Many people like to eat shrimp, but few people know what effect the shrimp has, and the shrimp head will turn black during the preservation process. Is it not fresh that the shrimp head turns black? 　　 Is it not fresh that the shrimp head turns black? To understand why shrimp heads turn black, you need to recognize a substance-tyrosinase, which is commonly found in animals and plants. Although the structure of tyrosinase in different organisms is very different, the basic function is similar. Under the action of tyrosinase, tyrosine can gradually form quinone substances, and then form black substances such as eumelanin and pheomelanin. Tyrosinase is distributed throughout the shrimp, but the tyrosinase activity in the head is the strongest, and the enzyme activity in the abdomen and tail is lower. Therefore, the shrimp head always turns black first, then the belly and tail. 　　 The blackening of shrimp is mainly due to the action of enzymes, which has little to do with microorganisms. Even under freezing or cold storage conditions, tyrosinase is still active, so blackening still occurs. The head of the shrimp is also where the viscera of the shrimp is. If the shrimp happens to eat a lot of dark algae, such as brown algae, before fishing, the shrimp may also have black hair. In addition, the undigested feed of farmed prawns may also appear black. Therefore, a black shrimp head does not necessarily mean that the shrimp is not fresh. From this point of view, the blackening of shrimp heads is not so scary, and it needs to be treated correctly. 　　How to judge the freshness of shrimp 　　1. The surface of fresh shrimp is dry 　　The surface of the fresh shrimp is clean, and it feels dry to the touch. But when the shrimp's body is about to deteriorate, the mucus-secreting granular cells under the carapace disintegrate, and a large amount of mucus seeps into the body surface, making it feel slippery to the touch. If the shrimp shell is sticky, it means the shrimp has gone bad. 　　2, fresh shrimps are bright in color 　　 There are different types of shrimps, and their colors are slightly different. Fresh prawns, roche prawns, and grass prawns are blue, sea-caught prawns are pink, and bamboo prawns and giwei prawns have black and white patterns and slightly pink. If the shrimp has black hair, it means that the shrimp is not fresh. The whole shrimp is darker and not bright, which means it has deteriorated. 　　3, fresh shrimps are curved 　　 The head and tail of fresh shrimp are intact, and the head and tail are closely connected to the body. The body of the shrimp is relatively strong, with a certain degree of flexibility and curvature. In the stale shrimp, the head and body, shell and meat are loosely connected, and the head and tail are easy to fall off or separate, and cannot maintain their original curvature. 　　4, fresh shrimp shells must be hard 　　 Fresh shrimps have hard shells, bright cyan color, sharp eyes, firm flesh and very strong smell. If the shell is soft, the color is gray, the eyes are sunken, and the shell flesh is slightly separated, it means that the shrimp has just died.