Basic principle of drying
Dried meat products, also known as meat dehydrated dry products, are a type of cooked meat made by pre-processing the meat to remove a certain amount of water from the meat by natural or artificial means, and reducing its water activity to the extent that the microorganisms are difficult to use. Products. The dehydration and drying of perishable foods such as meat is not only a storage method but also a processing method. For different types of dry meat products, dehydration and drying may be the main processing process, or it may be the entire processing technology. A link in the process.
The water content in meat is generally as high as 70%. After dewatering and drying, it not only greatly reduces the volume of the product, but also reduces the moisture content of the meat to 6% to 20%. It can be seen that the basic principle of meat drying is to eliminate the activity of microorganisms in the meat and the activity of the enzyme by removing a part of the water in the meat, thereby achieving the purpose of processing a novel product or prolonging the storage time.
Moisture is a kind of nutrient that is necessary for the growth and development of various microorganisms. However, not all water can be used by microorganisms. For example, adding a certain amount of sugar and salt aqueous solution, most of the water can not be used by microorganisms. Therefore, the water that can be utilized by microorganisms and enzyme chemical reactions is called effective water, and the water activity (Aw) is used to measure the amount of effective water. For each type of microbial growth, there is a minimum required water activity value. When the water activity value in the meat is lower than the lowest value, microbial reproduction is inhibited. When the water activity value in the meat is higher than the lowest value, Microorganisms are highly prone to reproduction.
Usually, fresh meat and fresh products have a water activity of about 0.99, while mold and yeast require water activities of 0.80 and 0.88, respectively, and the water activity required for bacterial growth is between 0.91 and 0.99. It can be seen that only cooking the meat can not achieve the purpose of long-term storage. Only after the cooked meat is dried, the water activity can be reduced to inhibit the growth of most microorganisms in the meat product. Extend the purpose of storage time. However, it must be pointed out that under normal dry conditions, the microorganisms in the meat products are not completely lethal, but the activities are inhibited. If the environment is suitable in the future, the microorganisms will continue to grow and reproduce. Therefore, meat should be properly treated on the one hand to reduce the amount of various microorganisms in the product; on the other hand, appropriate packaging materials and packaging methods should be adopted after drying to prevent moisture and pollution.
In addition to microbes, the preservation of dried meat products is also related to factors such as enzyme activity and oxidation of fat. As the water activity decreases, the stability of dry meat products increases, but the oxidation of fat is different from other factors. When the water activity is 0.2-0.4, the reaction rate is the slowest, and the reaction speed increases when it is close to the anhydrous state. The experiment proves that when the water content of the degreased dried meat product is 15%, the water activity value is less than 0.7, so it is suitable when the water content of the dried meat product is less than 20%.
Factors affecting the drying of meat products
The most basic phenomenon in the drying of meat products is dehydration. Dehydration of meat products is the result of alternating diffusion of two products; when the raw materials of meat products are exposed to a dry medium (heated air), the moisture on the surface of the meat products is heated to become water vapor and is largely evaporated due to contact with hot air. When the surface moisture is lower than the internal moisture, the difference in water vapor pressure between the inside of the meat product and the surface moisture, the water will be transferred from the inside to the surface, which is called water diffusion.
In general, the internal and external diffusion of moisture during the drying process is carried out simultaneously, but the speed is not equal, which varies depending on the variety, morphology and drying process conditions of the meat product. In the process of drying, the speed and diffusion of moisture should be coordinated and balanced as much as possible by the control and adjustment of the process conditions. If the external diffusion rate is much greater than the internal diffusion, the internal moisture will not be transferred to the surface, and the surface of the raw material will be Due to excessive drying, a hard shell (called "crust" phenomenon) is formed, which hinders the evaporation of water, and even surface coking and cracking, which reduces product quality.
During the drying process, the speed of drying plays a decisive role in the quality of dry products. When the other conditions are the same, the faster the drying speed, the less likely the product is to undergo bad changes, and the better the dry product quality. The factors affecting the drying of meat products mainly depend on the surface area of ​​the raw meat products, temperature, humidity, air circulation flow rate, atmospheric pressure and vacuum, and the load during drying.
