In fruits and vegetables, enzymatic browning occurs due to damage, such as bruising, and preparation operations (cutting, peeling, etc.). The resulting pigments (from tan to black) can appear very quickly, giving the product an unappetizing appearance. In intact tissue, the enzymes responsible for this, called phenolases because of their origin, are separated from the substrate, but when they come into contact as a result of damage, the naturally occurring phenolic compounds are enzymatically oxidized to give yellowish quinone compounds . This is followed by a series of polymerization reactions, resulting in the formation of brownish products (such as melanins).
The degree of darkening depends on the activity and amount of polyphenol oxidase in a particular fruit or vegetable and the presence of substrates, which can be, in particular, catechol, tyrosine or dopamine (oxygen is always necessary). A number of approaches have been used to prevent or slow down enzymatic browning. A decrease in the concentration of available oxygen was achieved in various ways: vacuum packing, which slows down the enzymatic darkening of potato strips ; packaging in the CWG, for example, for chopped salad and carrots ; the addition of an oxygen scavenger to the package, which slows down the enzymatic darkening and texture changes in the halves of apricots and peaches ; limiting the diffusion of oxygen into tissues by immersion in water, brine or syrup. High oxygen levels (70–100%) have been shown to reduce the destruction of ascorbic acid, lipid oxidation, and enzymatic browning in chopped lettuce (probably due to an increase in the overall antioxidant capacity of the material) . A more direct method of preventing enzymatic color changes is the use of enzyme inhibitors, although it may disrupt the image of a “fresh” product or be limited by law. The traditional use of sulfite by immersion in a metabisulfite solution in many cases has provided an effective means of preventing enzymatic browning. The introduction of restrictions on the use of sulfite stimulated the search for alternatives. The optimal pH for phenolase activity usually lies between pH 5 and 7. Lowering the pH below 4 with food acids inactivates this enzyme. Baths with citric or ascorbic acid slow down the darkening both by lowering the pH and by forming copper complexes necessary for the functioning of the enzyme. It was shown that the level of 10% ascorbic acid is effective for potatoes, and 0,5-1% for apples . Phenolases from most fruits and vegetables are easily inactivated by heating , but for salads and pre-cooked vegetables, heat treatment may be unacceptable due to concomitant changes in color and texture.
Glycolysis is a key metabolic pathway for intermediate metabolism found in almost all living organisms. Changes that occur during cattle slaughter and harvesting affect the path that the substrates assimilated along this path subsequently follow. Deviation from this path to produce lactic acid in meat and ethanol in vegetables significantly affects the quality of the food product subsequently.
Adenosine triphosphate (ATP) is consumed continuously by a living cell to maintain its structure and functions. It is produced as a result of glycogen metabolism through glycolysis and the Krebs cycle (citric acid cycle). With slaughter, the flow of blood, and therefore, the supply of oxygen to the muscles, ceases, but glycolytic activity continues using reserves in muscle cells. Glycogen is converted to pyruvate, but under anaerobic conditions, the Krebs cycle no longer functions, and pyruvate is reduced by the reduced form of nicotinamide adenine dinucleotide (NADH) to lactic acid. The intake of NADH is supported by glycolysis, which makes it possible to continue the conversion of glycogen to lactic acid until the glycogen stores are depleted. The destruction of each muscle glycogen glucose molecule leads to the formation of two lactic acid molecules. The accumulation of lactic acid gradually lowers the pH in the muscles, and this process ends when the muscle glycogen supply is depleted and the pH is approximately 5,5-5,6. When ATP is no longer produced, muscle fibers become stiff; this condition is known as rigor mortis. If there is a sufficient supply of glycogen during slaughter, the rate and degree of pH drop depends on the activity of key glycolytic pathway enzymes, competing reactions involving adenosine diphosphate and temperature. The lower the temperature, the longer it takes to reach the pH limit, as biochemical reactions slow down. The rate of fall and final pH can have a significant effect on meat quality . A decrease in muscle pH leads to protein denaturation and the release of a pink protein-containing fluid (called "drip" from the English. Drip - dew). A decrease in the rate of accumulation of lactic acid by rapid cooling of the carcass can dramatically reduce losses due to the release of this liquid [103,106], however, rapid cooling to temperatures below 12 ° C until cessation of anaerobic glycolysis leads to a condition called cold contraction, and the meat becomes stiff .
