What are the changes in protein during food processing?

The processing and storage of food involves cooling, heating, drying, fermentation, irradiation, chemical reagent treatment or various other treatments, in which inevitably will cause protein physical, chemical and nutritional changes, and at the same time will affect the quality of the product, so it is necessary for food developers to know more about the effects of the process on the proteins in food.
Changes under heat treatment
Most protein foods are sterilized by heating, and heat treatment is the treatment that has the greatest impact on proteins. The extent of the effect depends on factors such as the duration of the heat treatment, humidity, temperature, and the presence or absence of redox substances. Heat treatment involves chemical reactions such as thermal denaturation, thermal decomposition, amino acid oxidation, exchange between amino acid bonds, and formation of new amino acid bonds.
Heating on the nutritional value of food has a harmful side, but also has a favorable side, most food proteins only in a narrow temperature range to show biological activity or functional properties, most proteins heated to improve the nutritional value, in the appropriate heating conditions, the protein denaturation occurs after the original peptide chain due to heat and fracture, so that the original folded portion of the peptide chain loosened, so that it is susceptible to the action of digestive enzymes, improve the digestibility and essential amino acids. The action of the digestive enzymes, improve the digestibility and bioeffectiveness of essential amino acids.
Moderate heat treatment can also make some enzyme inactivation, enzyme inactivation can prevent the food from producing unsuitable color, texture, flavor changes and reduce the content of cellulose, and to ensure that the food in the preservation period does not occur in the rancidity, texture changes and discoloration.
Most of the natural protein toxins or anti-nutritional factors present in plant proteins can be denatured or blunted by heat. Protease inhibitors present in the seeds or leaves of leguminous foods inhibit protein hydrolytic enzymes in the body, which in turn affects protein utilization and its nutritive value. Legume and oilseed seeds can be inactivated by roasting and soybean meal can be inactivated by moist heat treatment with exogenous lectins and protease inhibitors to improve protein digestibility. Many proteins such as soybean globulin and ovalbumin are more digestible after moderate heat treatment.
However, sometimes certain unfavorable reactions can occur with excessive heat treatment. Protein or protein food without adding other substances in the case of heat treatment, can cause amino acid desulfurization, deamidation, isomerization and other chemical changes, and sometimes even accompanied by the production of toxic compounds; in the process of heat treatment, proteins will also be with the other components of food, such as sugars, lipids, pollutants and food additives and other reactions, resulting in a variety of favorable and unfavorable changes. Therefore, it is of great significance to choose appropriate heat treatment conditions in food processing to maintain the nutritional value of proteins.
Changes under low temperature treatment
Low temperature storage of food can delay or prevent the growth of microorganisms and inhibit enzyme activity and chemical reactions. Common low-temperature treatment of cooling and freezing, cooling is to control the temperature slightly above the freezing temperature, protein stability, microbial growth is also inhibited; freezing is to control the temperature below the freezing temperature, the flavor of the food is more or less some damage, but if the control is good, the nutritional value of protein will not decrease.
Meat food after freezing, thawing, cells and cell membranes are destroyed, the enzyme is released, with the rise in temperature enzyme activity to enhance the degradation of proteins, and protein-protein irreversible binding, instead of water-protein binding, so that the texture of protein changes, water retention is also reduced, but the nutritional value of protein has little effect. However, it has little effect on the nutritional value of the protein.
Freezing denaturation of proteins is mainly caused by changes in the dispersed density of protein plasmas, due to the lowering of the temperature, ice crystals gradually formed, so that the hydration film of proteins is weakened or even disappeared, and the protein side chains are exposed, and at the same time, due to the extrusion of the ice crystals, protein plasmas are combined close to each other, resulting in the agglutination of protein plasmas and precipitation. The degree of denaturation of proteins under freezing conditions is related to the freezing speed. Generally speaking, the faster the freezing speed, the smaller the ice crystals, the smaller the extrusion effect, the smaller the degree of denaturation. Rapid freezing can be used according to this principle to avoid protein denaturation and maintain the original flavor of the food.
Changes under dehydration
The purpose of food dehydration is to extend the shelf life of food, reduce the weight of food as well as increase stability, but at the same time there will be unfavorable reactions occur, when all the water in the protein solution is removed, due to protein-protein interactions, resulting in a large number of protein aggregation, especially at high temperatures to remove the water will lead to protein solubility and surface activity of the Decrease. The effect of drying conditions on the size of the powder particles as well as on the internal and surface porosity changes the wettability, water absorption, dispersion and solubility of the protein. Drying is usually the last process in the preparation of protein ingredients, and attention should be paid to the effect of drying treatment on the functional properties of proteins.
There are many dehydration methods commonly used in the food industry, and different drying methods cause different degrees of protein changes:
(1) Traditional drying method. To natural warm air drying, dehydration of meat, fish will become hard, shrinkage and poor recovery, cooking feel tough and no original flavor.
(2) Vacuum drying. This method has less damage to the quality of meat, because there is no oxygen, so the oxidation reaction is slower, at low temperatures can also reduce non-enzymatic browning and other chemical reactions.