(1) Surface area of ​​meat products
In the process of processing dried meat products, in order to accelerate the exchange of moisture and heat, the pre-cooked materials are usually cut into small pieces, strips, and granules, and then dehydrated and dried. After the material is cut into thin pieces or small particles, the heat transfer to the center of the meat block and the distance of the water from the center of the meat block are shortened, the surface area of ​​the meat product and the heating medium are increased, and the distance of the water inside the meat product is shortened. , thereby accelerating the evaporation of water and dehydration of meat products. Therefore, the larger the surface area of ​​the meat product, the faster the drying speed.
(2) Temperature
During the drying process, the greater the temperature difference between the drying medium and the meat product, the faster the heat is transferred to the meat product, and the water escaping speed is also accelerated. In air with a certain water vapor content, the higher the temperature, the more water vapor is required to reach saturation, and the faster the meat product is dried; on the contrary, the temperature is lowered, the water vapor required to achieve saturation is reduced, and the drying speed is lowered. However, the temperature should not be too high, otherwise the meat products will be coked and the value of the goods will be reduced.
(three) humidity
The effect of humidity on the drying speed of meat products is that the temperature is constant, the lower the humidity of the drying medium, the greater the difference in humidity of the air humidity, and the faster the dehydration speed of the meat. Increase the temperature, ventilate and dehumidify, reduce the humidity of the air, accelerate the dehydration speed, and reduce the water content of the dried meat products to the national standard.
(4) Air circulation flow speed
Accelerate the air circulation flow rate, not only because the amount of water vapor that can be accommodated by hot air will be higher than that of cold air, but also absorb more evaporated water, and it can also take away the saturated humid air accumulated near the surface of the meat product in time to avoid blocking the meat. Further evaporation of the moisture contained in the product, while also increasing the amount of air in contact with the surface of the meat product, significantly accelerates the evaporation of moisture in the meat product. However, the circulating flow rate of air should not be too large, preventing insufficient utilization of heat energy and increasing fuel consumption.
(5) Atmospheric pressure and vacuum
The boiling point of water will decrease with the decrease of atmospheric pressure. The lower the pressure, the lower the boiling point. Therefore, the lower the atmospheric pressure of the environment in which the meat is dried, the higher the vacuum, the drying process can be compared. The drying rate is faster at low temperatures.
(6) Loading capacity
In a certain unit area, the amount of raw materials is different, and the drying speed is also different. The loading of raw materials is usually closely related to its thickness. The more the loading, the greater the thickness, the more unfavorable the circulation of air, thus affecting the evaporation of water. Therefore, the optimum raw material loading must be determined according to the specifications of the drying equipment during the drying process.
Dry method
With the continuous development of science and technology, the drying method has been continuously improved and improved, and the drying equipment has been continuously updated. In summary, there are two main methods for dry processing of meat products, namely natural drying and artificial drying.
(1) Naturally dried
Natural drying is an ancient drying method that uses a method of solar radiation and dry air in natural conditions to dry the meat product. Natural dry can be divided into two types: drying and air drying.
Drying refers to direct drying of raw materials in the sun to dry them. The drying speed is mainly affected by the intensity of solar radiation. The intensity of solar radiation varies with regions and seasons. The low latitudes are stronger than the high latitudes. Summer and autumn are stronger than winter and spring.
Air drying refers to placing the raw materials in a well-ventilated room or under a shed to make them air dry. The drying speed depends mainly on the temperature, humidity and wind speed of the environment. It can be seen that natural drying requires simple equipment and low cost, but it is difficult to control temperature conditions due to natural conditions. Large-scale production is rarely used, but it is used as an auxiliary process in some product processing, such as dried sausage. Dry and so on.
(2) Artificial drying
Manual drying refers to a dry process that manually controls various drying process conditions. The equipment investment and energy consumption required for manual drying are relatively high, the cost is high, and the operation is complicated. However, compared with natural drying, manual drying introduces various advanced drying equipment, which greatly shortens the drying process. Time, dry conditions are easy to control, and the quality of dry products is high, which is the main direction of meat production. At present, the commonly used manual drying methods include drying and drying, drying in the drying room, drying in the tunnel, drying in the belt, and high-tech such as far-infrared drying, microwave drying, vacuum drying, and freeze drying. More and more widely used.
