【作者】 黄建立；

【导师】 郑宝东； 宋洪波；

【作者基本信息】 福建农林大学 ， 农产品加工及贮藏工程， 2010， 博士

【摘要】 银耳是我国主要的食用菌之一,也是重要的出口创汇特色农产品之一。目前银耳干制多采用传统的热风烘干,其干燥效率低、能耗大、产品质量差。微波真空干燥技术是一种高效、节能及环保的现代干燥技术,将微波真空干燥技术应用于银耳干制将为银耳产业的发展开创一条崭新的加工途径。本项目研制开发微波真空干燥设备,解决其干燥均匀性问题,研究银耳微波真空干燥动力学过程,探究银耳的微波真空干燥品质,研究不同干燥方式对银耳主要品质的影响,优化银耳热风-微波真空联合干燥工艺,并建立银耳中米酵菌酸的测定方法以及探讨微波处理对米酵菌酸的去除作用。1.微波真空干燥设备的研制。针对微波真空干燥不均匀以及采用实底载料盘干燥时间较长的问题,进行微波真空干燥机干燥系统的设计研究。研究确定了干燥室型式尺寸以及谐振腔模式:90×90×100cm3矩形谐振腔,其具有242个谐振频率模式;根据生产能力计算出微波总功率为4kW;采用干燥室上、下壁面对角交叉及均匀布置的方式进行微波馈入口布局,可保证回转干燥的均匀性;以葫萝卜为测试材料,进行载料盘对干燥均匀性影响的研究,发现采用筛底载料盘比实底载料盘可有效提高干燥速度、缩短干燥时间。2.银耳微波真空干燥动力学的研究。试验结果表明银耳微波真空干燥过程可分为加速干燥、恒速干燥及降速干燥3个阶段。在真空条件下对银耳进行微波干燥,能明显提高干燥速率,但通过提高真空度来加快干燥速率具有一定限度,真空度过低会影响干制银耳的品质,因此应选取合适的真空度进行银耳微波真空干燥。单位质量发射功率和初始含水率对银耳的干燥速率影响很大,单位质量发射功率越大,初始含水率越小,干燥速率越快,干燥时间越短。应用Matlab软件进行多项式曲线拟合,表明二次多项式能够有效地表达各干燥条件下的银耳干燥动力学曲线。利用BP神经网络模拟技术来减少实验次数,预测干燥结果的方法是可行的。神经网络预测值与实际值拟合效果良好,预测精度高。3.银耳微波真空干燥品质的研究。试验表明银耳干品的收缩率随单位质量发射功率的增大及真空度的升高而降低,复水比则随单位质量发射功率的增大及真空度的升高而增大;真空度对银耳收缩率及复水比的影响比单位质量发射功率的影响要小。单位质量发射功率增大,干制银耳的多糖含量降低;真空度升高,干制银耳的多糖含量也随着增加。真空度越低,银耳干品的中心越易出现焦化现象。为避免银耳多糖的损失,应尽量选择高真空度进行微波真空干燥。综合考虑单位质量发射功率不宜过高也不宜太低,以10W/g为宜,而真空度以-90kPa为宜。4.不同干燥方式对银耳主要品质影响的研究。分别采用热风干燥、微波干燥、真空干燥、微波真空干燥及冷冻干燥5种干燥方式干燥鲜银耳,并对银耳干品的收缩率、复水比、色泽、感官质量、多糖含量及组织结构等品质的影响进行分析,以及比较传统热风干燥与微波真空干燥银耳的干燥时间、干燥速率以及能耗。对比结果表明:微波真空干燥的银耳除亮度值低外,其它品质均较优,且干燥能耗低。微波真空干燥是一种值得推广应用的干燥方式。5.银耳热风-微波真空联合干燥工艺优化的研究。选取收缩率、复水比、感官质量以及单位能耗为评价指标,研究热风温度、转换水分含量及单位质量发射功率等因素对银耳品质以及干燥能耗的影响,通过正交优化确定银耳热风-微波真空联合干燥的较佳工艺参数为:前期热风温度70℃,转换水分含量30%,后期微波强度5W/g。将较佳工艺与热风干燥以及微波真空干燥进行比较,表明热风-微波真空联合干燥较传统热风干燥银耳的品质明显提高,能耗明显降低。6.微波真空干燥对银耳主要安全指标影响的研究。试验建立了测定变质银耳中米酵菌酸含量的方法—紫外分光光度法,并与高压液相色谱法相比,测定结果无显著差异,精密度与准确度均较好,且紫外分光光度法仪器成本低,操作简便,适宜推广应用;比较热风干燥、微波干燥以及微波真空干燥对变质银耳中米酵菌酸含量的影响,结果表明与传统热风干燥相比,微波干燥、微波真空干燥对银耳中的米酵菌酸均有一定的去除效果,去除率分别为23.8%和9.5%,微波干燥的去毒效果更明显。更多还原

