【作者】 王云阳；

【导师】 李元瑞；

【作者基本信息】 西北农林科技大学 ， 果树学， 2012， 博士

【摘要】 澳洲坚果(Macadamia tetraphylla)原产于澳大利亚，在澳大利亚、夏威夷、南非和南美洲都有商业化种植。据国际坚果委员会（INC）报道，在2010/2011年世界澳洲坚果产量约达到10万t。我国关于澳洲坚果的研发起步晚，20世纪70年代后，在广东、广西、云南和四川等省(区)的热带亚热带地区陆续引种。通过30多年的发展，我国澳洲坚果种植面积已接近6000hm2，当前主要的栽培区域是云南和广西。澳洲坚果具有厚且坚硬的外壳，导致其干燥周期很长，在工业化生产中干燥澳洲坚果需要一个多月的时间。此过程占地面积巨大，需要大量的人工操作，干燥费用也相当大。这就要求发展先进的加热技术，改造传统的干燥工艺，射频（RF）能量有潜力发展为传统干燥技术的替代者。射频加热对于农产品很有潜力进行均匀而快速地干燥，获得高质量的产品。为了得到热风辅助射频干燥澳洲坚果的工艺，本文进行了系统的试验研究。本文先后进行了澳洲坚果介电特性测定、解吸吸附等温线测定、澳洲坚果射频干燥和加热均匀性的试验研究。具体研究内容为：①采用开放末端同轴探头技术，在频率10MHz到1800MHz之间，温度25℃到100℃之间，水分含量3％到32％（干基，d. b.）之间，测量了坚果果仁的介电特性。②测定了澳洲坚果果仁粉和果壳粉在室温（25℃）下的解吸吸附等温线。使用统计分析软件SPSS (Version16.0, SPSS, Inc.)的非线性回归工具，分析评价了常见的6种模型对试验得到的解吸吸附等温线的拟合程度，以确定最佳拟合模型及其参数。③采用一个频率27.12MHz、功率6kW的射频加热系统，选用三个温度和三个电极板间距进行组合试验，以选择最佳的温度和极板间距。对热风辅助射频干燥与热风干燥澳洲坚果工艺进行了比较。使用软件SPSS的非线性回归工具，采用常见的六种干燥动力学模型，对热风辅助射频与热风干燥澳洲坚果的试验数据进行了拟合处理。在干燥过程中取出不同干燥时间的坚果，分析过氧化值和自由脂肪酸含量。④研究了射频加热和干燥澳洲坚果的均匀性。所用材料是已经预干燥的澳洲坚果和聚氨酯泡沫板，澳洲坚果的平均含水量为0.1056kg水·kg干物质-1。将堆叠起来的聚氨酯泡沫板水平或垂直放置在射频腔的下极板上的不同位置和不同高度，在通入热风和没有热风的条件下，研究加热均匀性和系统的电场分布。通过光纤传感器和红外成像测量温度，绘制温度分布等温线以及由试验数据计算出的均匀性指数进行对比评价。以坚果为原料，坚果放入容器中或放在堆叠起来的四层物料盘上，将盛有坚果的容器或物料盘放置在射频腔中的不同位置，在通入热风或者没有热风、移动或者静止的条件下，测定了热风辅助射频加热和干燥澳洲坚果的均匀性。主要结论如下：（1）果仁的介电常数和损耗因子，在频率10～300MHz之间，随着频率的增加迅速下降，而在频率300～1800MHz之间，随着频率的增加缓慢下降。介电常数和损耗因子随着水分含量和温度的上升而增大。随着射频频率、水分含量和温度的下降，介电穿透深度增大。以本项研究为基础，可以开发基于射频加热的均匀而快速的干燥技术，用于厚层澳洲坚果干燥。（2）根据国际理论和应用化学联合会（IUPAC）的分类，果仁粉解吸等温线属于第Ⅱ种类型，吸附等温线属于第Ⅲ种类型。GAB模型是最佳的解吸等温线拟合方程，Henderson模型是最佳的吸附等温线拟合方程。GAB模型拟合解吸等温线的参数A、B、C分别为8.2439、0.4815、1.3545。Henderson模型拟合吸附等温线的参数A、B分别为0.3006、0.8682。果壳粉解吸吸附等温线都属于第Ⅰ种类型。GAB模型是最佳的解吸等温线和吸附等温线拟合方程。GAB模型拟合解吸等温线的参数A、B、C分别为9.693、0.605、8.378，拟合吸附等温线的参数分别为9.695、0.635、3.268。（3）在热风辅助射频干燥过程中，极板距离对物料升温速度和平衡温度的影响比热空气大。最佳的组合为极板距离15.5cm，热风温度50℃。热风辅助射频和热风干燥澳洲坚果的干燥曲线呈现对数衰减模型。热风辅助射频干燥澳洲坚果需要360min，射频设备中的热风单独干燥需要750min，使坚果水分含量降到0.030kg水·kg干物质-1，与之对应的果仁水分含量为0.015kg水·kg干物质-1。Page模型是热风干燥最好的拟合模型，而Logarithmic模型拟合热风辅助射频干燥动力学效果最好。热风干燥和热风辅助射频干燥过程中，物料过氧化值和自由脂肪酸含量都随时间的增长而增大，但其值都在工业标准所要求的范围之内。结果表明热风辅助射频干燥澳洲坚果，干燥速度高、均匀性好、环保，很有潜力用于工业生产中。（4）采用聚氨酯泡沫板进行的试验表明，射频干燥腔中电场分布不均匀，存在角落和边缘加热效应，说明干燥腔的角落或边缘电场强度大。在所研究条件下，当堆叠的泡沫板在干燥腔的某一确定位置时，在水平和垂直方向上，都是中心温度高，周围温度低，呈现中心加热现象。热风有利于促进聚氨酯泡沫板射频加热的均匀性。坚果试验表明射频加热澳洲坚果也有角落和边缘加热效应。在所研究的试验条件下，移动物料，加热和干燥均匀性没有可察觉的明显提高。在同一位置，热风辅助射频加热坚果也呈现中心加热现象。热风是决定射频加热和干燥坚果均匀性的关键因素。更多还原

【Abstract】 Macadamia nuts (Macadamia tetraphylla) are native to Australia and growncommercially in Australia, Hawaii, South Africa, and South America. The world macadamiakernel production was estimated to be100,000metric tons in2010/2011according to thereport of International Nut Council (INC). Studies on macadamia nuts were developed late inChina. From1970s, macadamia nuts were introduced and planted in tropic and subtropicalzones, such as Guangdong, Guangxi, Yunnan and Sichuan provinces. After more than thirtyyears of developments, about6000hm2macadamia nuts were planted in China, with the mainplanting areas located in Guangxi and Yunnan. Macadamia nuts have thick and hard shells,which result in