Roasting
The baking and drying method is also called conductive drying. It is modified from the simplest manual drying equipment, the drying stove. It relies on the heat conduction of the wall and the like to transfer the heat to the wet material in contact with the wall, so that it is dehydrated and dried, because the wet material and the heated medium are not Direct contact, so this method is also known as indirect heat drying. The heat source for indirect drying may be water vapor, heat, hot air, or the like. This drying method can be carried out either at room temperature or under vacuum. For example, the frying in the processing of the meat is carried out at room temperature using this method.
2. Drying in the drying room
The drying room is a hot air convection drying device heated by flue gas. It has the characteristics of large drying capacity, fast drying speed and high production capacity. It is the most commonly used drying equipment for small and medium-sized township enterprises in China. Drying room drying method is to directly use high-temperature hot air as a heat source in the drying room to transfer heat to the wet material by means of air convection. Because of the direct contact between the wet material and the heated medium in this dry process, it is also referred to as direct heat drying. The hot air in the drying room is both a heat carrier and a wet carrier. This drying method is mostly carried out under normal pressure. Because in the case of vacuum drying, since the gas phase is at a low pressure, the heat capacity is small, and air cannot be directly used as a heat source, and other heat sources must be used. The temperature adjustment in the drying room is convenient, and the material will not be overheated, but the hot air still has a considerable amount of heat when leaving the drying room, so the drying room has a low utilization rate of heat.
3. Tunnel drying
Drying of the tunnel is to store the wet material in the vehicle, and the vehicle continuously passes through the tunnel along the track to dehydrate and dry the material. When the tunnel is used for drying, the temperature, humidity and flow rate of the air are easy to control, the drying time is short, the quality is good, and the production efficiency is high.
This drying method is carried out in a continuous hot air convection drying device, a tunnel dryer. The tunnel type dry machine is a rectangular parallelepiped made of metal plate. It consists of a heating room and a drying room. The heating room is equipped with a heater and a blower to send hot air into the drying room. The drying room is a long and narrow tunnel shape, generally 12 long. ~18m, width 1.8m, height 1.8~2.0m, the track is set at the bottom, and the raw materials to be dried are placed in the vehicle and slide along the track to dehydrate and dry. Dry tunneling is divided into three types: counterflow, downstream and hybrid according to the running direction of materials and hot air.
(1) Countercurrent drying
The running direction of the material carrier is opposite to the flow direction of the hot air, that is, the material enters from the low temperature and high humidity end, and exits from the high temperature and low humidity end. The starting temperature of the raw material drying is low (40 ~ 50 ° C), the temperature rises gradually, and the temperature reaches the highest (65 ~ 85 ° C).
(2) Downstream drying
Contrary to the former, the direction of the vehicle is the same as the direction of hot air flow. The material enters from the low-temperature end of high temperature (80-85 °C), the water evaporates quickly, the temperature gradually decreases, the humidity gradually increases, the evaporation of water slows down, and the temperature is lower at the end (55-60 ° C). This dry method is suitable for the dehydration of materials which require temperature change during the drying process of the meat.
(3) Hybrid drying
Hybrid drying is also known as convection drying. In the dry equipment, two heaters and two blowers are installed, which are respectively arranged at the two ends of the tunnel, and the hot air is blown to the middle from both ends. After the raw materials are passed, the hot and humid air is concentratedly discharged from the middle of the tunnel, and the other part is recycled. . When the vehicle loaded with materials is dry, the material first enters the downstream tunnel, meets the high temperature and rapid hot air, the water evaporates in a large amount, the vehicle travels forward, the temperature is gradually lower, the humidity is higher, and the evaporation speed of the water is slower, which does not cause The surface of the material is formed into a hard shell. After most of the moisture of the material is removed, it enters the counter-flow tunnel, and the more forward, the higher the temperature, the lower the humidity, and the more thoroughly the product is dried. After the material enters the counterflow tunnel, the air temperature must still be controlled to avoid coking discoloration of the dry product.
4. Belt drying
The belt type drying method is a method in which the wet material is placed on the moving steel mesh belt, and the hot air flows through the water vertically to remove the moisture and dry the material. During the drying process, the wet material is placed on the uppermost steel mesh belt. As the mesh belt moves, the material falls into the next mesh belt in turn, and the hot air is introduced from below, from the bottom to the top of the mesh belt, and the hot and humid air is from the upper part. The exhaust port is exhausted, and finally the dry material is discharged from the lower portion. In this dry method, the material is automatically flipped once when it falls from the upper layer to the lower layer, so that the drying is relatively uniform.
