微波冷冻干燥中试设备及关键技术研究
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摘要
微波冷冻干燥是以微波替代传统加热隔板作为热源的新型冻干技术,与常规冻干技术相比,在提高干燥速率、缩短干燥时间、降低能耗等方面具有较大优势,但由于过程中低压放电、干燥不均匀、工艺优化与过程控制难等技术难题的存在,该技术在大规模工业化应用方面仍面临诸多困难。本论文研制出微波冷冻干燥中试设备,针对微波冻干现有技术难题,选用当前面广量大、具有典型代表性的干燥蔬菜品种胡萝卜、作为试验原料,采用理论分析与试验研究相结合的研究方法,重点开展中试生产规模下的低压放电防控、干燥均匀性调控、真空冷却预处理技术等关键技术研究,综合多项条件和指标,对胡萝卜微波冷冻干燥工艺过程进行系统优化研究,以期为推动微波冷冻干燥技术工业化应用进程提供技术积累。主要研究结果分述如下：
1、研制的微波冷冻干燥中试设备干燥仓与捕水仓为筒形仓呈上下连体配置,中间通过屏蔽过流板相隔,可以允许水蒸气自由透过,同时也能防止微波进入捕水仓；微波系统采用小功率多口馈入方式,微波馈入口沿干燥仓周向交错排列,单只磁控管可独立启闭、功率连续可调,微波系统可实现精细化调节；设备测控系统具备光纤实时测温、实时称重、视频监视,能耗跟踪等功能,便于微波冻干干燥特性、干燥工艺优化等研究。
2、对研制设备的低压放电特性进行了试验研究和分析,并针对性提出低压放电防控措施。设备最易产生放电的干燥仓压强区域在150Pa附近,物料冻结温度越低、装载量越大,则低压放电临界功率密度越小；研究低压放电特性时,建议明确物料装载量,以微波放电功率为考量指标。微波放电特性随着冻干进程而不断变化,微波放电功率与升华干燥速率几乎同步变化；在冷冻干燥初期和后期,微波放电功率和安全加载功率较小,微波功率加载易受低压放电限制；而在干燥中期,微波放电功率和安全加载功率均明显高于干燥初期和后期,且呈先增后降趋势。在干燥初始和干燥末期测试表明,微波整体开启法、左右开启法对微波放电功率的影响差异不大。基于低压放电特性研究分析结果,提出干燥压强、物料冻结温度、预抽时间控制等工艺条件确定原则,提出冻干过程中“干燥前期小功率、中期高功率、后期小功率”的三段微波功率控制方案。
3、对研制设备的干燥均匀性进行了测试和分析,并据此确定了适宜的微波开启方式和干燥均匀性调控方案。在微波整体开启、交替开启两种方式下,不同料层间、同一料盘不同铺料区域均表现出相类似的干燥速度差异：20盘物料呈现中间层干燥速度慢、外层干燥速度快的趋势,同一料盘呈现周边物料干燥速度快、中心部位干燥速度慢的趋势；与整体连续开启方式相比,微波交替间歇开启方式虽能较好地改善料盘之间物料和同一料盘内物料的干燥均匀性,但总体来看,20盘物料存留的干燥速度差异仍影响物料整体的干燥效率和干燥品质,干燥均匀性与干燥成品要求还有一定差距。在微波交替开启控制方案的基础上,通过采用料盘中心部位空缺铺料法、冻干期间料盘2次出仓换位堆垛法,可以较好地改善不同层料盘间、同一料盘内物料因干燥速度差异而引起的干燥不均匀问题,料盘内物料含水率均匀度约为90%,料盘间物料平均含水率均匀度达94.5%,基本满足冻干制品的实际生产要求。
4、基于研制的微波冻干设备,提出采用真空冷却预处理的微波冻干加工工艺方法,可以一次完成冻干加工中的冷却、沥水、预干燥、冻结、冻干等5个工序,简化工艺过程,降低设备设施投入成本及生产成本,并开展试验研究。对40kg胡萝卜片物料进行60min真空冷却处理,物料温度呈现4个阶段变化：闪发前降压段、闪发冷却至冰点温度段、冰晶生成段、深层冻结段；处理结束后,胡萝卜片芯部温度接近-30℃,表层温度为-40℃以下,可以满足冷冻干燥对物料冻结温度的要求；真空冷却预处理可脱除物料中29%的初始水分,脱水速率在闪发现象出现后突然增加,达到高峰并维持一段时间后,随后快速下降,并在后期达到平衡,维持极低的脱水速率。过程中可能有冷量损耗和无效失水等现象的存在,导致真空冷却过程中理论脱水量与实测值存在一定的差异。
5、真空冷却后无冷库冻结(A组)、真空冷却后再冷库冻结(B组)、无真空冷却处理(C组)三组冻干工艺试验表明：经过真空冷却处理的A、B两组在0～1h干燥区间的微波加载功率可明显高于C组,可明显改善冻干初期的低压放电问题；按照设定的微波功率加载方案,A、B、C三组所用的干燥时间分别为8h、8.5h、10h,由于真空冷却处理的预先脱水和冻干初期低压放电问题改善效果等原因,A、B组比C组分别减少冻干时间2、1.5h；在干燥终点,三组工艺处理的整批物料平均含水率分别约为：A组14%、B组16%、C组22%,A、B、C三组物料含水率实际上均未达到干燥要求,各层料盘整体干燥进度不一致,其中C组整体干燥进度最慢；整个工艺过程中,三组处理的单位脱水量耗电大小依次为A 6、针对胡萝卜片微波冷冻干燥主要工艺环节与技术特点,从胡萝卜切片厚度、批次装料量、过程微波调控工艺、开仓换盘堆垛方案等方面对胡萝卜微波冷冻干燥工艺过程控制进行系统优化研究。综合考虑批次物料干燥程度、冻干生产率、单位能耗以及制品品质等指标,冻干工艺中胡萝卜切片厚度应控制在5mm左右,批次装料量为40kg,即每料盘铺料2kg。以冻干耗时、平均含水率、品质评分作为响应指标,对微波调控工艺进行响应面优化试验研究,微波调控工艺参数组合优化确定为：交替开启时间t1为10min,冷冻干燥中期微波功率Pm为6.5kw,干燥中一后期转化点含水率Rk为40%,产品干燥终点温度Te为60℃。通过在线称重系统测定冻干过程中的整批物料平均含水率Pt,并以此为开仓换盘堆垛操作的判定依据；冻干期间开仓换盘堆垛操作2次,按照制定的换盘堆垛方法,以干燥后物料含水率、含水率均匀度、品质评分作为响应指标,对冻干期间出仓换盘堆垛的物料含水率控制点Pt进行响应面优化试验研究,第一次出仓含水率控制点Pt1、第二次出仓含水率控制点Pt2优化确定为：Pt1(58.9%)、Pt2(30.1%)。
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.