a very long drying cycle, often more than one month to complete the industrialdrying. The long process occupies enormous areas, with excessive handling of the in-processmaterial and considerable costs involved. It is desirable to develop advanced heatingtechnologies to speed up the conventional drying process. Radio frequency (RF) energy hasthe potential to be one of the alternatives since RF heating provides fast and uniform dryingwith acceptable product quality. To develop the optimal hot air assisted RF (RFHA) dryingprotocol of macadamia nuts, experiments were conducted systematically.In this study, the following experiments were conducted successively on macadamia nuts:determining dielectric properties, developing adsorption and desorption isotherms, andstudying RFHA drying of macadamia nuts and RFHA heating uniformity. The detailedresearch contents were completed as follows:①Dielectric properties of the macadamia nutkernels were measured between10and1800MHz using an open-ended coaxial-line probetechnique at temperatures between25and100°C and moisture contents between3%and32%on a dry basis (d. b.).②Adsorption and desorption isotherms of macadamia nut kernelsand shells at room temperature (25℃) were determined. Non-linear regression tool of thesoftware SPSS for Windows (Version16.0, SPSS, Inc.) was used to evaluate the fitness of thesix commonly used models based on the experimental data, and to determine the best modelsand their parameters.③A pilot-scale27.12MHz,6kW RF system was used. Experimentscombined with three temperatures and three electrode gaps were done to select optimal hot airtemperature and electrode gap. Comparisons were made between RFHA drying of nuts andhot air (HA) drying. Six drying kinetic models were fitted with the data of HA and RFHA drying of nuts, using nonlinear regression tool of the software SPSS. Peroxide value and freefatty acid were analyzed of nuts obtained during drying.④Experiments were conducted todetermine the heating and drying uniformity of macadamia nuts. The material was pre-driedmacadamia nuts with averaged moisture content of0.1056kg water·kg dry solid-1andpolyurethane foams. Stacked polyurethane foam sheets were located at different positionshorizontally or vertically and different height from the ground electrode in the RF cavity withor without hot air to investigate the heating uniformity and electric field distribution of thesystem. Comparrisons were made of temperatures measured by fibre optic sensors andinfrared imaging, contour plot and uniformity index derived experimentally for the RF unit.Heating and drying uniformity was conducted with nuts in containers or four layers placed atdifferent positions in the RF cavity with or without hot air under move or static conditions.The main results were described as follows.