5. Far infrared drying
Far-infrared drying refers to a method in which far-infrared rays are emitted by a radiating element installed in a drying chamber, and are absorbed by the material into heat energy to achieve dehydration drying. Infrared is an electromagnetic wave between visible light and microwave. The wavelength is 0.75-1000μm. The 40-1000μm band is called far-infrared. The far-infrared rays are absorbed, refracted and reflected when they are irradiated onto the surface of the material. The absorbed part is absorbed. Converted to heat, raising the temperature of the material. It also has a strong penetrating power that allows both inside and outside of the material to be heated. Therefore, far infrared drying has the characteristics of fast drying speed, high efficiency, good quality and energy saving.
6. Microwave drying
When the meat is dried by the above-mentioned dry method, the heat energy is transmitted from the surface of the material to the inside, the surface temperature of the material is higher than the interior, and the moisture is diffused from the inside to the surface, and the surface of the material first becomes a dry solid heat insulation during the drying process. The layer increases the resistance of heat transfer and vaporization and diffusion of internal moisture, so the drying time is long and the internal coke is easily wet. The above problems can be effectively solved by the new microwave drying method.
The microwave electron tube can emit ultra-high frequency electromagnetic waves with a frequency of 300 to 300,000 MHz and a wavelength of 0.001 to 1.0 m between the radio wave and the light wave. The ultra-high frequency electromagnetic wave forms an electric field exhibiting a wave-like change with positive and negative electrodes. . There are a large number of positively and negatively charged molecules (water, salt, sugar) in the material. Under the action of the electric field formed by the microwave, the negatively charged molecules move toward the positive electrode of the electric field, and the positively charged molecules move toward the negative pole of the electric field. The intermolecular motion generates a large amount of heat, allowing the material to dry. Although the microwave frequency is from 300MHz to 300,000MHz, it is actually not possible to use the frequency within this range. Because this frequency band includes frequencies for broadcasting and communication radars, Internationally, the frequency used for industrial use is only two frequency bands of 915 MHz and 2450 MHz.
The microwave has a strong penetrating ability and can be produced simultaneously inside and outside the material, and does not require heat conduction, radiation, and convection, and can achieve the purpose of drying in a short time. The inside and outside of the material are evenly heated, and the surface is not easy to be irritated. However, the investment cost of microwave drying equipment is relatively high, and the characteristic flavor and color of dry meat products are not obvious.
7. Vacuum drying
Vacuum drying is also known as vacuum drying. The boiling point of water decreases with the pressure drop. Under vacuum conditions, the moisture of the material can be removed by using a lower temperature to achieve the purpose of drying. When vacuum drying, the vacuum pump draws the drying chamber into a vacuum, and the steam is heated into the heating plate to heat the material. Under vacuum, the internal diffusion of the moisture of the material is dried together with the external evaporation. Therefore, compared with the atmospheric pressure drying method, the vacuum drying has the advantages of short drying time and small surface hardening; however, the escape of the aromatic components of the material and slight heat denaturation due to evaporation during the drying process are also unavoidable.
8. Freeze drying
Freeze drying, also known as freeze-sublimation drying, is the process of rapidly freezing the material below the freezing point to make the water into solid ice, and then dehydrating and drying the ice directly into the water vapor without liquid in a high vacuum state. . This drying method has almost no adverse effects on color, taste, fragrance and shape, and is the most advanced drying method in modern times.
At present, the application of China's freeze-drying method in the processing of dried meat products has only started, and it is believed that it will develop rapidly. The method of operation is as follows: the meat pieces are rapidly frozen to -40 to -30 ° C, and placed in a drying chamber having a vacuum of 13 to 133 Pa, in which state the ice of the meat pieces is directly sublimated into water vapor. dry. The freeze-dried meat block has the same volume, the internal tissue is loose and porous, and has good rehydration. It is an ideal excipient for instant noodle food. Therefore, the freeze-dried meat pieces are also very easy to absorb water during storage, and the loose porous state and the air contact area are increased, and are easily oxidized and deteriorated during storage, especially materials having a high fat content.
In addition, the artificial drying method is also spray drying, radiation drying, dielectric heating drying, etc., since these drying methods are rarely used in the processing of dried meat products, they are not described here.
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