【Abstract】 White fungus is one of main edible fungus in China, and it is also one of important characteristic agricultural products exported to earn. At present, drying method of white fungus usually adopts traditional airflow drying, which consumes high drying consumption and presents low drying efficiency and poor product quality. Microwave vacuum drying (MVD) technology is one kind of high-efficiency, energy-conservation and environmental protection drying technology. The application of MVD will start a new treating approach to white fungus drying. In this study, MVD equipment was designed to solve the problem of drying uniformity, the MVD kinetics process of white fungus was studied, MVD quality of white fungus was explored, effects of different drying methods on the quality of white fungus was studied, combined airflow and MVD technology of white fungus was optimized, testing method of Bongkrekic acid (BA) in degenerative white fungus was established and removal effect of BA by microwave treatment was investigated.1. MVD equipment was developed. MVD system was designed to solve drying uniformity and long-drying-time consumption with load tray of solid bottom. The type with size of drying chamber and resonator was determined as follows: rectangular resonator with the volume of 90×90×100cm3, which had 242 resonant frequencies. The total microwave power was calculated base on the productivity was 4kW. It could guarantee rotary drying uniformity by using diagonal crossover of upper and underside walled surface and uniform arrangement for designing microwave fed entrance. Using carrot as testing material, it was found that load tray of solid bottom could enhance drying rate and shorten drying time than load tray of ethmoidal bottom by studying effects of load tray on drying uniformity.2. MVD kinetics of white fungus was studied. The experiment results showed that the MVD process of white fungus was divided into three stages that were speed-up , constant-speed and speed-down. Vacuum condition could obviously enhance drying rate, however, which was limited. If vacuum degree was too low, the quality of dried white fungus would be influenced. Therefore, appropriate vacuum degree should be chosen to drying white fungus. Unit mass microwave power and initial moisture content had great influence on drying rate of white fungus. The higher unit mass microwave power, the less initial moisture content was, and then the quicker the drying rate was, the shorter the drying time was. Quadratic polynomial could express drying kinetics curves of every drying condition by using Matlab software for polynomial curves fitting. It was feasible to reduce experiment times and predict drying results by applying BP neural network simulation technique. Predictive value of neural network and actual value had good fitting results, and prediction precision was good.3. MVD quality of white fungus was studied. Shrinkage of dried white fungus decreased when unit mass microwave power and vacuum degree increased, rehydration ratio increased when microwave intensity and vacuum degree increased, and vacuum degree had less important effect on the shrinkage and rehydration ratio of white fungus than unit mass microwave power. Polysaccharide contents of white fungus increased when microwave intensity decreased and vacuum degree increased. The lower vacuum degree was, the easier the coking phenomenon occurred in the center of dried white fungus. In order to avoid the loss of polysaccharide, high vacuum degree should be chosen as possible. Comprehensively considered, unit mass microwave power was neither too high nor too low, using 10W/g, and vacuum degree used -90kPa.4. Effects of different drying methods on the quality of white fungus were studied. Five drying methods, viz., airflow drying(AD), vacuum drying, microwave drying(MD), MVD and freezing drying were applied to drying fresh white fungus, effects on shrinkage, rehydration ratio, sensory quality and organization structure of dried white fungus were analyzed, and the drying time, drying rate and consumption between airflow drying and MVD were compared. The comparison results showed that MVD was the most advantageous method and worthy of popularization, for it consumed lower energy and generated a better quality in spite of deficient gloss.5. Optimization of combined AD and MVD technology for white fungus was studied. Choosing shrinkage ratio, rehydration ratio, sensory quality of dried white fungus and unit energy consumption as evaluation indexes, effects of airflow temperature, conversed moisture content and unit mass microwave power on the quality of dried white fungus and drying energy consumption were studied. By orthogonal optimization, the optimal combined AD and MVD technology of white fungus was determined as follows: airflow temperature was 70℃, conversed moisture content was 30%, and unit mass microwave power was 5 W/g. The better technology of combined AD and MVD for white fungus was compared with single AD and single MVD. It showed that combined AD and MVD could greatly improve the quality of dried white fungus and obviously reduce the energy consumption than single AD.6. The effect of MVD on main safety index of white fungus was studied. Testing method of BA content in degenerative white fungus, that was uv spectrophotometry, was established by experiment. There was no significant difference in testing results between uv spectrophotometry and high pressure liquid chromatography. Precision and accuracy of uv spectrophotometry were both good, and this method was suitable to popularize due to its low equipment cost and convenient operation. Effects of AD, MD and MVD on BA content in degenerative white fungus were compared. Results showed that MD and MVD both had removal effect compared with AD, and removal rate was respectively 23.8% and 9.5%, which signified detoxification effect of MD was more obvious.更多还原