its basic function is similar. Under the action of tyrosinase, tyrosine will gradually produce a quinone, and then produce black substances, such as eumelanin and brown melanin. This is the reason why the shrimp head becomes darker in color.
Tyrosinase activity is highest in the head of the shrimp, while the enzyme activity in the abdomen and tail belly is relatively low; so we always see the head of the shrimp darken first, and then the abdomen and tail only begin to change color.
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If stored at room temperature, the shrimp head gradually blackened over time, which can reflect the freshness of the shrimp to some extent. However, the shrimp head blackened after all is the shrimp's own biological enzymes, and micro-organisms have little to do, so blackening does not mean that the shrimp spoiled. The enzyme tyrosinase is still active even under frozen or refrigerated conditions, and thus black changes can still occur.
Shrimp is a highly perishable food, once spoiled will produce volatile amines, shrimp meat becomes soft and lose elasticity and other phenomena, these are shrimp is not fresh.