| Microwave freeze-drying is a new freeze drying technology using microwave heating system instead of conventional radiant heating panels. Compared with the conventional freeze-drying technology, it has some advantages in terms of improving drying rate, shortening drying time, reducing energy consumption, and soon on. However, due to the existence of technical problems including glow discharge, uneven drying, difficulties in process optimization and process control, there are many difficulties before the large-scale industrial application of microwave freeze-drying. In this paper, a microwave freeze-drying pilot test equipment was developed. Based on the existing technical problems of microwave freeze-drying, and employing the research method of a combination of theoretical analysis and experimental study, some key technologies such as glow discharge prevention, drying uniformity regulation and vacuum cooling pretreatment were investigated under the pilot production scale of carrots freeze-drying, and the drying technological process was systematacially optimized by considering of all kinds of conditions and indices. The above studies were aiming at providing some technical accumulation for promoting the forward course of industrial application of microwave freeze-drying technology. The results were as follows:1. Cylinder-shaped drying chamber and cold trap chamber of the pilot test equipment were connected in top-bottom position configuration, and were separated by the shielding pore plate which can allow the freedom passing through of water vapor and also prevent microwave’s penetrating into clod trap chamber. For the microwave system, low-power and multi-port feed entrances were staggered along the circumference of the drying chamber. Every microwave magnetron can be turned on or off independently, and is continuously adjustable in the power, which enable the microwave system achieve a fine adjustment. The measurement and control system many real-time functions including material temperature measuring, water loss weighing, video surveillance, energy consumption tracking, etc., which contribute to the studies on the drying characteristics and the process optimization of microwave freeze-drying.2. Some experimental studies and analysis were carried out on the glow discharge characteristics of the developed equipment, and the prevention and control measures were proposed subsequently. When the drying chamber pressrue was around150Pa, glow discharge was most prone to occurring. The lower temperature of freezed materials and the more materials loading, the smaller the critical power density of glow discharge was. Microwave discharge power at specific material loading was recommended as the consideration index for the studying of glow discharge characteristics. Discharge characteristics of the freeze-dryer changed along with the freeze-drying process, during which microwave discharge power almost varied synchronously with sublimation drying rate. During the initial and latter process of freeze-drying, microwave discharge power and safe loading power were comparatively smaller, and microwave power loading was susceptible to the restriction of glow discharge. However, the microwave discharge power and safe loading power during the medium-term drying process were evidently higher, and represented a variation trend of first-increasing and succedent-decreasing. The testing results of the initial and latter process of freeze-drying showed that there was no significant difference between the influences on microwave discharge power by two kinds of microwave operating methods, i.e., the whole working method and the two-group alternative working method. Based on the above research results, the principles of deteriming the technological conditions involving drying chamber pressure, freezing temperature, and pre-vacuumizing time and soon on were established. And the three-stage microwave power control scheme was determined for the drying process controlling, that is, low power for the initial term of drying process, high power for the medium term, small power for the latter term.3. Testing and analysis of drying uniformity were performed on this pilot-test equipment, and the appropriate microwave operating method and regulation scheme of drying uniformity were accordingly determined. For both the whole working method and the two-group alternative working method of microwave system, there were similar drying speed differences among the20layers of material trays or different areas in the material trays. The results concluded that out layers of20material trays were dried faster than inner ones, and as for the materials within the same layer tray, peripheral materials were dried faster than central ones. Compared with the whole working method, although the two-group alternative working method can improve the drying uniformity among the different layers or the different material areas of the same layer, however, the remaining drying speed differences among the20layers of materials still brought out great influences on the drying efficiency and drying quality of the batch of materials as a whole, so that the drying uniformity didn’t achieve the requirements of freeze-dried products. Based on the two-group alternative controlling scheme of microwave system, tray-center-vacant materials loading method and trays’transposition and restacking method were adopted to realize a considerable improvement on the problems of uneven drying, with the result that the uniformity degrees of material content in single