As a food additive, nitrite has a positive effect on improving the quality of meat products, but excessive use will cause nitrite poisoning. Under the action of microorganisms, nitrite can be converted into nitrosamines in the stomach, which is a strong carcinogen. In addition, nitrogen-containing substances in meat varieties can be transformed into nitrite through amination and nitrification of bacteria. Fresh fish will produce some nitrite in the traditional pickling process, mainly because microorganisms such as Escherichia coli, Staphylococcus aureus and mold in pickled fish have strong reducing ability, which can reduce nitrate in fish to nitrite. At the same time, nitrogen-containing substances such as protein in fresh fish are decomposed, and amine substances such as trimethylamine will further form nitrosamines for nitrite, which is easy to cause cancer to human body.

Ammonium nitrite is formed by nitrosation of nitrite or nitrogen oxides with nitrogen-containing substances (such as amines and amino acids) in acidic environment. Nitrosamines mainly include: N- dimethyl nitrosamine, N- diethyl nitrosamine and nitrosopyranone. A large number of studies show that pickled vegetables, meat, fish and other foods contain more N- nitroso compounds. Fresh fish itself does not contain volatile N- nitroso compounds, but such substances are often detected in pickled fish. The main reason is that the protein decomposition of pickled fish produces a large number of amine substances, and the raw salt used in pickling may contain nitrite. Under suitable conditions, amines react with nitrite to produce more nitroso compounds in pickled food, which can accumulate in human body for a long time, leading to nasopharyngeal carcinoma, esophageal cancer, gastric cancer and bladder cancer. Fish is rich in protein and water. Due to the decomposition of bacteria and enzymes, the excessive decomposition of protein will lead to the deterioration of protein in fish. Under the action of bacterial protease and endoprotease, protein in fish will decompose to produce protein aging and skin, and the breakage of skin chain will obviously increase the content of amino acids. Then, due to deamination and deamination of related enzymes, a large number of volatile basic nitrogen, histamine, trimethylamine, ammonia, amines, hydrogen sulfide, noise and methyl noise are produced, which leads to fish deterioration.

Pickling has a certain effect on the protein of fish. During the curing process, the higher structure of protein was destroyed, which caused the denaturation and structural changes of protein, mainly because the secondary structure of actin was affected by salt. The higher the salt concentration, the lower the CAZ+~ ATPase activity of actin, and the transition temperature of actin and myosin in fish will move to a low value. The inactivation rate of ATPase in muscle fibrin CaZ increased accordingly.

The contents of soluble protein, amino nitrogen and sodium chloride in fish sauce are positively correlated with curing time, curing temperature and salt water concentration. Pickling temperature is positively correlated with amino nitrogen content in brine and sodium chloride content in fish. With the increase of salt water concentration, the soluble protein and amino nitrogen content in salt water decreased significantly, while the sodium chloride content in fish increased significantly. With the increase of salt content, the content of volatile basic nitrogen in preserved fish decreased; With the increase of curing temperature, TBA value and volatile base nitrogen content of pickled fish increased; Reducing the amount of salt, increasing the curing temperature and prolonging the curing time can increase the content of some free amino acids in pickled fish. Pickling can promote the formation of the characteristic flavor of preserved fish, and the volatile components in preserved fish with different salt water concentrations are significantly different. In addition, the preserved fish dried at low temperature has better flavor.

During the processing of salted fish, various components of the fish body will change. It was found that the content of soluble calcium in salted fish increased obviously after curing, which was 65,438 06.67 times higher than that of fresh fish. The content of nitrite is very low, which are 0.65, 438+0.465 and 438+0.2g/kg respectively. The content of protein decreased from 63.33% to 665438 0.86%. The fat content decreased from 19. 18% to 17.35%. In the drying stage, protein and fat in preserved fish will undergo obvious degradation and oxidation reactions, and the contents of methionine, serine, methionine, glutamic acid, threonine and TBA value will increase obviously. Fish contains a certain amount of adipose tissue, and salted fish will undergo complex chemical changes during processing and storage, resulting in unpleasant smell, yellowing and stickiness. This is the result of the interaction of microorganisms, light, oxygen, humidity, temperature and other factors on the surface of fish.

There are two ways of fat corruption, one is fat hydrolysis and the other is fat oxidation. Generally, these two processes exist at the same time, which leads to food spoilage. Low molecular weight fatty acids produced by fat hydrolysis mainly include formic acid, caprylic acid, aldehyde acid, acetic acid, nonanoic acid, azelaic acid and so on. There is nothing bad to smell. The final products of fat oxidation are mainly short-chain carbon-based compounds, which cause bad flavor of food. In most cases, these products are harmful to human body. The dominant bacteria in pickled products processing are lactic acid bacteria, yeast, micrococcus, staphylococcus and mold. Studies have shown that lactic acid bacteria, yeast, staphylococcus and micrococcus in preserved fish will grow and multiply in large quantities during processing. Twenty strains of lactic acid bacteria were isolated and identified from preserved fish, among which strain H was Lactobacillus Sauvignon, which was the dominant lactic acid bacteria in preserved fish processing. There are mainly five kinds of lactic acid bacteria, which are Lactobacillus foodborne, Lactobacillus plantarum, Pediococcus lactis, Lactobacillus curvaturus and Pediococcus pentosaceus. Among them, Pediococcus lactis has better crude protease activity and low temperature and high salt tolerance than other strains, and is suitable for processing preserved fish.

Common food-borne pathogens in aquatic products include Staphylococcus aureus, Listeria monocytogenes, Salmonella, Vibrio cholerae, Vibrio vulnificus, Escherichia coli with intestinal hemorrhagic diseases, Vibrio parahaemolyticus and Vibrio alginolyticus, which can cause human intestinal diseases. Pathogenic microorganisms in fermented meat products include Salmonella, Staphylococcus aureus, Clostridium botulinum and Listeria. Pseudomonas, Alcaligenes. Micrococcus. And streptococcus. It can cause meat to be sticky, and these bacteria produce volatile formic acid, acetic acid, propionic acid and butyric acid during metabolism, resulting in odor. In order to control the microbial contamination of pickled fish products, measures such as controlling the initial number of bacteria in raw materials, sanitary conditions and temperature during processing can be taken. In addition, efficient sterilization technologies such as ultraviolet sterilization, high pressure sterilization and microwave sterilization can be adopted to ensure the quality and safety of pickled fish products from raw materials to end consumers.