(1) Both dielectric constant and loss factor of the kernels decreased sharply withincreasing frequency over the RF range (10～300MHz), but gradually over the measuredMW range (300～1800MHz), which were largely enhanced by increasing moisture contentand temperature. Penetration depth decreased with increasing frequency, moisture content,and temperature. Based on this study, uniform drying of macadamia nut kernels in thicklayers could be effectively developed using RF energy.(2) For macadamia nut kernels, the obtained desorption isotherm was of type Ⅱ andadsorption of type Ⅲ according to IUPAC’s classification. GAB model was the best fitted fordesorption isotherm and that for adsorption was Henderson model. The parameters (A, B, C)of GAB model were8.2439,0.4815, and1.3545, respectively, for desorption isotherm. Theparameters (A, B) of Henderson model were0.3006and0.8682for adsorption isotherm. Formacadamia nut shells, the obtained desorption and adsorption isotherms were of type Ⅰ,according to IUPAC’s classification. The hysteresis loop was of type H3. GAB model was thebest fitted for desorption and adsorption isotherms. The parameters (A, B, C) of GAB modelfor desorption isotherm were9.693,0.605and8.378respectively, and those for adsorptionisotherm were9.695,0.635and3.268.(3) Gap of electrodes played more important role in the heating rate and equilibriumtemperature of samples than hot air in RFHA drying. The optimal combination of gap and hotair temperature was15.5cm and50°C. The drying curves showed an exponential decay andrequired750min and360min to achieve the final moisture content of0.030kg water·kg drysolid-1in whole nuts and of0.015kg water·kg dry solid-1in kernels for hot air and RF heating,respectively. The drying kinetics of the nuts could be well described by Page Model for hotair drying but by Logarithmic model for RF drying. Peroxide value and free fatty acid increased with the drying time both for hot air and RF drying but remained within acceptablerange required by the nut industry. The RF treatment holds potential to provide rapid, uniformand environmentally friendly drying technology for the nut industry.(4) Experiments with foam showed that electric field was not uniformly distributed in RFcavity. There existed corner or edge heating in RF cavity, in which the electric field wasstrong at the corner or the edge of the cavity. When foam stacks was placed at the sameposition horizontally or vertically in RF cavity, it showed that temperatures were higher in thecenter than that all around. There appeared center heating pattern of the foam stacks under thestudied conditions. Hot air was helpful to facilitate the RF heating uniformity of stackedpolyurethane foam cubes. Experiments with nuts showed that edge heating also existed in RFcavity. Under the experimental conditions, moving did not brought noticeable improvementon the heating and drying uniformity. There appeared center heating pattern of RFHA heatingof nuts. Hot air was vital factor for uniform heating and drying of nuts. The determinedRFHA heating and drying conditions were useful for further detailed drying studies of nuts.更多还原