(3) Freeze-drying. Freeze-drying when the food is frozen, under low pressure so that the water is removed by direct sublimation of ice. Freeze-dried food can maintain the original shape and size, has a porous nature, has a better recovery. Freeze-drying is the best method of meat dehydration, but will still make part of the protein deterioration, meat tough, poor water retention, but its essential acid content and digestibility and fresh meat is not much difference.
(4) Spray drying. Eggs, milk dehydration commonly used spray drying method, the liquid will be sprayed in mist into the fast-moving hot air, resulting in the next granular, this method of protein damage is less.
(5) Drum film drying is the raw material is placed on the surface of a steam-heated rotating drum, dehydration into a film, often because it is not easy to control the proper and make the product slightly burnt flavor, the solubility of proteins is also reduced.
Changes under irradiation treatment
The method of irradiation to preserve food has been adopted by many countries, but different foods and different purposes of irradiation require different doses of irradiation. High doses (10-50 kGy) can sterilize meat or meat products; medium doses (1-10 kGy) can prolong the shelf-life of refrigerated fresh fish, chickens, fruits, and vegetables; and low doses (<1 kGy) can prevent the germination of potatoes and onions, delay the ripening of fruits, and kill cereals, peas and vegetables. ripening, and killing insects in cereals, peas, and kidney beans, among others.
When a substance absorbs ionizing radiation, it first forms ions that react with excited molecules and ions degrade or react with neighboring molecules causing chemical bonds to break and produce free radicals, which can bind to each other or diffuse into the bulk medium to react with other molecules. The effect of irradiation on proteins is also related to water content, oxygen, pH, temperature, and irradiation dose, etc. In general, radiation has little effect on the nutritional value of amino acids and proteins.
Changes under alkali treatment
Concentration, separation, bubbling, emulsification, or the linking of proteins in solution into fibers often depends on alkali treatment, and alkali treatment of food, especially when carried out at the same time as heat treatment, has a great effect on the nutritional value of proteins.
After alkali treatment, proteins can undergo many reactions to generate a variety of new amino acids, such as those leading to the formation of various lysyl aminoalanine, lanolinothionine, ornithine, and inter- or intramolecular covalent cross-linking bonds, which are generated by the condensation of residues such as lysine, cysteine, or ornithine, etc., with aminoacrylic acid residues.
Under alkaline heat treatment, amino acid residues will also undergo isomerization, from L-type to D-type, the nutritional value is reduced; at the same time, the functional properties of proteins will be changed, such as the use of moderate alkaline pH, to promote the dissociation of oligomeric proteins, and then prepared by spray drying of sodium caseinate salt or soy protein salt, which has a high degree of solubility, and has good water absorption and surface properties. This depolymerization method can be used to solubilize and extract insoluble plant proteins, microbial proteins or fish proteins.
Changes under oxidative treatment
Proteins are often in contact with lipids during food processing, and auto-oxidation of lipids produces hydroperoxidation, peroxyl radicals and oxidation products that can oxidize and cross-link with protein side-chain moieties. The occurrence of protein oxidation reaction leads to the reduction of the nutritional value of proteins and even produces harmful substances.
Photo-oxidation of sulfur-containing amino acids can easily occur under aerobic and light conditions, especially if there are also natural photosensitizers in the food; polyphenols are present in many plants, which are easily oxidized to quinones under neutral or alkaline pH conditions, and when they come into contact with proteins, the reaction of protein residues being oxidized occurs; hot-air drying and the blasting of air in the fermentation process in the food can also lead to amino acid oxidation.
Changes under Mechanical Treatment
Mechanical treatments have a large effect on proteins in food, e.g., sufficiently dry milled protein powders or concentrates can form small particles and large surface areas, which can improve their water absorption, proteolysis, fat absorption, and effervescence when compared to those that have not been milled finely, and mechanical forces also play an important role in the weaving process of proteins.
Protein suspensions or solution systems can be subjected to strong shear forces that can cause protein aggregates to split into subunits, which can improve protein emulsification. Applying shear at the air interface usually causes denaturation and aggregation of proteins, and partial denaturation of proteins can make the foam more stable.
Changes under enzymatic treatment
Enzymatic treatment is the current research focus of protein modification. Enzymatic treatment has the advantages of fast enzymatic reaction speed, mild conditions, strong specificity, no amino acid destruction or racemization, complete preservation of active ingredients in raw materials, no by-products and harmful substances, no environmental pollution, and controllable process of action. At present, the enzyme treatment has enzyme hydrolysis method and enzyme synthesis method, with the former as the main method.
Enzymatic treatment is the use of protease endocytosis and exocytosis, the degradation of protein molecules into peptides and smaller amino acid molecules in the process, the physical and chemical properties of the product than the original protein has changed. Protein enzymatic processing can be targeted to improve the functional properties of protein processing, the main factors affecting enzymatic digestion are: enzyme properties, pH, the denaturation range of proteins, substrate concentration, enzyme concentration, ion concentration, temperature, inhibitors, etc., of which the enzyme properties are the key factors, which affects the site and region of the peptide chain of protein enzymatic digestion.