The freeze-dried beef can maintain the original flavor and nutritional value of the product after rehydration, and can be used for special occasions such as astronautics, mountaineering, navigation, exploration, and military field battles, and can also be used for the production of convenience food. Kong Lingyuan of Ningxia University studied the low-consumption and high-speed freeze-drying process of dry-cut cooked beef.
Experimental design of freeze-drying beef
In previous studies, the eutectic point, eutectic point, and melting point of dry-cut beef freezing have been determined to be -21, -18, and -3, respectively. In the freeze-drying single factor test of dry-cut beef, it was found that the effect of drying chamber pressure on the drying rate would change significantly at the eutectic point and the melting point during the whole freeze-drying process, which was determined by the special material characteristics of dry-cut beef. Therefore, in the experiment, the whole process of freeze-drying is divided into three stages with the eutectic point and the melting point as the demarcation point (i.e., the stage from the final temperature of prefreezing to the eutectic point, the stage from the eutectic point to the melting point, and the melting point to the end of sublimation drying). Based on the effects of pressure and material thickness on drying energy consumption and production efficiency at these three stages, a quadratic quadratic regression orthogonal combination test of four factors and five levels was carried out (see Table 7-12) to study the effects of drying chamber pressure and material thickness on drying rate and energy consumption.
In the experimental study, the dry cut beef made of semimembranous muscle, longissimus dorsi and other parts was selected, the fat and connective tissue attached to the cooked beef were removed, and the block shape was cut into a length of 50mm, a width of 40mm, and a thickness of 6 30mm, and the moisture content was measured to be 53 before the test4%~56.8%, approximate 55%. The cut beef is placed in a tray for later use with a fill factor of =08。
According to the actual controllable pressure of the freeze dryer and combined with the requirements of production efficiency in production, the upper limit of the pressure in the drying chamber was determined to be 120Pa and the lower limit was 20PaThe upper limit of material thickness is 30mm, and the lower limit is 6mm.
Through the study of the effects of refrigeration temperature and freezing rate on the energy consumption of pre-freezing and drying during the pre-freezing process, it was determined that the test was carried out by slow freezing, and the final temperature of pre-freezing was -32. When the cold trap is opened, the material that is to be placed in the tray is inserted into the center and surface with a temperature probe and placed on the freezer plate for pre-freezing, and the output temperature in the center of the material is -32, and then frozen for 1h to end the pre-freezing.
After the pre-freezing, the material is moved from the freezer plate to the bottom of the heating plate (that is, on the weighing scale), the drying bin door is closed, the vacuum pump is turned on, the experiment starts and the ** weighing system is opened, and the heating plate is heated when the vacuum degree drops to the set value, and the temperature of the heating plate is set to be constant at 80. The sublimation phase ends when the temperature probe output value is -21 (eutectic point). The first pressure swing drying enters the sublimation stage, when the temperature probe output value is -3 (melting point), most of the free water in the material has been removed, and the sublimation drying is considered to be over. The second pressure swing drying is transferred to the stage (analytical drying), when the core temperature and surface temperature of the material are the same and there is no change in the weight of the material within half an hour, the drying is considered to be over. During the experiment, the start of the experiment, the time of two transforms, and the readings of the three-phase electricity meter were recorded to calculate the energy consumption and drying time of each stage.
Optimization of beef freeze-drying and low-consumption process
According to the quadratic regression orthogonal experimental design of four factors and five levels, 27 experiments were arranged, of which the center point was repeated 3 times, and the experimental results are shown in Table 7-13.
The analysis results show that within the experimental range, the factors affecting the drying rate are in the following order: material thickness, stage drying chamber pressure, stage drying chamber pressure, and stage drying chamber pressureThis is because the stage is mainly the sublimation stage of moisture, about 65% of the moisture in the wet material is removed at this stage, and the lower drying chamber pressure can greatly improve the sublimation rate of moisture, which is the main way to improve the drying rate. The order of factors affecting energy consumption per unit of moisture is as follows: material thickness, stage drying chamber pressure, stage drying chamber pressure, and stage drying chamber pressure. This is because in the freeze drying process stage for the analysis stage, the main drying combined with water, accounting for about 10% of the total moisture, but the drying time is longer than the stage, so select the appropriate stage drying chamber pressure, shorten the drying time, to reduce energy consumption has a key role.
The regression analysis of the above experimental data was carried out by SAS software, and it can be seen that the interaction between the pressure of the drying chamber and the thickness of the material at the stage has a significant effect on the drying rate and energy consumption (see Figure 7-15). As can be seen from Figure 7-15, the drying rate of thin material is much less affected by the pressure of the drying chamber in the stage than that of the thick material, and when the pressure of the drying chamber in the stage is small, the drying rate decreases with the increase of the thickness of the material, and the drying rate reaches the maximum when the thickness of the material is the thickest and the pressure of the drying chamber is the largest in the stage, because the stage is the analytical stage of freeze drying, and the heat transfer is mainly used to determine the drying rate for dry cut beef, and increasing the pressure of the drying chamber is conducive to the progress of heat transferThe surface area of thick materials (including upper surface and side surface) is larger than that of thin materials, and the increase of surface area is conducive to the escape of water vapor and the increase of drying rate. As shown in Figure 7-16, with the increase of the pressure of the drying chamber and the increase of the thickness of the material in the stage, the energy consumption per unit of water is reduced, and in the case of thicker material, increasing the pressure of the drying chamber can significantly reduce the energy consumption per unit of water, because the drying rate mentioned above increases with the increase of the thickness of the material and the pressure of the drying chamber, thus shortens the drying time, thereby reduces the energy consumption per unit of moisture.
The purpose of the experimental study was to find the optimal process operating conditions that could ensure the drying rate without significantly increasing the energy consumption, because the drying rate was the main consideration, and the energy consumption per unit moisture was the secondary consideration, so the drying rate weight of 1=07. The weight of energy consumption per unit of water is 2=03。The regression analysis of the comprehensive weighted value was carried out, and the maximum value was taken, and the optimal process operating conditions of low consumption and high speed were obtained: the pressure of the three-stage drying chamber was 20Pa (-1.).546），44.18pa（-0.8070），120pa（1.546), material thickness 30mm (1546), at which point the drying rate is 14269g h, the energy consumption per unit of moisture is 3435794kj/kg。
Each optimization result was carried out 3 parallel experiments, the specific experimental conditions and measurement results are shown in Table 7-14, from the experimental results, the optimized maximum drying rate conditions are about 20% higher than the same pressure in the whole drying process, and the minimum drying conditions can save 25% of the energy consumption for each removal of 1k**, it can be seen that the optimized operating conditions are very obvious whether it is to improve the drying rate or reduce the drying energy consumption, indicating that different drying chamber pressures are taken in stages to improve the drying Both the drying rate and the reduction of drying energy consumption have a significant effect. In particular, the results of the optimization of low consumption and high speed (Experiment 3) were reduced by only 46%) and reduced the energy consumption per unit of moisture (21%), and the adjustment of freeze-drying process parameters has more practical reference value.