Protein spoilage of fish meal is mainly due to the oxidative decomposition of harmful microorganisms such as spoilage bacteria, which produces some toxic substances such as histamine, volatile base nitrogen, biogenic amines (putrescine, cadaverine, chlorpromazine, etc.), indole, mercaptan, hydrogen sulfide, fecal odor, etc.
VBN (Volatile Salt-based Nitrogen): Some volatile amines (such as dimethylamine and trimethylamine, mainly trimethylamine, which are produced by microbial reduction of Quaternary amines) and ammonia, which are the main components of rotten fish meal odor, the decomposition products of bacterial reproduction, amino acids and other nitrogen-containing compounds, are found in fish meal. It will increase with the decrease of freshness. They have low boiling point, volatility and alkalinity (microbial action during raw fish decay, fish meal processing and storage). Generally speaking, the freshness is usually excellent when the VBN content of fish meal is less than 50mg per 100g, and more than 150 mg indicates that it has started to decay. The VBN of fresh fish meal is generally not more than 110mg/100g.
Biogenic amines are usually synthesized from decaying microorganisms and are generally considered to be potentially toxic. By-products with a certain degree of decay (including meat and bone meal, blood meal, feather meal, chicken meal and fish meal) are usually the most abundant sources of biogenic amines. There are many kinds of biogenic amines, including histamine (a local hormone synthesized from histidine), tyramine (synthesized from tyrosine), cadaverine (synthesized from lysine) and mammalian polyamines (including putrescine, arginine and spermine) synthesized from ornithine. Biogenic amine research has been carried out earlier in livestock and poultry, and many valuable conclusions have been obtained. The relative research on fish and shrimp feed is very limited, and the conclusion is also quite different from that in livestock and poultry.
Studies have warned us that biogenic amines should not always be considered as toxic or chemically induced foods, but can also be used as non-hormonal growth promoters.
Domestic fish meal VBN is one of the indicators of freshness of raw fish, but it will evaporate partially during high temperature processing of fish meal. Free amino acids form biogenic amines under various microbial actions, such as histidine histamine, lysine cadaverine, arginine putrescine and tryptophan tryptophan. These biogenic amines are non volatile and can effectively display the freshness of raw fish products such as fish meal and other drying products. But in the process of producing fish meal, some of the biogenic amines will be dissolved in the waste liquid. Fat in fish can also be oxidized to aldehydes and ketones and decomposed into glycerol and various fatty acids. Acid value is used to determine the decomposition of fat into fatty acids. In addition to volatile nitrogen and biogenic amines, other substances can also be produced in the process of fish body proteolysis and deterioration.