Detailed explanation of the most popular food spoi

Detailed explanation of food spoilage and its control technology (IV)

1.2 chemical process of food spoilage

the process of food spoilage is essentially the catabolism of contaminated microorganisms such as protein, carbohydrate and fat in food or some biochemical processes carried out by their own tissue enzymes. For example, the ripening of fresh meat and fish, the respiration of grain and fruit, etc. can cause the decomposition of food components, the collapse of food tissue and the fragmentation of cell membrane, providing conditions for the extensive invasion and action of microorganisms, resulting in the corruption and deterioration of food. Because the decomposition process and products of food components are very complex, it is still difficult to establish a quantitative detection of food spoilage

⑴ decomposition of protein in food

protein rich foods such as meat, fish, eggs and bean products are mainly characterized by protein decomposition. Spoilage of protein foods caused by microorganisms is usually called spoilage

proteins are first hydrolyzed into polypeptides and then cleaved into amino acids under the action of animal and plant tissue enzymes, as well as protease and endopeptidase secreted by microorganisms. Amino acids gradually show their potential through decarboxylation, deamination, polycarbonate polyurethane (PCU), desulfurization and other functions, and further decompose into corresponding (1) ammonia, amines, organic acids and various hydrocarbons caused by abnormal operation and human or natural disasters. Food shows the characteristics of corruption

decomposition of amino acids amino acids are decomposed by deamination and decarboxylation

① deamination reaction

in the deamination reaction of amino acids, carboxylic acids and a-ketoacids are generated by oxidative deamination, unsaturated fatty acids are generated by direct deamination, and organic acids are generated by reductive deamination. For example:

rch2chnh2cooh (amino acid) + O2? Rch2cocooh (a-ketoacid) + nh3

rch2cnh2cooh (amino acid) + O2? Rcooh (carboxylic acid) + NH3 + CO2

rch2chnh2cooh (amino acid)? Rch=chcooh (unsaturated fatty acid) + nh3

rch2chnh2cooh (amino acid) + H2? Rch2ch2cooh (organic acid) + NH3

② decarboxylation reaction

amino acid decarboxylation to produce amines; Some microorganisms can deamination and decarboxylation at the same time, and generate ethanol, fatty acids, hydrocarbons, ammonia, carbon dioxide, etc. by adding water for decomposition, oxidation and reduction. For example:

ch2nh2cooh (glycine)? CH3NH2 (methylamine) + co2

ch2nh2 (CH2) 2 chnh2cooh (ornithine)? Ch2nh2 (CH2) 2 ch2nh2 (putrescine) + co2

ch2nh2 (CH2) 3 chnh2cooh (arginine)? Ch2nh2 (CH2) 3 ch2nh2 (cadaverine) + CO2

histidine? Histamine + CO2

(CH3) 2chchnh2cooh (valine) + H2O? (CH3) 2CH CH2OH (isobutanol) + NH3 + CO2

ch3chnh2cooh (alanine) + O2? CH3COOH (acetic acid) + NH3 + CO2

ch2nh2cooh (glycine) + H2? CH4 (methane) + NH3 + CO2

③ decomposition of amines

amines generated in corruption are decomposed by bacterial amine oxidase, and finally ammonia, carbon dioxide and water are generated

rch2nh2 (amine) + O2 + H2O? RCHO + H2O2 + NH3

hydrogen peroxide is decomposed by catalase. At the same time, aldehyde is also decomposed into carbon dioxide and water by acid

formation of mercaptan

mercaptan is formed through the decomposition of sulfur-containing compounds. For example, methionine is decomposed by methionine mercaptan deaminase as follows

ch3sch2chnh2cooh (methionine) + H2O? CH3SH (methyl mercaptan) + NH3 + ch3ch2cocooh (a-ketoacid)

④ production of methylamine

trimethylamine oxide, a normal component of fish, shellfish and meat, can be reduced by bacterial trimethylamine oxidoreductase to produce trimethylamine. This process requires substances (organic acids, sugars, amino acids, etc.) that can make bacteria carry out oxidative metabolism as hydrogen donors

