How to keep shrimp fresh?

The purpose of preservation is to take measures to keep the freshness of freshly caught prawns. Common methods for keeping prawns fresh are:

First of all, add ice to keep fresh (chilled)

Adding ice to keep fresh is a traditional method for keeping fresh aquatic products, and chilled ships have been used in coastal areas of China for hundreds of years. Because chilled aquatic products have basically maintained their original biological characteristics, this method is still adopted in all countries of the world. De-icing method mainly uses the method of adding ice to keep shrimp fresh, that is, directly remove the broken ice on the surface of shrimp. Its advantage is simple operation, and the melted ice water can also wet the surface of shrimp to prevent it from oxidation and blackening. In operation, the caught prawns are washed first, then a layer of ice is taken out from the plastic fish box, and then a layer of prawns is loaded. It is required to remove the ice cubes of the layer of ice prawns and thin ice prawns evenly, so that the prawns can be cooled quickly, evenly, fully and thoroughly. There should be a drain hole at the bottom of the fish tank to let the melted ice water flow out. After the ice storage is completed, the fish box should be loaded into the refrigerated compartment (or insulated fish box or insulated car) in time, and then shipped out. It is not suitable to use water ice or cooling seawater for shrimp preservation, because long-term soaking will lead to water absorption and expansion of shrimp meat, which is more likely to deteriorate.

At present, massive ice (ice made of salt water ice bucket) is widely used, and it is crushed before use. In order to avoid damaging the shrimp, it is best to roll the shrimp into small ice cubes with uniform size. Flake ice, barrel ice, plate ice, granular ice and snowy ice can all be directly used for ice shrimp, while the latter is especially suitable because of the uniform and delicate ice crumbs. The ideal temperature of ice is -0.5℃, because using ice with lower temperature can hardly improve the refrigeration capacity, while producing ice with lower temperature consumes more energy.

Processing method of chilled shrimps with heads: Fresh shrimps with heads need to be freshly caught for export, packed into plastic fish boxes or fish baskets according to the method of layered chilled shrimps, covered with ice cubes, and transported to the refrigeration plant for processing as soon as possible. After arriving at the factory, pour the chilled shrimps into plastic boxes with holes in batches, put them into ice buckets, and gently rotate and shake them to minimize the damage of shrimps during cleaning. The ice water for washing shrimps should be replaced in time after it becomes dirty.

Pour the cleaned prawns on the operating table and grade them according to the customer's requirements. When grading, soft-shelled prawns and shrimps with serious mechanical damage are excluded. Put the graded shrimp into a plastic box with holes according to the specifications, and then immerse it in an ice sink to cool for about 20 minutes, so that the temperature of the shrimp can be reduced to about 2℃. Soak the cooled shrimp in a 2% (KH-D) fresh-keeping ice water solution tank for 2 minutes to prevent the shrimp from turning black.

Drain the shrimp soaked in the fresh-keeping liquid for 5 minutes, then weigh it, and add 1.5 kg of crushed ice to every 3 kg of shrimp. The weighed shrimps should be randomly sampled to check whether their weight, specifications and quality meet the requirements. Mix the weighed prawns with crushed ice and put them in a food film bag. After extrusion and exhaust, tie the bag mouth tightly with a rubber band and put it into a 49×32×9 cm polystyrene plastic box for precipitation. Every 4 boxes are stacked into boxes, and a cover with a thickness of 1.8 cm is added. After being covered with a large plastic bag, they are put into an outer carton and sealed with adhesive tape. Tie the packing tape tightly to the carton with a strapping machine, write down the specifications, brush the marks and affix the air label. The outer boxes should be sent to low temperature refrigeration in time and stacked separately according to the specifications. After the whole batch of processing is completed, it will be counted, loaded, shipped abroad and exported by air.

Second, drug preservation.

Drug preservation is a fresh-keeping method of aquatic products with the help of sterilization or bacteriostasis of various drugs, alone or in combination with other fresh-keeping methods.

In 1950s, a variety of broad-spectrum antibiotics (tetracycline, chlortetracycline, oxytetracycline, etc. ) is used to keep fish and shrimp fresh, and the methods of use include splashing, soaking, antibiotic ice, etc., all of which have obvious fresh-keeping effects. Later, due to the problem of antibiotic residues and bacterial resistance in fish and shrimp, its application value was denied from the perspective of public health.

Since the 1960s, attempts have been made to use drugs such as sulfite to prevent shrimp from turning black, and some countries have already formed commercial sales. The prawns were soaked in 0.7% sodium sulfite solution for 10 minute, then washed with seawater for 20-30 seconds, and kept at 2-5℃ for 72 hours without blackening. Whether to use anti-browning drugs in domestic frozen shrimp production can be handled according to customer requirements. According to March 1989 of Taiwan Province Province's China Aquatic Products, in the presence of polyphosphate, the mixture composed of water-soluble antioxidant sodium erythorbate, amino acids and peptides with similar structure to butyric acid has obvious effects on inhibiting the blackening and freshness decline of prawns, and prawns have no SO2 residue, so they have the function of color protection.

The method of drug preservation is simple and effective. If it can be used at low temperature, the effect will be obvious. However, due to the strict management of food hygiene in countries all over the world, we should be cautious when promoting the drug preservation of prawns.

Third, radiation protection.

Co-60γ -ray and high-energy electron beam (4MeV) irradiation can prolong the storage time of prawns. For example, shrimp was irradiated at a dose of 45 ~ 56 kgy (1gy =100 rad) and stored at 0.5 ~ 1.5℃ for 7 months, and the sensory test (9-point evaluation) was good. Although irradiated food has been sold in 1976, there are not many projects approved or recommended by the international health organization.