1、研制的微波冷冻干燥中试设备干燥仓与捕水仓为筒形仓呈上下连体配置,中间通过屏蔽过流板相隔,可以允许水蒸气自由透过,同时也能防止微波进入捕水仓；微波系统采用小功率多口馈入方式,微波馈入口沿干燥仓周向交错排列,单只磁控管可独立启闭、功率连续可调,微波系统可实现精细化调节；设备测控系统具备光纤实时测温、实时称重、视频监视,能耗跟踪等功能,便于微波冻干干燥特性、干燥工艺优化等研究。
2、对研制设备的低压放电特性进行了试验研究和分析,并针对性提出低压放电防控措施。设备最易产生放电的干燥仓压强区域在150Pa附近,物料冻结温度越低、装载量越大,则低压放电临界功率密度越小；研究低压放电特性时,建议明确物料装载量,以微波放电功率为考量指标。微波放电特性随着冻干进程而不断变化,微波放电功率与升华干燥速率几乎同步变化；在冷冻干燥初期和后期,微波放电功率和安全加载功率较小,微波功率加载易受低压放电限制；而在干燥中期,微波放电功率和安全加载功率均明显高于干燥初期和后期,且呈先增后降趋势。在干燥初始和干燥末期测试表明,微波整体开启法、左右开启法对微波放电功率的影响差异不大。基于低压放电特性研究分析结果,提出干燥压强、物料冻结温度、预抽时间控制等工艺条件确定原则,提出冻干过程中“干燥前期小功率、中期高功率、后期小功率”的三段微波功率控制方案。
3、对研制设备的干燥均匀性进行了测试和分析,并据此确定了适宜的微波开启方式和干燥均匀性调控方案。在微波整体开启、交替开启两种方式下,不同料层间、同一料盘不同铺料区域均表现出相类似的干燥速度差异：20盘物料呈现中间层干燥速度慢、外层干燥速度快的趋势,同一料盘呈现周边物料干燥速度快、中心部位干燥速度慢的趋势；与整体连续开启方式相比,微波交替间歇开启方式虽能较好地改善料盘之间物料和同一料盘内物料的干燥均匀性,但总体来看,20盘物料存留的干燥速度差异仍影响物料整体的干燥效率和干燥品质,干燥均匀性与干燥成品要求还有一定差距。在微波交替开启控制方案的基础上,通过采用料盘中心部位空缺铺料法、冻干期间料盘2次出仓换位堆垛法,可以较好地改善不同层料盘间、同一料盘内物料因干燥速度差异而引起的干燥不均匀问题,料盘内物料含水率均匀度约为90%,料盘间物料平均含水率均匀度达94.5%,基本满足冻干制品的实际生产要求。
4、基于研制的微波冻干设备,提出采用真空冷却预处理的微波冻干加工工艺方法,可以一次完成冻干加工中的冷却、沥水、预干燥、冻结、冻干等5个工序,简化工艺过程,降低设备设施投入成本及生产成本,并开展试验研究。对40kg胡萝卜片物料进行60min真空冷却处理,物料温度呈现4个阶段变化：闪发前降压段、闪发冷却至冰点温度段、冰晶生成段、深层冻结段；处理结束后,胡萝卜片芯部温度接近-30℃,表层温度为-40℃以下,可以满足冷冻干燥对物料冻结温度的要求；真空冷却预处理可脱除物料中29%的初始水分,脱水速率在闪发现象出现后突然增加,达到高峰并维持一段时间后,随后快速下降,并在后期达到平衡,维持极低的脱水速率。过程中可能有冷量损耗和无效失水等现象的存在,导致真空冷却过程中理论脱水量与实测值存在一定的差异。
5、真空冷却后无冷库冻结(A组)、真空冷却后再冷库冻结(B组)、无真空冷却处理(C组)三组冻干工艺试验表明：经过真空冷却处理的A、B两组在0～1h干燥区间的微波加载功率可明显高于C组,可明显改善冻干初期的低压放电问题；按照设定的微波功率加载方案,A、B、C三组所用的干燥时间分别为8h、8.5h、10h,由于真空冷却处理的预先脱水和冻干初期低压放电问题改善效果等原因,A、B组比C组分别减少冻干时间2、1.5h；在干燥终点,三组工艺处理的整批物料平均含水率分别约为：A组14%、B组16%、C组22%,A、B、C三组物料含水率实际上均未达到干燥要求,各层料盘整体干燥进度不一致,其中C组整体干燥进度最慢；整个工艺过程中,三组处理的单位脱水量耗电大小依次为A
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.
引文
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