| 苹果冷冻-热风联合干燥体积收缩机制 |
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| 引用本文： | 任广跃,朱乐雯,段续,刘文超,李格格,卫新雨.苹果冷冻-热风联合干燥体积收缩机制[J].农业工程学报,2024,40(2):63-71. |
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| 作者姓名： | 任广跃 朱乐雯 段续 刘文超 李格格 卫新雨 |
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| 作者单位： | 河南科技大学食品与生物工程学院，河南 洛阳 471023;河南省农产品干燥装备工程技术研究中心，河南 洛阳 471023 |
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| 基金项目： | 国家自然科学基金项目（32172352）；中原科技创新领军人才项目（234200510020） |
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| 摘 要： | 为降低冻干苹果能耗，同时获得具有良好外观的脱水产品，该研究将冷冻-热风联合干燥应用于苹果脱水加工，并从水分迁移角度探究此过程中产品的收缩机制。选取4个水分转换点（干基含水率分别为1.00、0.76、0.53和0.33 g/g）对苹果进行联合干燥处理，并对脱水产品收缩率、质构特性、微观结构、孔隙分布及样品在热风干燥阶段的水分迁移与分布进行测定及分析。结果表明，联合干燥样品的收缩情况显著（P<0.05）优于单一热风干燥样品，且转换点对样品收缩率影响较大（收缩率6%～45%），当转换点干基含水率低于0.53 g/g时，联合干燥样品没有出现明显的体积收缩现象。随着转换点干基含水率的升高，样品的收缩程度增大，并出现不同程度的中心塌陷，且孔隙率逐渐减小，但相应能耗降低。产品收缩主要发生在热风干燥过程的升速阶段，在此阶段样品自由水含量大幅减少，结合水与不易流动水未发生明显改变，样品内部水分在湿度差的作用下向表面迁移，这是导致联合干燥样品发生体积收缩的关键机制。该研究结果可为冷冻-热风联合干燥高效生产良好外观的脱水苹果提供数据支撑及理论参考。
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| 关 键 词： | 冷冻干燥|热风干燥|联合干燥|收缩|水分迁移|微观结构 |
| 收稿时间： | 2023/6/26 0:00:00 |
| 修稿时间： | 2023/9/18 0:00:00 |
Volume shrinkage mechanism for combined vacuum freezedrying-hot air drying of apples |
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| REN Guangyue,ZHU Lewen,DUAN Xu,LIU Wenchao,LI Gege,WEI Xinyu.Volume shrinkage mechanism for combined vacuum freezedrying-hot air drying of apples[J].Transactions of the Chinese Society of Agricultural Engineering,2024,40(2):63-71. |
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| Authors: | REN Guangyue ZHU Lewen DUAN Xu LIU Wenchao LI Gege WEI Xinyu |
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| Institution: | College of Food and Biological Engineering, Henan University of Science and Technology, Luoyang 471023, China;Agricultural Products Drying Equipment Engineering Technology Research Center, Luoyang 471023, China |
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| Abstract: | The combination of vacuum freeze drying (FD) and hot air drying (HAD) has been observed to markedly reduce the volumetric shrinkage of hot air dehydrated products. While the FD can be attributed to concurrently optimizing energy consumption. However, the appearance quality of combined drying materials is still unstable at present. This study aims to clarify the shrinkage mechanism of apple products during the FD-HAD. Four points of moisture transition (moisture contents of dry basis were 1.00, 0.76, 0.53, and 0.33 g/g, respectively) were selected for the combined drying of apples. Shrinkage, texture, microstructure and pore distribution were determined in the dehydration products. The water migration and distribution of the samples were analyzed at the HAD stage using low-field nuclear magnetic resonance (LF-NMR). The results showed that the shrinkage of FD-HAD samples was significantly (P<0.05) better than that of HAD ones. The moisture transition point shared a significant (P<0.05) effect on the shrinkage of the samples (6%~45%). There was no outstanding volumetric shrinkage when the moisture content at the transition point was below 0.53 g/g. The samples were more shrinkable, harder and less crisp, as the moisture content increased at the transition point. There was a great variation in the center collapse. The decrease was found in the porosity and average pore size, with the increase of moisture content at the transition point. FD-HAD treatment saved energy more substantially (23.58%~28.95%), compared with the FD. The unit energy consumption decreased gradually with the increase of water content at the conversion point. The shrinkage of the sample in the FD-HAD occurred in the HAD when the moisture content at the conversion point was greater than 0.53 g/g. The sample was divided into two phases of rising and falling in the HAD, where the volumetric shrinkage of the sample mainly occurred in the rising phase. A three-phase transition of water was found in the sample from the ice crystals to the liquid and water vapor, which was then removed from the sample. There was a rapid decrease in the free water, and little change was measured in the content of bound and immobile water. There was a larger humidity difference between the center and the surface of the sample. The water was migrated from the center to the surface. The ice crystals inside the sample were basically removed by sublimation, and then to reduce the melting of ice crystals, when the moisture content at the transition point was less than 0.53 g/g. The small amount and range of free water and its migration resulted in a small shrinkage of the sample. In summary, the controllable transition point of co-drying controls shrinkage can be expected to reduce the amount and extent of ice crystal melting, as well as the free water migration in the HAD phase of the sample. There was less damage to the microscopic pores caused by moisture migration. This finding can provide the basic data and theoretical reference for the precise regulation of the appearance quality of FD-HAD samples and energy saving. |
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| Keywords: | freeze drying|hot air drying|combined drying|shrinkage|moisture migration|microstructure |
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