【作者】 王玉川；

【导师】 阿润·莫占达；

【作者基本信息】 江南大学 ， 农产品加工与贮藏工程， 2013， 博士

【摘要】 莴苣是营养价值较高的蔬菜之一，尤其莴苣茎营养更为丰富，富含K+，Na+，Ca+等多种离子。本文以莴苣茎颗粒作为果蔬原料的代表进行负压微波高效节能均匀干燥机理及工艺实验研究。为了分析莴苣颗粒介电特性与电特性变化，以及对负压微波干燥特性及品质影响的目的，进行热水漂烫（HWB）及微波漂烫（MWB）莴苣颗粒在冷却、冻结、解冻及冻干（FD）过程介电特性、电导率变化趋势研究，以及HWB与MWB莴苣颗粒微波冻干（MFD）干燥特性与品质研究。实验结果表明：漂烫处理可以明显地改变莴苣颗粒细胞中电解质区域化分布，MWB莴苣颗粒的电导率值在常温下是HWB电导率值的2倍，是未漂烫（NOB）电导率值的5倍；MWB导致莴苣颗粒介电常数的减小及介电损耗的增加；HWB与MWB莴苣颗粒的介电常数及介电损耗因子随着温度的降低而快速下降；在FD过程中，介电常数及介电损耗因子随着干燥时间先上升再下降； MWB莴苣颗粒MFD干燥时间（4.5h）比HWB缩短30%以上。为了从干燥机理上分析导致负压微波干燥低压气体放电原因的目的，进行了MFD机理研究。通过热水、微波两种漂烫方式对莴苣颗粒冻结点与融化点温度、未冻结水分布与流动性研究、以及结合上述对莴苣颗粒介电特性、电导率及MFD特性影响研究，发现莴苣颗粒在MFD过程中吸收的微波能量与莴苣的组织结构、未冻结水的含量及分布有直接的关系。基于冻结产品中未冻结水的数量及分布、介电特性、离子特性，以及有效地防止低压气体放电，根据热量的供应方式，MFD过程分成三个干燥阶段，即预升华阶段、微波升华阶段及解吸阶段。为了开发高效均匀负压微波干燥实验装置的目的，在介电特性、电导率及MFD机理研究的基础上，通过分析影响负压微波干燥均匀性的各种因素，提出了颗粒物料负压微波脉冲喷动干燥概念，并进行实验装置设计、研制。实验结果表明：负压微波脉冲喷动干燥装置设计合理，解决了低压气体放电现象，并且改善了微波干燥均匀性。为了验证负压微波脉冲喷动干燥装置性能的目的，进行了莴苣颗粒脉冲喷动微波真空干燥及冻干均匀性、干燥特性及品质的工艺实验研究。实验结果表明：与传统负压微波干燥装置相比，负压微波脉冲喷动干燥可以明显地提高莴苣颗粒负压微波干燥均匀度（90%以上）及复水后的弹性，缩短干燥周期50%（微波真空）及20%（微波冻干）以上，单位干燥产品的能耗比传统的真空干燥节能50%以上。更多还原

【Abstract】 Stem lettuce is one of the highly valued vegetables for its ediblestems, especially beingrich in K+, Na+, and Ca2+. In this paper, mechanism and technology of negative pressuredrying assisted by microwave for stem lettuce cubes as a model for plant tissue were studiedwith efficiency, energy-saving, and uniformity。Effects of boiling water and microwave blanching methods on the dielectric properties,electrical conductivity, drying characteristics as well as quality of stem lettuce cubes werestudied. Results showed that the electrical conductivity of samples blanched by microwavewas two times higher than that of boiling water blanched ones and five times higher than thatof unblanched samples. Apart from that, dielectric constant (ε ’) and loss factor (ε ’’) of thestem lettuce cubes decreased significantly after freezing. MFD duration of microwaveblanched samples was approximately4.5h, reduced by30%compared to boiling waterblanched ones. MFD process in a circular conduit yielded products of high quality comparedto that in a tray.Mechanism of microwave freeze drying (MFD) was studied by investigating the changesin the ice-melting and freezing point temperature as well as distribution and migration ofunfrozen water in stem lettuce cubes using differential blanching methods. Results showedthat the microwave energy supplied and specimen temperature profiles during MFD werestrongly influenced by the tissues, unfreezable water content, moisture distribution, and ioniccharacteristics of the samples. It was also found that the MFD process involving the conduitscan be divided into three steps based on the amount and distribution of unfrozen water in thefrozen samples and how heat is supplied in order to prevent corona discharge.Based on the studies above, a laboratory scale apparatus was designed and developedtaking into account some key factors which influence uniformity of quality ofnegative-pressure microwave dried products, and the energy-consumption for dried stemlettuce slices was also analyzed. Results showed that this negative-pressure pulse-spoutedmicrowave dryer can prevent corona discharge and improve drying uniformity as well asreduce energy-consumption by more than50%when compared to conventional vacuumdryer. In addition, this apparatus owns simple structure, small investment and easy operation.Technology, Drying uniformity，drying characteristics，and quality of stem lettuce slices were studied using the newly developed laboratory scale negative-pressure pulse-spoutedmicrowave dryer containing microwave vacuum drying (MVD) and MFD. Results showedthat negative-pressure pulse-spouted microwave drying resulted in dried stem lettuce sliceswith more uniform (90%), higher rehydration capacity as well as greater hardness afterrehydration over a shorter drying time reducing by50%for MVD and20%for MFD relativeto those obtained in a conventional negative-pressure microwave dryer.更多还原