Animals that were depleted at the time of slaughter have depleted glycogen stores and produce less lactic acid during rigor mortis. Pork, which has a hardening pH of more than 6,0-6,2, is a "dark hard dry" meat (DFD), which spoils due to the action of microorganisms for 3-5 days due to the high pH. Animals that were stressed at the time of slaughter to such an extent that their breathing becomes anaerobic can reach the rigor pH within 1 hour after slaughter. Pork, whose pH drops to 5,8 within 45 minutes after slaughtering, is “pale, friable, watery” meat (PSE). It is characterized by excessive losses due to fluid and pallor due to the release of fluid from the membranes and the denaturing of proteins. The shelf life of such meat is reduced due to the increased growth of microorganisms and the oxidation of phospholipids.
Protease activity may, depending on the situation, have a positive or negative effect.
Meat proteases play an important role in the loss of stiffness that occurs after rigor mortis and is called relaxation (autolysis). Traditionally, it begins in a slaughterhouse and should flow until the meat is soft and acceptable to the consumer. Ideally, it takes 2-3 weeks at low temperature, but uncooled carcasses lose their stiffness faster, since proteases act faster at higher temperatures. In beef, the relaxation rate increases with temperature up to 45 ° С (210–2,4), then it goes at a lower speed up to 60 ° С . The role of proteases in autolysis has been considered in a number of works [33,87]. Meat proteases can be classified based on their preferred pH level. Proteases active at acidic pH (for example, cathepsins) are found in small organelles - lysosomes located on the periphery of muscle cells. The stability of lysosomes decreases with decreasing pH, while it becomes possible for proteases to enter the cell and, ultimately, into extracellular spaces. It is believed that a protease that is active at neutral pH and participates in autolysis is calpain (CaIprag I), which requires free calcium ions to function. When rigor mortis occurs in meat, the absence of ATP as an energy source for “pumping out” calcium ions from cells leads to an increase in the level of free calcium and the creation of favorable conditions for protease activity. The duration of rigor mortis depends on the type of animal and is about 1 day for beef, half a day for pork, and 1-2 hours for chicken. The reasons for these differences are not entirely clear. In chicken and pork, which autolize rather quickly, cathepsin levels are higher , while in beef myofibrillar structure is more resistant to cathepsin . We have yet to understand in more detail which proteases are involved in these processes and what are the conditions that determine their activity.
In cheese making, the addition of proteases found in rennin and the culture of sourdough to milk during maturation causes the formation of a specific taste and texture. Chymosin (aspartyl protease contained in renin) cleaves a single peptide bond in k-casein, a protein of milk, which leads to its coagulation. The combination of the action of chymosin and protease culture of the starter culture decomposes casein to peptides. Many of these peptides may have a bitter or sour taste (or no taste at all), but intracellular proteases from starter cultures break down the peptides further to small amino acids and peptides that have the ability to improve taste.
The opposite result of protease activity may be the bitter taste of dairy products. Peptides, consisting predominantly of non-polar amino acids, are sometimes bitter. It is very likely that under conditions conducive to proteolysis and the accumulation of intermediate products of peptide decomposition, fermented dairy products will turn out to be bitter.
In fish, proteases cause an effect known as abdominal rupture. Intensive nutrition before catch increases the concentration and activity of digestive tract enzymes. If the fish is not gutted or cooled shortly after catching, the activity of proteases weakens the intestinal walls, which makes it possible for the contents to enter the surrounding tissues. Herring and mackerel are very susceptible to this process, and herring may be unsuitable for smoking in a day. In crustaceans (such as lobsters and shrimps), this process proceeds even faster - intestinal enzymes destroy the meat within a few hours after catching, which requires quick cooling and processing.
The hydrolysis of triglycerides at the oil-water interface is catalyzed by lipase (Fig. 9.5). The specificity (selectivity) of lipases is different: some of them can destroy esters in triglycerides in all three positions, and some only in positions 1 and 3.
The activity of lipases of both endogenous and microbiological origin leads to changes in the functional properties of some dairy products - for example, to a deterioration in the ability of milk to skim when receiving skim milk and to a deterioration of cream cream, and also, especially important, to the appearance of a soapy or rancid taste. Usually a soapy flavor is associated with long chain fatty acids, and an unpleasant rancid taste is associated with short chain fatty acids; for example, the smell of valerianic acid is described as “the smell of sweaty feet,” and hexanoic acid is described as “goat's.” The aromatic threshold of these compounds is usually low (for example, 14 ppm for hexanoic acid), and therefore even very weak lipolytic activity can significantly affect the quality of the product.
