2. When the temperature drops below freezing point, the water in the microorganism and its surrounding medium is frozen, which increases the cytoplasm viscosity, electrolyte concentration, pH value and colloid state of the cell, leading to cell degeneration and cell membrane damage due to the mechanical action of freezing. These changes in internal and external environment are the direct cause of microbial metabolic activities being blocked or dying. Cold storage of meat refers to lowering the temperature of the deep part of the product to about 0 ~ 65438 0℃, and then storing it at about 0℃. Due to the activity of low-temperature bacteria, the shelf life of chilled meat is not long. Generally, pork can be stored for about 1 week. In order to prolong the shelf life of chilled meat, the deep temperature of the product can be reduced to about -6℃. However, due to the different types of raw materials, the conditions of cooling treatment are also different.
1. cooling method
Before each feeding, the temperature in the cooling chamber should be lowered to -2 ~-3℃ in advance, and it should be cooled for about 14 ~ 24h after feeding. When the meat temperature reaches about 0℃, the temperature in the cooling chamber is kept at 0 ~ 65438 0℃. Under the condition of natural circulation with air temperature of about 0℃, the cooling time of pig and cattle carcasses and by-products is 24 hours, that of sheep carcasses is 18 hours and that of poultry is 12 hours.
2. The storage of chilled meat and the change of storage period.
The storage of chilled meat refers to the storage of chilled meat at about 0℃. The purpose of cold storage of chilled meat, on the one hand, can complete the ripening process of meat, on the other hand, it can achieve the purpose of short-term preservation. Short-term processed meat should not be frozen and refrigerated. Because frozen meat and thawed meat, even under good conditions, the dry consumption and gravy loss after thawing are greater than that of chilled meat.
(1) Refrigeration conditions The time for meat to be refrigerated in a cooled state depends on the temperature and humidity of the refrigeration environment. According to the recommendation of the Fourth Committee of the International Institute of Refrigeration, the cold storage conditions and duration of frozen animal meat are shown in table 1-4- 1.
Meat should be kept at constant temperature and humidity during cold storage, and the air velocity should be 0.1~ 0.2m/s.
(2) Changes of meat during cold storage Meat and poultry refrigerated at low temperature, due to the action of microorganisms, make the surface of meat sticky, moldy, soft, discolored and smelly.
Sticky and moldy: Sticky and moldy is the most common phenomenon in frozen meat. When the temperature is 0℃, when the initial number of bacteria contaminated on the meat surface is 0/00 per square centimeter/kloc-0, it becomes sticky at 16d. At 10, it only takes 7 days to be sticky. With the increase of temperature, the viscosity time is obviously shortened.
The humidity of air also has a great influence on viscosity. When the temperature is kept at 4℃, the relative humidity is reduced from 100% to 80%, and the time of forming viscosity is prolonged by 1.5 times. Table 1-4- 1 Refrigeration temperature and shelf life of chilled meat (℃) Relative humidity (%) Estimated shelf life (d) Beef meat
calf
mutton
pork
cured meat
bacon
Edible non-staple food
Take out the dirty chicken-1.5 ~ 0
- 1~0
- 1~0
- 1.5~0
-3~ 1
- 1~0
- 1~0
0 90
90
85~90
85~90
80~90
80~90
75~80
85~90 28~35
7~2 1
7~ 14
7~ 14
30
120~ 180
three
7 ~ 1 1 meat color change: under the condition of low temperature and low humidity, the muscle can keep bright red for a long time. When the humidity is 100%, the muscle turns brown at 16℃ in less than 2 days. At 0℃, it can be extended by more than 10d; If the temperature is the same, under the conditions of 4℃ and humidity of 100%, the bright red can be kept for more than 5 days, and it can be shortened to 3 days when it is 70%.
The high velocity of air will promote the dry consumption of meat surface, thus promoting the oxidation of meat. In order to improve the refrigeration effect, modified atmosphere refrigeration has been applied in the field of meat refrigeration.
In addition, a few will turn green, yellow, cyan and so on. This is a special phenomenon that protein decomposes due to the propagation of bacteria and molds.
