真空冷却预处理在微波冻干胡萝卜片中的应用

摘要

为了探讨真空冷却预处理在微波冻干工艺中应用的可行性,该文以WDG-5型微波冻干设备为平台,以胡萝卜为试验原料,开展真空冷却预处理工艺试验以及3组不同工艺方法包括真空冷却后无冷库冻结、真空冷却后再冷库冻结、无真空冷却处理对微波冻干的影响研究.真空冷却工艺试验中,在真空泵和冷阱制冷机组满负荷运转的工况下,预处理22 min后胡萝卜片达到冰点,物料失水16.5％,在预处理60 min后,物料表层和芯部温度分别为-41.2、-29.5℃,物料总失水20％.3组冻干试验中,与无真空冷却处理组相比,真空冷却后无冷库冻结组、真空冷却后再冷库冻结组的干燥初期微波安全加载功率相对较高,冷冻下燥时间缩短了2h,总耗电分别节省了17.3％、19.9％;3组试验的冻干胡萝卜片在含水率、复水率、颜色等指标方面均无显著差异,真空冷却后无冷库冻结组、真空冷却后再冷库冻结组的胡萝卜片维生素C保存率均超过80％,而无真空冷却处理组维生素C保存率为68％.结果表明,在胡萝卜片微波冻干工艺中应用真空冷却预处理技术,有效改善了冻干初期的微波低压放电问题,缩短干燥时间,降低能耗,并提高维生素C保存率.%In order to discuss the feasibility of vacuum-cooling pretreatment applied to microwave freeze drying process, the experiment of vacuum-cooling pretreatment process and three different processing techniques of microwave freeze drying were conducted on WDG-5 type microwave freeze dryer using the material of carrot slices, which involved in 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. After 22 minutes of vacuum cooling with full capacity of the vacuum pump and cold trap, the temperature of carrot slices declined to freezing point as well as 16.5% water content removed. 60 minutes of vacuum cooling pretreatment removed 20% water content and decreased surface temperature and core temperature of the carrots to -41.2℃ and -29.5℃, respectively. Compared with group C, group A and B increased microwave power in initial drying stage, shortened the drying time by 2 h, and respectively reduced 17.3% and 19.9% of the total power consumption. There was no significant difference among three groups' dried carrot slices in moisture content, rehydration rate and color indices. The retention rates of Vitamin C in group A and B were both more than 80%, higher than the 68% retention rate in group C. It is concluded that vacuum-cooling pretreatment applied to microwave freeze drying can alleviate the problem of glow discharge in initial drying stage, reduce the drying time and power consumption and improve the retention rate of Vitamin C.