tray and between all the trays were about90%and94.5%,respectively.4. A new technological process of applying vacuum-cooling pretreatment to microwave freeze drying was proposed on the basis of the developed microwave freeze-drier, during which5process steps including cooling, draining, pre-drying, freezing and freezing-drying were accomplished at a time on the drier, simplifying the original processes, reducing equipments and facilities investments and production costs.60min vacuum-cooling pretreatment experiments were performed on40kg carrot slices, resulting in4-phase variation of material temperature, that is, pre-flashing phase, flashing to ice point phase, ice crystals generating phase, deep freezing phase. After the vacuum-cooling pretreatment, the core and surface temperatures of carrot slices were close to-30℃and below-40℃, respectively, satisfying the requirements for the freezing temperature of freezing drying. The whole vacuum-cooling pretreatment removed29%of original moisture in the materials, during which dehydrate rate rose rapidly to the peak after the appearing of flash point, then sustained the peak for a period of time, run a deep decrease and leveled off with a very low rate at the latter period. The pretreatment process may have the phenomena of refrigerating capacity loss and invalid moisture loss, leading to a certain difference between the theoretical moisture dehydration and the actual measurement value.5. Three different microwave freeze-drying technological tests were conducted using the material of carrot slices, which involved group A:vacuum cooling substituting the process of freezing in refrigerator, group B:vacuum cooling followed by freezing in refrigerator, and group C:no vacuum cooling pretreatment. The test results showed that: compared with group C, group A and B had a evident increase on the microwave loading power during the initial1hour drying stage and significantly improved the glow discharge problem at the initial term of the freeze-drying; under the scheduled microwave power loading scheme, the demanded drying time of group A, B and C were8h,8.5h and10h, respectively. Due to the pre-dehydration function and the improvement of discharge problem in initial freeze-drying stage by vacuum cooling treatment, compared with group C, group A and B shortened the drying time2h and1.5h, respectively. At the drying end point, the average moisture content of the entire batch of materials in group A, B and C were14%,16%and22%, respectively, all of which didn’t meet the requirements of freeze-dried products. The dehydration rate of all material trays were inequable, that of group C was slowest. During the entire process, power consumption of every unit dehydration in the three groups ranked according to the increasing sequence of A, B and C, i.e., energy utilization efficiency in group A was highest, that of group C was lowest. There were no significant differences on quality indices of the top tray materials in the three groups, including rehydration ratio and color, etc. Loss ratios of Vitamin C in group A, B and C were17.6%,19.0%and31.6%, respectively, which showed that vacuum cooling pretreatment significantly reduced the loss of vitamin C in the freeze-drying process. In conclusion for the above experimental results, vacuum cooling pretreatment can accomplish4process steps of material cooling, draining, pre-drying and freezing, improve the initial-term glow discharge problem, reduce freeze-drying time and energy consumption, enhance the preservation ratio of Vitamin C and had no significant influence on the other freeze-drying qualities.6. Based on the process steps and technical characteristics in microwave freeze-drying of carrot slices, the drying process optimization was comprehensively investigated including thickness of carrot slices, batch loading capacity, microwave regulation technology during the drying process, scheme of trays’transposition and restacking, etc. Considering the comprehensive factors including drying degree of batch materials, freeze-drying productivity, energy consumption of unit materials and quality indices, about5mm-thick carrot slices and40kg of batch loading capacity, i.e.,2kg materials for every tray were adopted in the freeze-drying process. Response surface optimization experiments on microwave regulation process were investigated with the response indices of freeze-drying time, average moisture content and quality evaluation of the dried products. The optimized parameters of microwave regulation process were as flows:alternative intermission time ti was10min, microwave power Pm during the medium-term freeze-drying process was6.5kw, moisture transition point of medium-latter drying term Rk was40%, drying endpoint temperature Te was60℃.Real-time average moisture content of the batch materials Pt during the freeze-drying process was measured by online weighing system, which was viewed as the criterion for the operation of trays’transposition and restacking. According to the scheduled twice operation method of trays’ transposition and restacking, response surface optimization experiments on control points of Pt for trays’ transposition and restacking were investigated with the response indices of moisture content of materials, uniformity degree of tray’s moisture content and quality evaluation of the dried products. And the first moisture content control point Ptl and the second moisture content control point Pt2were optimized to be58.9%and30.1%, respectively. |
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