面条热风干燥的湿热传递机理及耦合模型研究

Drying is the key process affecting noodle quality, production efficiency and energy consumption, but few literature was focused on the moisture and heat transfer, as well as microstructure analysis during the drying process. Most papers were focused on the qualitative analysis based on the hypothesis of moisture and heat transfer mechanism. In order to investigate the mechanism of moisture and heat transfer during the drying process, model noodles were made with wheat gluten and wheat starch in different proportions. The moisture status and moisture distribution of the model noodles in different drying time were determined by NMR and MRI, wheat gluten network and starch granules distribution were determined by SEM, and the main ingredient affecting the moisture transfer and moisture transport pathways could be confirmed. Model noodles with various porosity were dried, drying curves were compared and analyzed, and the effect of porosity on moisture transfer during drying could be confirmed. The temperature field during drying was determined by the infrared thermal imager, the moisture status and moisture distribution were also determined, and then the relation between moisture transfer and heat transfer was analyzed. The effective moisture diffusion coefficients and other thermal physical parameters related with ingredient, porosity, moisture content and temperature were obtained. Finally, the two dimensional moisture and heat transfer coupled model for the noodle drying process was built based on the local thermal non-equilibrium method, and the model could be used for the drying conditions optimizing and automation controlling for the noodle drying process.

干燥是影响挂面质量、生产效率和生产能耗的关键工序，而目前对面条干燥过程中水分和热量传递机理及微观结构分析的研究相对较少，主要以机理假设为主进行定性分析。本项目采用蛋白质/淀粉的不同配比制作模型面条，利用核磁共振技术和磁共振成像技术观察不同干燥时刻面条中水分的存在状态和水分分布，并利用扫描电子显微镜观察面筋网络结构和淀粉颗粒分布，揭示影响挂面组织结构和水分传输的主要原料成分及水分传输途径；采用变孔隙率法制作面条，绘制不同孔隙率的面条的干燥曲线，分析孔隙率对面条中水分传递的影响；利用红外显微热成像仪测定干燥过程中面条横断面的温度分布，并测定水分存在状态和水分分布，分析热量传递与水分传递的相互关系；确定基于原料成分、孔隙率、含水率、温度的水分扩散系数及其他热物理参数；最终，根据质量、能量守恒，建立面条干燥过程的局部非热平衡的二维湿热耦合传递模型，为干燥条件优化、干燥过程自动控制提供理论支持。

干燥是影响挂面质量、生产效率和生产能耗的关键工序。目前，对挂面干燥过程的研究以实验室的优化设计为主，对干燥过程中水分传递机理及微观结构的分析也以机理假设为主，进行定性分析。本研究主要开展了以下研究：1）采用谷朊粉/小麦淀粉按不同质量配比制作模型面条，测定了面条干燥过程中的干燥特性，并利用低场核磁共振和磁共振成像技术，测定了不同干燥时间挂面内部水分的结合状态和水分分布，利用扫描电镜观察其微观结构变化；2）将挂面看作虚拟连续介质体，根据质量、能量守恒，建立了挂面干燥过程的湿热耦合传递四参数数学模型，并基于实验原料测定了模型参数与蛋白含量等因素的关系；3）验证了模型的准确性，并应用该模型对挂面干燥过程进行了模拟和分析。结果表明：1）挂面内部水分可分为三类，即强结合水、弱结合水和自由水，其中弱结合水占比最大，大于80%。鲜面条中，淀粉含量对水分的结合能力影响较大，而干燥时，谷朊粉含量对水分传递影响较大。T22随谷朊粉和含水率的增加而增加。干燥开始时，T22为4~6ms，干燥1h后，T22降至2~3ms，干燥1.5h后，T22降至1~1.7ms，并维持稳定。高谷朊粉比例引起的孔隙率降低，阻碍了水蒸汽在挂面内部的传递，导致整体干燥速率降低。2）面条干燥过程的湿热耦合传递四参数数学模型，能够模拟挂面骨架及内部孔隙中空气的水分和温度变化，且基于实验原料测定的模型参数，包括有效水分扩散系数、孔隙度特性、比热容、导热系数、密度等，与蛋白质含量、水分、温度之间存在相关性，提高了模型的准确性。3）模型经过验证，模拟值与试验值吻合较好。干燥过程中最外侧水分先脱除，挂面温度先升高；干燥过程中存在含水率从表面开始的梯度界面，界面不断内移，10min到达挂面几何中心；水分场的不均匀性持续时间较长，约120min，而温度场的不均匀性持续时间较短，约20min。干燥过程中内部水分传递是影响挂面干燥的主要因素。