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Explain the phenomenon of black spot in shrimp, conditions and control measures

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Nov 3, 2021
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In shrimp, the polyphenoloxidase enzyme system is located in the transparent membrane under the shell. When shrimp die, the membrane is broken, and polyphenoloxidase is released, which catalyzes the oxidation reaction of phenolic compounds to form black complexes. These are called black spots in shrimp.


Explain the phenomenon of blackening in shrimp

In aquatic animals, especially in fish and shrimp, the enzyme polyphenoloxidase (PPO) exists quite commonly. This enzyme often causes blackening of fish and shrimp. The PPO enzyme system in shrimp is mainly tyrosinase, which exists in the transparent membrane under the shrimp shell. When shrimp die, the protease in shrimp breaks down this membrane, promoting the release of PPO. After PPO is released with an oxygenated environment and a suitable temperature, it will catalyze the oxidation reaction of phenol-containing compounds to create a black-brown complex (melanine), which is the phenomenon of blackening of PPO. shrimp.

The phenol-based substances in fish and shrimp are usually tyrosine or phenylalanine. The phenomenon of blackening in fish and shrimp is very common and it significantly reduces product quality. Thus, the conditions for the formation of melanin due to 3 factors are the presence of enzymes such as tyrosinase, the amino acid substrate tyrosine and oxygen that are abundant in the air, so to limit the phenomenon of blackening, we must control 3 conditions. above.

Conditions for the occurrence of black spot process

pHth (E) = 6, the oxidation rate is high in black tiger shrimp (monodon).
pHth (E) = 8 with white shrimp (according to Simpon et al.).
th(E) = 6.5 for lobster (Chen et al.).
t° = 40°C with Khanh Hoa black tiger shrimp (monodon).
t° = 30 ÷ 35°C with lobster.
t° = 45°C with Taiwanese black tiger shrimp.

The suitable temperature of the enzyme polyphenoloxidase for white shrimp is 40°C. Thus, the average suitable temperature range of enzymes to promote black shrimp process is 30 ÷ 40°C.

At 4°C, the polyphenoloxidase enzyme activity was 19.1% lower than the original. Thus, when preserving shrimp in ice, the blackening phenomenon still occurs but slowly.

The activity of polyphenoloxidase enzyme in different organs on shrimp body is different. The activity of this enzyme in the head is highest. If the polyphenoloxidase enzyme activity in the tail is considered to be 1, it is 3.1 in the body and 9.8 in the head.

Measures against blackening in shrimp

Preservation of ice combined with chemicals

  • If preserved with ice at a ratio of 2:1, the storage time is 3 days.
  • If preserved by ice combined with Potassiumsorbate 0.1%, the storage time is 6 days.
  • If stored in the following mixture, the shelf life is 17 days. Includes the following compounds in proportion: Potassiumsorbate (KS) 0.1%, Sodium tripolyphosphate (Na5P3O10) 0.05%, NaCl 3%, Ice 2/1.
  • If preserved with the following mixture: 4hexyresol (hexyResolcinol) 50ppm, citric acid 1%, KS 0.1%, Na5P3O10 0.05%, Ice 2/1, the preservation time is 20 days.

In addition, it can be preserved with BL7P, NaHSO3, citric acid...

To limit the phenomenon of blackening in shrimp, we store raw shrimp at 0-2°C in ice or cold water to inhibit the activity of tyrosinase, reduce the penetration of air or use chemicals. substances that limit the penetration of oxygen such as sulfites or some other specific chemicals. It should be noted that after coming out of the water for 2-4 hours, if not preserved well, black pigment will appear. This phenomenon of blackening occurs when the shrimp is still very fresh, but because the shrimp has blackened, it is no longer valuable.

Source: Collector
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