(CH3)3NO + NADH ? (CH3) 3N + NAD +

decomposition of fat in food

although the deterioration of fat is mainly caused by chemical action, many studies have shown that it is also closely related to microorganisms. The deterioration of fat is characterized by the generation of acid and irritating "Hara" smell. People generally call the deterioration of fat rancidity

the chemical reaction of oil rancidity in food is mainly the oxidation process of oil itself, followed by water hydrolysis. The self oxidation of oil is a free radical oxidation reaction; Hydrolysis is the decomposition of neutral fat in food into glycerol and fatty acids under the action of lipolytic enzymes in microorganisms or animal tissues. However, the chemical reaction of oil rancidity is still under study, and the process is complex, and some problems need to be clarified

⑵ self oxidation of oil

self oxidation of oil is a free radical (free radical) oxidation reaction, which mainly includes: fatty acids (rcooh) are activated under the action of heat, light or copper, iron and other factors to form unstable free radicals R ·, H ·, which form peroxide free radicals with O2; Then, free radicals are transferred repeatedly to generate new free radicals. In this series of oxidation processes, hydroperoxides, carbonyl compounds (such as aldehydes, ketones, low molecular fatty acids, alcohols, esters, etc.), hydroxyl acids, fatty acid polymers, and condensates (such as dimers, trimers, etc.) are generated

fatty acid hydrolysis

the biggest obstacle for carbon fiber lightweight composites to enter the Chinese automotive market lies in the lack of rapid mass production technology and professional utilization skills and knowledge, including the water decomposition of fat, which produces free fatty acids, glycerol and their incomplete decomposition products. Such as monoglyceride and diglyceride

fatty acids can then break the chain to form ketones or ketoacids with unpleasant taste; Unsaturated bonds of unsaturated fatty acids can form peroxides; Fatty acids can also be re oxidized and decomposed into particularly smelly aldehydes and aldehydes, that is, the "Khara" smell of Wei. This is the reason why sensory properties change after rancidity of edible oils and fatty foods

the complex decomposition products produced by the self oxidation of fat and the decomposition of water make edible fat or fat in food have several obvious characteristics: first, the peroxide value increases, which is the earliest indicator of fat rancidity; The second is the increase of acidity, and the carbonyl (aldehyde ketone) reaction is positive. In the process of fatty acid rancidity, due to the decomposition of fatty acid, its inherent iodine value (value), freezing point (melting point), specific gravity, refractive index, saponification value and so on will inevitably change, so the unique "Khara" flavor of fatty acid rancidity; Excessive oxidation and yellowing of fat in meat and fish food; The "oil burning" phenomenon of fish is often used as a more practical indicator in the identification of oil rancidity

the rancidity of fat and edible oil in food is affected by fat saturation, ultraviolet light, oxygen, moisture, natural antioxidants, copper, iron, nickel ions and other catalysts. Unsaturation of fatty acids in oil and residues of animals and plants in oil can promote the rancidity of oil; However, when the content of fatty acid saturation, vitamin C, e and other natural antioxidants and aromatic compounds of oil is high, oxidation and rancidity can be slowed down

⑶ decomposition of carbohydrates in food

carbohydrates in food include cellulose, hemicellulose, starch, glycogen, disaccharides and monosaccharides. Foods containing more of these ingredients are mainly grains, vegetables, fruits, sugars and their products. Under the action of various enzymes and other factors in microorganisms, animals and plants, this is caused by a variety of reasons. These food components are decomposed into low-grade products such as monosaccharides, alcohols, aldehydes, ketones, carboxylic acids, carbon dioxide and water. The deterioration of sugars caused by microorganisms is customarily called fermentation or fermentation

the main characteristics of food deterioration with high carbohydrate content are increased acidity, gas production, slightly sweet taste, alcohol smell, etc. Different kinds of food also show the increase of sugar, alcohol, aldehyde, ketone content or gas production (CO2), sometimes with the unique smell of these products. Pectin in fruits can be decomposed by pectinase produced by Aspergillus and Clostridium multienzyme, and can soften fresh fruits and vegetables with less enzyme