The release of 1-1,5% fatty acids from triglycerides into milk can make it unpleasant in taste (Table 9.2). If lipolysis occurs before the heat treatment of milk and the total number of viable microorganisms is less than 106 / ml, it is most likely that endogenous milk lipases are the cause of this phenomenon. Changes in the taste / smell of milk due to endogenous lipases are rare. Endogenous lipases are denatured during pasteurization, but extracellular lipases secreted by psychrotrophic bacteria (such as Pseudomonas subspecies) are heat resistant. They tolerate pasteurization and (in some cases) short-term processing at high temperature (HTST). Since psychrotrophic organisms can grow at 2-4 ° C, that is, at the storage temperature of milk or cream in tanks, a significant level of lipase can be achieved. Heat-resistant lipases may take weeks to affect the quality of the product, and they are usually more important for the quality of products stored at ambient temperature or for products with a long shelf life.
In cheese making, hydrolysis may be required to obtain the desired taste / odor due to the activity of lipases in rennet . Almost all cheeses with strong
Fig. 9.5. The action of lipase on triglycerides
Table 9.2. Concentrations of free fatty acids in dairy products and threshold flavoring values (according to )
|Product||The concentrations of free fatty acids (mg eq. / G of fat)|
|normal||you may experience problems|
|Powdered milk||0,3 – 1,0||1,5 – 2,0|
|Ice cream||0,5 – 1,2||1,7 – 2,1|
|Butter||0,5 – 1,0||2,0|
taste / aroma (Stilton, Roquefort, Gorgonzola, Parmesan) this taste / aroma depends on free fatty acids. To obtain the exact proportion of fatty acids that determine the desired taste / aroma, when using microbial proteases to replace rennet starter, it is necessary to add lipases with appropriate selectivity. Problems associated with achieving the required selectivity and the required amount of enzyme have been identified for cheddar cheese, but differences in the content of fatty acids, giving a normal and rancid cheddar, may occur despite very slight differences in the lipase content .
Features of physical and chemical reactions
Physico-chemical reactions affecting the quality of chilled products occur as a result of physical changes in the product or subsequent chemical or biochemical reactions. So, this category includes the migration of components as a result of diffusion or osmosis and the absorption of light by natural or artificial pigments.
Some of the most important physical and chemical reactions in the refrigerated products
migration of components
In mayonnaise-based salads, such as coleslaw (raw cabbage, carrot and onion salad) and potato-based salads, the observed quality changes are organoleptic changes associated with the distribution of oil and water between mayonnaise and plant tissue . In the case of coleslaw, an increase in extractable solids from cabbage by 13,5% and an increase in the transparency of cabbage indicated that it absorbed oil from mayonnaise within 6 hours after mixing . In mayonnaise, a change in the oil content was expressed in an increase in the polydispersity of the size of the fat globules. In addition, the migration of water from cabbage to mayonnaise, due to the difference in osmotic potential, led to the fact that during the same time during which cabbage became more transparent, mayonnaise became liquid and non-enveloping. Studies of differences between cabbage varieties in terms of oil absorption showed that during storage of Dutch cabbage, there was no change in the estimation of mayonnaise in terms of creaminess-oiliness, while fresh English cabbage showed a significant decrease in this parameter. Other ingredients with a large difference in the osmotic potential relative to mayonnaise (for example, celery and raisins) can also cause problems due to moisture migration leading to the separation of water in the form of droplets on the surface of the mayonnaise.
One of the most well-known quality problems associated with the migration of water is the “wetting” of sandwiches. To provide a barrier to moisture, its migration from the filling to bread can be reduced at the interface by using fat-based pastes . In products based on dough products (for example, pies and pizza), moisture migration from fillings and decoration into a flour product causes similar problems. The migration of moisture or oils may be accompanied by the migration of dyes dissolved in them. For example, in a pizza filling where the cheese and sausage come into contact, red stripes become visible on the cheese, and in multilayer cakes, color migration between layers can worsen their appearance (if you do not use the appropriate coloring method). The migration of enzymes from one component to another (for example, when sliced unplaned vegetables come into contact with dairy products) can lead to problems with taste and aroma, color or texture (depending on the enzymes and substrates used) .
