One of these enzymes, a polyphenol oxidase, catalyzes the formation of a bluish black pigment. Other enzymes and bacteria enhance pigment formation by producing small building blocks that the polyphenol oxidase uses to produce the pigment. The bluish black pigment begins to form quickly and is usually visible inside the head and body of the shrimp within a day or two. The intensity of the color increases with time, changing the bright colored body to black and reducing consumer appeal for the shrimp. Enzyme reactions are affected by temperature.
Low temperatures slow the reactions.
Rapid chilling of the shrimp will slow the enzyme reactions, but does not stop them. Sodium bisulfite is a inhibitor of polyphenol enzymes, but appears to be ineffective in slowing discoloration in ridgeback shrimp. This is probably due to the inability of the sodium bisulfite to penetrate the shrimp's shell. The best way to prevent darkening of ridgeback shrimp is to keep the shrimp alive on the vessel and throughout distribution. Dead shrimp darken within a day or two and should have their heads removed as soon as possible.