Shrimp, especially in the cephalothorax contains PP0 (polyphenol oxidase), PP0 catalyzed by a series of biochemical reactions, in the presence of oxygen PP0 will be oxidized with the surface of the shrimp body of the colorless compounds of monophenol into colorless biphenol, biphenol and into colored quinones, quinones are very easy to shrimp amino acids in the combination of formation of a black-brown complex.

The oxides have some toxicity but are negligible

, not a big problem, you can eat it.

Can make the shrimp not black method:

First fresh shrimp with boiling water or boiling oil to break the raw, dry a dry and put in the refrigerator. So that even if the shrimp died after its red color will not disappear, and the main thing is that this can be done both to preserve the freshness of the shrimp, taste and live shrimp no difference.