Many chilled products are sold unpacked directly from the windows (especially for cooked fresh meat, fish, pastes, pies and cheese). The shelf life of such products differs significantly from their wrapped counterparts - 6 hours compared with several days and weeks. The most common reason for this reduction in shelf life is loss due to evaporation, leading to a change in appearance to such an extent that the consumer chooses the last products laid out in the display case. In practice, the laying-out time of uncooked meat products is determined by surface color changes that can make the product look unattractive. Changes in appearance are associated with losses in mass due to evaporation (table. 9.3). The direct costs of evaporation losses from unpacked products in refrigerated display cabinets were, according to some estimates, in 1986 over 5 million pounds . In stores with a high turnover rate, the average weight loss will be higher due to the fact that freshly moistened surfaces are constantly exposed to air flow.
Losses of mass from the surface of non-wrapped products depend on the rate of evaporation of moisture from the surface and the rate of diffusion of moisture from the product. Temperature, relative humidity and air velocity are factors that significantly affect
Table 9.3. Weight loss for evaporation from the open surface of the cut beef and its corresponding appearance when displayed in a display case for 6 hours (according to )
|evaporation loss (g / cm)||Changing the appearance of the surface|
|Until 0,01||Red, attractive and still moist; may lose some brightness|
|0,015 – 0,025||The surface becomes drier; still attractive but darker|
|0,025 – 0,035||Clearly noticeable darkening; It becomes dry and rough|
|0,05 – 0,10||Black|
weight loss. Weight loss during storage of fruits and vegetables occurs mainly due to evaporation. Most products have an equilibrium moisture content of 97-98%, and if they are stored at lower humidity, they lose moisture. For practical reasons, the recommended storage humidity range is 80–100% . The rate of moisture loss depends on the difference in the pressure of water vapor created by the product and the pressure of water vapor in the air, as well as the speed of air above the product. A loss of only 5% moisture by weight leads to wrinkling or withering of fruits and vegetables. With increasing air temperature, the amount of water needed to saturate it increases (approximately doubling for every 10 ° С rise in temperature). If products are placed in hermetically sealed containers, they will lose or gain moisture until the humidity in the container reaches the value characteristic of this product at a given temperature. If the temperature rises and the amount of water vapor in the atmosphere does not change, the air humidity drops. To prevent moisture loss in such a situation, it is very important to minimize temperature fluctuations.
Damage during cooling
Although storing fruits and vegetables at low temperatures is considered the most effective method of preserving the quality of perishable foods of plant origin, it can be more beneficial for sensitive crops. Most tropical and subtropical fruits and vegetables are damaged when exposed to low temperatures above freezing temperature (10-15 ° C) . Some temperate fruits and vegetables are also prone to damage, but at lower threshold temperatures (below 5 and up to 10 ° C) .
Damage due to cooling is indicated by a number of signs that adversely affect quality. The appearance of "smallpox" pit is caused by dehydration and low temperatures. This phenomenon is most pronounced in mangoes, avocados, grapefruit and lime, in which the outer shell is harder and denser than the inner layers. Surface discoloration is common in fruits with a thin soft peel (for example, sweet pepper, eggplant and tomatoes). Uneven or incomplete ripening occurs in tomatoes, melons and bananas. Most often, internal tissue destruction and weakening makes fruits or vegetables susceptible to damage due to post-harvest pathogens. If temperatures are well below the critical level, damage due to cooling can occur within a fairly short time. In some cases, signs of damage can develop and become apparent only after the products have been removed from the place of refrigerated storage and when stored at warmer temperatures, which makes it difficult to determine if damage has occurred precisely due to exposure to low temperatures.
Although changes in the physical structure that occur during injuries due to low temperatures are described, their relationship with the development of signs of damage in most cases has not been established. Changes in the lipid structure and composition of membranes , changes in the structure of the cell cytoskeleton, and conformational changes in some regulatory enzymes and structural proteins leading to a loss of compartmentalization within cells were reported. As a result, changes in plant physiology occur, including loss of membrane integrity, leakage of solutions, stimulation of ethylene formation  and a sharp increase in respiration .
Methods to mitigate the effects of damage caused by cold, significantly depend on the type of fruit or vegetables . The most obvious way is to avoid exposure to low temperatures on fruits that are sensitive to low temperatures. However, as already noted, cooling provides a means of reducing the rate of respiration, evaporation and transpiration, thereby increasing the shelf life. In some cases, heat treatment is effective - for example, conditioning before storage at temperatures immediately above the temperature threshold (acclimatization) (used for cucumbers and bananas), periodic warming during storage (used for apples and stone fruits) or storage at ambient temperature for short time before refrigerated storage. It has been shown that in some cases storage in the CSG is useful - for example, for avocados , peaches  and okra (okra, gumbo, gombo) , but it is believed that it exacerbates cold damage, creating an additional load on products due to a decrease in the concentration of oxygen and a high concentration of carbon dioxide .