Dry consumption: meat is refrigerated, and the initial dry consumption is large. As time goes on, the dry consumption per unit time decreases. The cold storage period exceeds 72 hours, and the daily weight loss is about 0.02%. In addition, the dry consumption during cold storage is related to air humidity. With the increase of humidity, dry consumption decreases.
(3) Methods of prolonging the storage period of chilled meat The methods of prolonging the storage period of chilled meat include applying CO2, antibiotics, ultraviolet rays, radiation and ozone, and using gaseous nitrogen instead of air medium. The practical application in 20 1 1 is as follows:
CO2: At low temperature, when the CO2 concentration is 10%, the growth of mold on meat will be slow; 20% will stop mold activity. The solubility of CO2 is very high, which increases with the decrease of temperature, and it can also penetrate the cell membrane well. Meat fat, protein and water can absorb CO2 well. Therefore, in a short time, the concentration of CO2 can be increased enough to inhibit the growth of microorganisms, not only on the surface of meat, but also in deep tissues. Due to the high solubility of CO2 in fat, the oxygen content in fat is reduced, thus delaying the oxidation and hydrolysis of fat. The storage period of chilled meat at 0℃ and CO2 concentration of 10% ~ 20% can be prolonged by 1.5 ~ 2.0 times (longer than that in nitrogen).
The disadvantage of CO2 method is that when the concentration exceeds 20%, the color of meat becomes darker because of the combination of CO2 with hemoglobin and myoglobin. In addition, CO2 storage requires a storage chamber with a special structure.
Ultraviolet irradiation: The conditions of ultraviolet irradiation on chilled meat are: air temperature 2 ~ 8℃, relative humidity 85% ~ 95% and circulating wind speed 2m/min. The shelf life of chilled meat irradiated by ultraviolet rays can be doubled.
The disadvantage of ultraviolet irradiation is that it can only sterilize the surface layer of meat; Radiation will make some vitamins (such as vitamin B6) ineffective; Due to the changes of myoglobin (Mb) and hemoglobin (Hb) and the transformation of oxygenated myoglobin (MbO2) into methemoglobin (MeTMb), the surface of meat is dark. Due to the formation of ozone, the oxidation process of fat is significantly enhanced; It is difficult for the carcass to be evenly irradiated; Ultraviolet rays are harmful to human eyes and skin. When the temperature is above freezing point, it can only inhibit the activities of enzymes and microorganisms and various changes of meat to a certain extent, but can not stop their activities. Therefore, meat can only be stored for a short time after cooling. If the meat is to be preserved for a long time, it needs to be frozen, that is, the temperature of the meat is reduced to below-18℃, and most of the water in the meat (above 80%) forms ice crystals. This process is called meat freezing.
1. Frozen pretreatment of meat
The treatment before freezing can be roughly divided into three ways: ① the carcass is cut in half and then packaged and frozen directly; (2) carcass segmentation, deboning, packaging, boxing and freezing; ③ The corpse was divided, boned and then frozen in a freezer.
2. Freezing method
The general freezing point of meat is-1.7 ~-2.2℃. When this temperature is reached, the water in the meat begins to freeze. In the freezing process, the supercooled state is completed first. The phenomenon that the temperature of meat drops below freezing point without freezing is called supercooling. In the supercooled state, only nearly crystalline aggregates are formed. This state is very unstable. Once it is destroyed (the temperature is reduced to the point where ice cores or vibrations begin to appear), it will immediately release latent heat and convert it into ice crystals, and the temperature will rise to freezing point and the ice crystals will precipitate. The temperature at which stable crystal nuclei form during cooling, or the lowest temperature at which they begin to rise, is called critical temperature or supercooling temperature. The supercooling temperature of livestock and fish is -4 ~-5℃. When meat is at supercooled temperature, water precipitates to form stable aggregates, then rises to freezing point and begins to freeze.
The crystallization of gravy during freezing is mainly composed of pure water in gravy. In which soluble substances are concentrated in the remaining liquid phase. With the freezing of water, the freezing point drops. When the temperature drops to -5 ~- 10℃, about 80% ~ 90% of the water in the tissue has frozen into ice (Table 1-4-2). The temperature before this is usually called the maximum ice crystal formation zone. With the continuous decrease of temperature, the freezing point is also decreasing. When it reaches the ice crystal point of gravy, all the water freezes into ice. The ice crystal point of gravy is -62 ~-65℃. Table 1-4-2 Freezing temperature of meat and freezing rate of water in gravy (℃)-1.5-2.5-5-7.5-10/7.5-20-25-32.5 Freezing rate (%)
The freezing speed in general production is often distinguished by the required time. For example, the semi-carcass of medium-fat pig is frozen from 0 ~ 4℃ to-18℃, and the quick freezing time is less than 24 hours; 24 ~ 48h for medium-speed freezing; If it exceeds 48 h, it is slow freezing.
