莴笋渗透脱水传质动力学及渗后热风干燥特性研究

【作者】 王妮；

【导师】 杨薇；

【作者基本信息】 昆明理工大学 ， 农业机械化工程， 2011， 硕士

【摘要】 渗透脱水是将果蔬等多孔性物料浸入高渗溶液中,在高渗压引起的驱动力作用下水分自发向外转移的过程。采用渗透脱水预处理能减少后续干燥时间和对产品品质的热损害,但目前国内外的渗透脱水对象集中在水果类,所得结果对蔬菜的研究指导作用有限,且与渗透脱水相关的数学模型还可进一步探讨。因此本课题提出对莴笋渗透脱水的传质动力学及渗后热风干燥特性进行研究,希望为渗透脱水的理论研究和实际生产提供一定参考。本课题以莴笋为研究对象,以失水率、固形物增加率、有效扩散系数、湿基含水率、干燥速率为指标,试验采用回归正交旋转设计和均匀设计方法,同时使用SPSS、Matlab及SYSTAT软件作为统计分析、绘图和优化软件。首先,设计了一个五元(葡萄糖浓度、氯化钠浓度、温度、厚度和时间)二次回归正交旋转试验,研究了莴笋渗透脱水失水率和固形物增加率的传质回归方程,优化了工艺参数；其次,以葡萄糖浓度和温度为影响因素,根据均匀试验结果,建立了莴笋渗透脱水的动力学模型,拟合了有效扩散系数与因素间的回归方程；最后,研究了风温、风速、铺料密度对渗后莴笋热风干燥特性的影响,比较了渗后和未渗样品的干燥特性、产品外观品质及复水特性。结果表明：第一,葡萄糖浓度、渗透时间增加能显著提高失水率和固形物增加率；在葡萄糖+氯化钠组合溶液中,氯化钠浓度对失水率无明显影响但对固形物增加率有极显著促进作用；温度升高不利于失水但利于固形物增加；而厚度增加既不利于失水也不利于固形物渗入；莴笋渗透脱水的最优工艺为：葡萄糖浓度32.5%、氯化钠浓度2%、温度35℃、厚度5mm、时间139min；在此条件下,失水率的预测值及试验值分别为52.08%和53.54%、固形物增加率的预测值及试验值分别为9.83%和9.14%；失水率和固形物增加率的回归方程可用于莴笋渗透脱水的预测。第二,Azuara模型可计算平衡时刻的失水率和固形物增加率；两指数模型比Page模型更适于描述莴笋渗透脱水的动力学过程；根据曲面拟合方法得到的莴笋渗透脱水有效扩散系数与葡萄糖浓度和温度的回归方程拟合性高；水分、固形物有效扩散系数大小与平衡时刻的失水率和固形物增加率无直接关系,有效扩散系数表示的是失水率和固形物增加率达到平衡时刻的快慢程度。第三,风温、风速增加能减少莴笋渗后热风干燥时间,而铺料密度增加会延长干燥时间；莴笋经渗透脱水后,干燥速率虽有所降,但干燥时间大为减少；莴笋渗后样品在热风干燥或复水后,其外观品质均优于未渗样品；渗后样品在高温下复水较快而未渗样品适宜在室温状态下复水,渗后样品的复水能力好于未渗样品；莴笋渗后样品复水时间可控制在1h内。更多还原

【Abstract】 Osmotic dehydration is based on the principle that a driving force for water removal sets up because of the high osmotic pressure of the hypertonic solution when porous material (such as fruits and vegetables) are immersed in a hypertonic aqueous solution. Osmotic dehydration could minimize the heat damage to quality and reduce total drying time. But the main research materials about osmotic dehydration are fruits. Because results from those studies could not be used in vegetables absolutely and models of osmotic dehydration could have a further study. So the object of this paper is to study the kinetics of mass transfer during osmotic dehydration of asparagus lettuce and following air-drying characteristic so as to provide references for research and actual processing.In this paper, asparagus lettuce was chosen to be experimental subject. And experimental indicators included water loss, solid gain, effective diffusion coefficients, wet basis and drying rate. Regression orthogonal rotatable design and uniform design were used in this study. Besides, SPSS, Matlab and SYSTAT were used to statistical analysis, drawing and optimization.First, a quadratic regression orthogonal rotatable design (factors including glucose concentration, sodium chloride concentration, temperature, thickness and dehydration time) was used to study the regression equations of water loss and solid gain on mass transfer of osmotic dehydration and processing parameters were optimized. Secondly, glucose concentration and temperature were decided to be factors. According to the results of uniform design, kinetics models of asparagus lettuce during osmotic dehydration were built and regression equations which were related to effective diffusion coefficients and factors were obtained. Last, effects of air temperature, air velocity and paving density on drying characteristics of asparagus lettuce treated with osmotic dehydration were studied. Also, drying characteristics, exterior quality of products and rehydration characteristics of samples with and without treated by osmotic dehydration were investigated.The results showes that, first, water loss and solid gain increase significantly by the increasing of glucose concentration and time. In the combined of glucose and sodium chloride solution, the increasing of sodium chloride concentration does not affect water loss apparently but increasing solid gain significantly. The increasing of temperature could decrease water loss but it increases solid gain. Thickness has negative effects on two indicators. And the optimum operating conditions for osmotic dehydration of asparagus lettuce are found to be glucose concentration of 32.5%, sodium chloride concentration of 2%, temperature of 35℃, thickness of 5mm and dehydration time of 139min. At this optimum point, predict and experimental values of water loss are 52.08% and 53.54% respectively and the values of solid gain are 9.83% and 9.14%. So regression equations of water loss and solid gain can predict the mass transfer of osmotic dehydration of asparagus lettuce.Secondly, Azuara model could calculate water loss and solid gain of equilibrium. The two-term exponential model is more suitable for the kinetics of osmotic dehydration than Page model in this research. Regression equations based on surface fitting represent the relationship between effective diffusion coefficients and factors (glucose concentration and temperature) well. And effective diffusion coefficient does not have direct relationship to water loss or solid gain of equilibrium, however, it represent the rate of water loss and solid gain reaching to the equilibrium.Thirdly, the increasing of temperature and air-velocity could reduce drying time, but paving density has a negative effect on it. Samples pre-treated with osmotic dehydration consume less drying time, but they have a lower drying rate comparing with untreated ones. And the exterior qualities of pre-treated samples after air-drying or rehydration are better than those untreated samples. The pre-treated samples rehydrate faster in water solution of high temperature, however, untreated samples are suitable for rehydrating under room temperature. In addition, pre-treated samples have a better rehydration capability than untreated samples. For samples which are treated by osmotic dehydration, the rehydration time can be controlled within 1h.更多还原