脉冲电场预处理对苹果冷冻干燥崩塌温度及干燥品质的影响

During the freeze drying process, when the drying temperature was reached the collapse temperature, the scasoning check and collapse phenomena were appeared and thus reduce the drying effect. Pulsed electric field as non-thermal processing for drying on fruits and vegetables in the freeze drying not only can increase the diffusion coefficient of water and the drying layer temperature, but also can improve product quality and reduced energy consumption. To prevent the collapse of tissue, improve the drying rate and quality, the drying experiments were conducted with the typical fruits and vegetable, such as apple, which were pretreated by pulsed electric field. The scanning electron microscope observations were performed combined with low temperature drying microscopy techniques to analysis the effect of pulsed electric field pretreatment on drying temperature and micro-structural of apple tissues, and determined the collapse temperature. Moreover, the simultaneous heat and mass transfer model was established to predict the drying temperature and moisture distribution in apple tissues that pretreated by pulsed electric field, during freeze drying using fractal geometry theory. And the mechanics mechanism of pulsed electric field on collapse temperature of apples was revealed. According to study the effect of the parameters of pulsed electric field and drying processing on the quality of apple, the optimal parameters of pulsed electric field and vacuum freeze drying processing technology were obtained. The results can reduce energy consumption and improve product quality, thus it provide a strong theoretical support for thermal control of agricultural products drying.

果蔬冷冻干燥达到崩塌温度时，存在组织断裂、细胞塌缩等现象，影响干燥品质。脉冲电场作为非热加工预处理技术，与冷冻干燥结合，能加快水分扩散传质过程，提高果蔬干燥层温度，改善品质。本研究选取典型果蔬苹果为研究对象，以改善细胞崩塌，提高干燥速率与品质为目的，开展不同脉冲电场参数预处理后苹果的冷冻干燥试验，通过低温冻干显微镜等技术手段探索经脉冲电场预处理后苹果微观结构与干燥层温度之间的关系，确定预处理前后崩塌温度的范围；基于分形几何理论，结合苹果的结构与物性参数，建立预处理后苹果冻干过程的传热传质模型，分析冻干过程中崩塌温度和样品含水率的变化规律，揭示脉冲电场预处理对苹果冷冻干燥崩塌温度的影响机理；通过研究脉冲电场和冻干过程参数对苹果干燥品质的影响，获得优化的脉冲电场和冻干过程参数组合，达到提高干燥速率，改善干燥品质的目的。

针对苹果在冷冻干燥过程中存在组织断裂、细胞塌缩等现象，影响其干燥品质的实际问题，研究应用脉冲电场预处理苹果，与冷冻干燥结合，能加快水分扩散传质过程，提高苹果干燥层温度，改善品质。项目进行了脉冲电场作用苹果冷冻干燥试验及对其干燥特性和品质的影响研究，在脉冲强度为1.5 kV/cm，脉冲个数为10个时，干燥能耗降低23.06%，干燥时间缩短了15.60%，有效水分扩散系数提高了21.16%，复水率提高了55.31%，而对Vc含量的影响则不明显。探讨了电场预处理对苹果微观结构的影响，苹果微观结构的完整性随着脉冲电场参数的增大而降低，未处理的干燥苹果结构相对完整，但细胞具有明显的褶皱。通过研究电场预处理对苹果力学性能的影响可知电场预处理后苹果组织的硬度和脆度均下降，且硬度和脆度均随着电场强度和脉冲个数的增加而增加。探讨了脉冲电场对苹果崩塌温度的影响，脉冲电场预处理可以提高苹果样品的崩塌温度20℃左右，经过脉冲电场预处理后苹果的崩塌温度为70℃～75℃，还探索了电场作用苹果样品对传热传质过程的影响，研究结果可为脉冲电场在冷冻干燥中的广泛应用提供基础支持。