The freezing process of meat is that the water between muscle cells first freezes and supercooled, and then the water in the cells freezes. This is because the vapor pressure between cells is lower than that in cells, the salt concentration is also lower than that in cells, and the ice is higher than the freezing point in cells. Therefore, intercellular water first forms ice crystals. Subsequently, the concentration of the solution near the crystal increases, and the water in the cell permeates outside the cell through the action of osmotic pressure. Around the ice crystals, the ice crystals grow up and become large ice particles. Until the temperature drops, the liquid in the cell is frozen into ice crystals.
Quick freezing and slow freezing have different effects on meat quality. In the process of slow freezing, the maximum ice crystal formation zone (-1 ~-5℃) stays for a long time, and a large amount of water in the fibers seeps out of the cells, which increases the concentration of intracellular fluid bodies and lowers the freezing point, resulting in larger and larger ice crystals between muscle fibers. When water turns into ice, its volume increases by 9%, causing mechanical damage to muscle cells. This kind of frozen meat is almost irreversible when thawed, resulting in a large amount of gravy loss. So slow freezing has a great influence on meat quality; During quick freezing, the temperature drops rapidly, and water passes through the largest ice crystal formation zone quickly, so the redistribution of water is not obvious. The formation speed of ice crystals is faster than the diffusion speed of water vapor, and the time of staying in supercooled state is short, so the ice crystals move faster from the surface to the center. The result is that the water inside and outside the cell freezes almost at the same time, and the formed ice crystal particles are small and uniform, which has little influence on meat quality, great reversibility during thawing and less juice loss.
The best freezing time of meat depends on the biochemical changes of meat after slaughter. Frozen before, during and after rigor mortis, the quality of meat and the loss of gravy are different. Frozen before rigor mortis, due to the high content of ATP, glycogen, creatine phosphate and actin, low content of lactic acid and glucose, high pH value, no plasma separation on the muscle surface, tight myofibrils, neat myofibrils and clear transverse stripes, when frozen quickly, a small amount of ice crystals are formed and exist in cells. When thawing slowly, the reversibility is great and the gravy loss is less. However, rapid thawing will cause a large loss of juice.
Frozen before rigor mortis and stored for a short time, the meat quality lacks firmness and flavor when thawed, and needs to be improved when matured after thawed.
Frozen in the state of rigor mortis is easy to cause the loss of gravy because of the low water retention capacity of meat. By comparing the quality of frozen meat at different times, Xiwei thinks that 1d frozen meat is the best after slaughter, and 3d frozen meat is better, and the quality will decline in the later stage. Frozen after thawing, because the water retention capacity is partially restored, the hardness is reduced, and the gravy is less lost. It is easier to separate after thawing than zombie meat.
4. Freezing method
The freezing process includes primary freezing and secondary freezing.
(1) Fresh meat frozen for the first time is directly sent to the freezing room for freezing without cooling. When the indoor temperature in the freezer is -25℃, the wind speed is 1 ~ 2m/s, the freezing time is 16 ~ 18h, and the deep body temperature reaches-15℃, the freezing process is completed and sent out to the warehouse for storage.
(2) After the second freezing, the slaughtered fresh meat is first sent to the cooling room, cooled at 0 ~ 4℃ for 8 ~ 12h, then transferred to the freezing room and frozen at -25℃. Generally, the freezing process is completed in 12 ~ 16h.
Compared with secondary freezing, primary freezing can shorten the processing time by about 40%, reduce a lot of handling, improve the utilization rate of freezing room and reduce the loss of dry consumption. However, freezing beef and mutton sensitive to cold shrinkage once will lead to cold shrinkage and thawing stiffness, so some countries do not adopt the method of freezing beef and mutton once. The secondary frozen meat has good quality and is not easy to shrink. The thawed meat has good water retention, less juice loss and good tenderness.