It has been shown that chemical treatment is effective for some fruits and vegetables. Since damage to the cold is caused by changes in the structure of the membranes, treatment was applied aimed at changing the components of the cell membranes or protecting them. Processing tomato seedlings with ethanolamine increases the levels of unsaturated fatty acids included in membrane phospholipids, which reduces damage to cellular components during cooling . Free radical scavengers or antioxidants (ethoxikine and sodium benzoate, diphenylamine and butylated hydroxytoluene) have been shown to be effective for cucumbers, sweet peppers  and apples . Coating fruits with solid hydrocarbons (wax, paraffin) or oils (provided that they are approved for use in food) before cooling is effective because it prevents moisture loss and reduces the amount of oxygen that can be used for oxidation. It was shown that the introduction of benomyl or thiabendazole (TB2) fungicides into such a coating provides additional advantages for peaches and nectarines . The maximum objective to mitigate damage caused by cooling is to select, breed or produce genetically engineered fruits and vegetables to eliminate their sensitivity to cooling. To create targeted genetic engineering programs aimed at solving this problem, a better understanding of the mechanisms of Cold Damage is needed, but it is unlikely that the consequences of such damage caused by various causes will be overcome with the help of universal solutions.
The release of fluid or the slow spontaneous movement and separation of fluid from a colloidal semi-fluid mass is called syneresis. It occurs as a result of physico-chemical changes in carbohydrates or proteins, affecting their ability to retain moisture.
In food products, starch performs several necessary functions - it thickens, gels and stabilizes emulsions, regulates the migration of moisture and affects the texture. Natural starches and flour have a limitation in their insufficient stability at low temperatures and with changes in temperature. At low temperatures, starches become susceptible to fluid evolution or syneresis.
Natural starch is a complex carbohydrate consisting of homopolymers, amylose and amylopectin. Amylose is a linear chain molecule consisting of 1,4-a-£) -glucopyranose blocks. Amylopectin has the same main chain as amylose, but 1,6-bridges make its structure branched, and therefore its hydration ability is greater than that of amylose. Thus, the ratio of amylopectin to amylose affects the properties and texture of starch. For example, wheat starch (a traditional thickener used in various sauces) gives the desired taste and opacity, but it does not have low temperature stability, and its cooling leads to syneresis. When the starch grains swell, the linear amylose molecule dissolves and reassociates into aggregates bound by hydrogen bonds. The newly formed amylose pushes water, which leads to an increase in transparency and syneresis. Cooling and freezing leads to constriction of the overall structure, significantly increasing the rate of syneresis.
Syneresis problems often arise as a result of the wrong choice of starch type. The introduction of stabilized corn starches, which harden at low temperatures, in products that need to be cooled, increases their resistance to spoilage and syneresis. Another possible way to prevent syneresis is to use starches specially modified with monofunctional blocking groups to prevent compounds between molecules of dissolved amylose. The use of modified starch in combination with wheat flour ensures the stability of the final product.
Syneresis in milk is known as separation of serum. It is necessary for cheese making, but in dairy products such as yogurt, it is not desirable. Homogenization of milk to produce yogurt reduces syneresis, increasing hydrophilicity and ability to hydrate by enhancing the interaction of casein and fat in the membrane of fat globules, as well as other protein-protein interactions . The heat treatment for the production of yogurt (85 ° C for 30 minutes, or 90-95 ° C for 5-10 minutes) is unique to dairy production. It is believed that it leads to important changes in the physicochemical structure of proteins that minimize syneresis, and allows to achieve maximum clot density.
The market of sandwiches with a wide range of fillings requiring refrigerated storage has grown significantly, but the staling of bread is one of the few reactions with a negative temperature coefficient , that is, bread at lower temperatures becomes stale faster . The term “stale” as applied to bread is used to describe the increase in the density of the crumb and the rigidity of its texture, the loss of friability and increase in the rigidity of the crust, as well as the disappearance of the taste and aroma of fresh bread and the appearance of the taste and smell of stale bread. Despite extensive studies of the mechanism of staling, most researchers are willing to agree only with the fact that density changes are due to physicochemical reactions of the starch component (mainly due to the amylopectin contained in it); some researchers note the participation of flour proteins in this process.
The shelf life of industrially produced bread is considered equal to two days , and this shelf life decreases when stored at low temperature. It is believed that the speed of stale bread reduces the use of packaging in CWGs, especially in carbon dioxide .