5. Refrigeration of frozen meat
The air temperature in the cold storage of frozen meat is usually kept below-65438 08℃, and the temperature change range is not more than 65438 0℃ under normal circumstances. In the process of bulk purchase and delivery, the temperature shall not exceed 4℃ for one day and night. The shelf life of frozen meat depends on the storage temperature, quality, type and fatness before storage, in which temperature is the main factor. Therefore, we should pay attention to the safe storage of frozen meat, implement the principle of first-in first-out, and check the products frequently. See table 1-4-4 for the cold storage conditions and duration of frozen meat. Table 1-4-4 Frozen Meat Types Temperature in Cold Storage (℃) Relative Humidity (%) Storage Period (Month) Beef
calf
pork
pork
pork chop
pork
mutton
rabbit meat
poultry
Viscera (packaging)-18 ~-23
- 18
- 18~-23
-29
- 18
- 18
- 18~-23
- 18~-23
- 18
- 18 90~95
90~95
90~95
90~95
90~95
90~95
90~95
90~95
90~95
90~95 9~ 12
8~ 10
7~ 10
12~ 14
6~8
3~ 12
8~ 1 1
6~8
3~8
3 ~ 4 6. Effects of freezing and storage on meat quality
The quality changes of meat during freezing include changes in tissue structure, colloidal properties and other changes. These changes are affected by freezing speed and storage time after freezing. When stored for a long time, the influence of time factor is greater than that of freezing speed.
The structural change of (1) is mainly due to the mechanical damage of ice crystallization. During the freezing process, the moisture in the fiber moves outward, which causes the fiber to dehydrate and shrink, and promotes the protein particles in the fiber to approach and gather. After the water in muscle tissue is frozen, the volume increases by about 9%.
Therefore, when the meat is frozen, the ice crystals formed in the meat will inevitably exert certain mechanical pressure on the tissue. In the case of rapid freezing, the pressure per unit area is relatively small because the frozen crystals produced are small. Moreover, because muscle has certain elasticity, it will not cause muscle tissue damage. However, if it is frozen slowly, the frozen crystals formed are large in volume and uneven in distribution, so the pressure per unit area generated by ice crystals is large, resulting in damage and destruction of organizational structure. At the same time, the fibers are squeezed and aggregated. This kind of tissue damage caused by ice crystallization is mechanical and therefore irreversible. Thawing will cause a large loss of gravy.
(2) Changes in Colloidal Properties Freezing will destroy the colloidal properties of muscle protein, thus reducing the quality of meat. The reason why the colloidal properties of protein were destroyed was that protein denatured during freezing. The reasons for protein's degeneration are as follows:
Salting out: In the process of meat freezing, the pure water is frozen first, and then the solution is diluted. Therefore, when most of the water is converted into ice, the concentration of residual solute in the unfrozen part gradually increases, that is, the concentration of salt in the residual liquid increases, which makes protein salting out and precipitate out of the solution. Protein after salting-out still retains its natural properties in the initial stage, and it can still be dissolved if diluted. But if the salting-out time is too long, it will gradually become irreversible degeneration.
Hydrogen ion concentration: the dissociation degree of acids in meat is very small (mainly phosphoric acid, lactic acid and creatine), and meat protein itself is an amphoteric electrolyte with strong buffering effect, so the change of acidity has little effect on the hydrogen ion concentration in this solution.
When the meat is frozen, the acid concentration in the residual liquid increases with the increase of ice crystal precipitation. At this time, on the one hand, due to the increase of salt concentration, protein salted out, which gradually reduced the soluble protein in the solution. On the other hand, water freezing caused mechanical damage to protein, so the buffering effect of protein in the solution gradually weakened. The concentration of hydrogen ions in solution tends to increase. Therefore, even a small increase or decrease of acid in meat after freezing has a great influence on the concentration of hydrogen ions, thus promoting the denaturation of protein. For example, when the pH value of beef juice is about 6 ~ 7, the denaturation degree is low and stable, and when it is lower than 6.0, it increases rapidly.
Binding water freezing: The colloidal state of protoplasm in muscle fibers, in which the main dispersion is protein. Protein molecules are surrounded by bound water, which has a strong affinity for protein. In the process of freezing, free water freezes first. As the temperature continues to drop, the amount of frozen water gradually increases. When the amount of frozen water exceeds a certain range, the combined water freezes. The freezing of bound water leads to mechanical damage to the structure of colloidal particles, which weakens protein's affinity for water. During thawing, this part of water can no longer be absorbed by protein particles, which makes protein lose bound water and become dehydrated protein. In this way, protein particles are easy to agglutinate and precipitate, and lose their reversibility, so that the intracellular protoplasm can not return to the colloidal state before freezing.
In recent years, due to the development of deep freezing (such as liquid nitrogen), this explanation has been questioned, that is, although the freezing temperature is very low, the reversibility of frozen food is much better than that of food frozen above -25℃. It is difficult to explain this problem with the theory of combined water freezing. On the other hand, the experiment made by Love et al. (1962) provides a basis for combining the theory of water freezing. Therefore, in 20 1 1, it is considered that the freezing speed, which is the key factor affecting the reversibility of meat quality, is secondary to the final freezing temperature.
Variation of dispersion density of protein particles: Due to the formation of ice crystals and the freezing of some bound water, the hydration layer of protein molecules weakened or even disappeared, and the side chains were exposed. At the same time, combined with the extrusion of ice crystals formed in freezing, the protein particles are close to each other and combined, and the protein particles are agglomerated and precipitated. This effect is closely related to the freezing speed. The faster the freezing speed, the smaller the extrusion and the lower the degree of denaturation.
(3) Other changes of meat during freezing and cold storage.
Drying shrinkage: the degree of drying shrinkage varies with air conditions (temperature, humidity, flow rate), meat grade and size, and packaging state. When the temperature is high, the humidity is low, the air velocity is fast, the cold storage time is long, the fat content is low, the shape is small and there is no packaging, the drying shrinkage rate increases significantly. When the above conditions are obviously unfavorable at the same time, the meat can be turned into a sponge, and the meat and fat are seriously oxidized. This is because the drying shrinkage during freezing and refrigeration is similar to the sublimation of ice. In this process, there is no movement of water. Therefore, a dehydrated sponge layer will be formed after the water on the surface of frozen meat evaporates. The ice crystals under the sponge layer continue to sublimate and penetrate the surface layer in the form of water vapor, and the sponge layer deepens.
On the other hand, the diffusion of air makes the air accumulate in the gradually deepening dehydrated sponge layer, which leads to the formation of a highly active surface layer in the meat, where strong oxidation occurs and various odors are adsorbed.
Reducing the shrinkage loss of meat is not only beneficial to quality, but also has important economic significance. If 5000 tons of frozen meat are refrigerated every year, the shrinkage loss will be reduced by 0.5% during refrigeration, and the loss of 25 tons of meat can be recovered.
Discoloration: the color of frozen meat gradually darkens during storage, mainly due to the oxidation of heme and the evaporation of surface water, which increases the concentration of pigment substances. The lower the freezing temperature, the smaller the color change. Almost no discoloration at -50 ~-80℃.
Juice loss: when frozen meat is thawed, the ice crystals inside melt into water, but the water at this time can not be completely absorbed by the tissue, so it is called juice loss when it flows out of the tissue. The loss of juice can be used as one of the indexes to judge the quality of frozen meat. The so-called juice loss generally refers to the juice that naturally flows out during and after thawing, which is called freedrip. The juice flowing out under the pressure of 98 ~ 1862 kPa is called squeezable dripping except for the loss of freedom. Both are always called juice loss.
The total loss of fruit juice and the ratio of free loss to squeezable loss are related to pre-freezing treatment, types and forms of raw materials, freezing humidity, freezing speed, time and temperature during freezing and cold storage, management and thawing methods.
The raw materials are fresh (except for the stiffness caused by thawing), the freezing speed is fast, the freezing temperature is low and stable, the freezing time is short, and the general juice loss is less. If it is thawed immediately after freezing, there will be almost no juice loss. Juice loss increases with time in frozen state, but it does not increase after reaching a certain maximum. The reason of juice loss is basically that the protein colloid changes irreversibly, so that the water in the gel structure cannot be retained and flows out of the tissue.
For beef, the ripening time after slaughter is related to the loss of juice. The loss of juice frozen 5 hours after slaughter is less, but it increases after 24 hours to 5 days. During this period, people who have been frozen are similar to those who have been frozen for 24 hours. Mature beef, even if it forms large ice crystals, loses less juice when thawed because of its good water retention. Changes in fat: At low temperature, although the activation ability of oxygen molecules has been greatly weakened, it still exists. Therefore, fats will still be oxidized, especially those containing more unsaturated fatty acids. Among all kinds of meat, animal fat is the most stable, followed by poultry fat, and fish fat is the worst. After stored at -8℃ for 6 months, pig fat turned yellow and had greasy taste. After 12 months, these changes spread to the depth of 25 ~ 40 mm; However, after storage at-65438 08℃ for 65438 02 months, no bad phenomenon was found in the fat.
Microorganisms and enzymes: Microorganisms are not easy to grow and reproduce at extremely low refrigeration temperature. However, if the frozen meat has been contaminated by bacteria or mold before refrigeration, or when it is refrigerated under poor refrigeration conditions, bacterial and mold colonies will also appear on the surface of the frozen meat, especially where it melts.
As for the activity of cathepsin after freezing, it is reported that the activity increases after freezing, and the activity of repeated freezing and thawing is greater.
7. Thawing of frozen meat
Thawing is a necessary step before frozen meat is used or further processed, and it is a technological process to convert water in ice crystal state in frozen meat into liquid and restore the original state and characteristics of frozen meat. Thawing is actually the reverse process of freezing. The quality of thawed meat is related to thawing speed and thawing temperature. There is a big difference between slow thawing and rapid thawing.
The results showed that when the temperature was 15℃, the loss of beef 1/4 carcass was 3%. When slowly thawed at 3 ~ 5℃, the loss is only 0.5% ~ 1.5%. Therefore, slow thawing can reduce the loss by 65438 0.5% ~ 2.5%. The longer the meat is preserved, the higher the thawing temperature, and the greater the loss of gravy. The loss is 1 1.5% at 40℃, 4.35% at 7℃ and 2.55% at 1℃.
There are many ways to thaw, but the following are commonly used:
(1) air thawing method
The frozen meat is moved to the thawing chamber and thawed by heat exchange between the air medium and the frozen meat. Generally speaking, thawing in 0 ~ 5℃ air is called slow thawing, and thawing in 15 ~ 20℃ air is called rapid thawing.
(2) liquid thawing method
The methods of liquid thawing mainly include soaking or spraying with water. Its advantage is that the thawing speed is faster than that of air thawing. The disadvantage is that it consumes a lot of water, and at the same time, it will also make part of protein and extract lose, and the meat color will fade and the aroma will weaken. The water temperature is 65438 00℃, the water temperature is 20℃ after thawing for 20 hours, and it is thawed10 ~11h. The thawed meat needs to be dried at an air temperature of about 65438 0℃ because its surface is wet. If it is packed in polyethylene bags and thawed in water, the quality of meat can be guaranteed. Thawing in salt water, salt will penetrate into the shallow layer of meat. This method can be used to thaw cured meat. When pork is thawed in 6℃ salt water 10h, the gravy loss is only 0.9%.
(3) Steam thawing method
The advantage of steam thawing method lies in its fast thawing speed, but the loss of gravy is much greater than that of air thawing. However, due to the condensation of steam, the weight will increase by 0.5% ~ 4.0%.
(4) microwave thawing method
Microwave thawing can greatly shorten the thawing time. At the same time, it can reduce the loss of gravy, improve sanitary conditions and improve product quality. This method is suitable for thawing half or a quarter of the carcass. The meat pieces with equilateral geometry are better in this way. Because in the microwave electromagnetic field, the whole meat is heated at the same time. Microwave thawing can be carried out with packaging, but the packaging material should meet the corresponding capacitance and have sufficient stability to high temperature. It is best to use polyethylene or polystyrene instead of metal sheets. (5) vacuum thawing
The main advantage of vacuum thawing method is that the thawing process is uniform and there is no dry consumption. Beef with a thickness of 0.09 m and a weight of 3 1 kg only needs 60 minutes to pass through the vacuum thawing device.