基于转换点调控的怀山药多相态微波干燥及品质特性

    Multiphase microwave drying and quality characteristics of Chinese yam based on conversion point regulation

    • 摘要: 为解决冻干和冻干-微波真空联合干燥技术存在的耗时长、能耗高、设备成本高等问题,同时获得品质较好的干制品,以怀山药为对象开展多相态微波干燥(multiphase microwave drying,MMD)研究,通过不同微波功率加载方案实现转换点调控,探究转换点干基含水率(0.36、0.59、0.79 g/g)对怀山药干制品品质的影响。并以真空冷冻干燥、微波冷冻干燥和微波真空干燥为对照,研究不同干燥方式对怀山药干燥特性、能耗和品质特性(复水性、收缩率、色泽、硬度、脆度、微观结构)的影响。结果表明:随着微波功率水平的增加,怀山药MMD转换点干基含水率增大,产品复水性降低,收缩程度增大,硬度变大,细胞结构破损程度增加。MMD方案Ⅰ(转换点干基含水率0.36 g/g)干燥时间与微波冷冻干燥相比缩短31.3%,能耗相比真空冷冻干燥、微波冷冻干燥分别降低68%、34%,同时,所得怀山药干制品具有良好的品质和均匀的微观孔隙结构,其复水比(2.44±0.04)、收缩率(0.88±0.02)、色泽、硬度(4.95±0.45)N、脆度(2.48±0.51)N与微波冷冻干燥无明显差异(P>0.05)。微波真空干燥虽所需能耗低,但其产品复水性最差,收缩最为严重。综合考虑高效低能耗干燥与产品品质提升的需求,可通过转换点调控的多相态微波干燥实现高品质怀山药加工。

       

      Abstract: Freeze drying and combined freeze drying-microwave vacuum drying can be required for high-quality dried products, due to the long processing time, high energy consumption and equipment cost. In this study, three schemes of microwave power loading were used for the multiphase microwave drying (MMD) of Chinese yam. A systematic investigation was implemented to clarify the effects of conversion point moisture content on a dry basis (0.36, 0.59, and 0.79 g/g) on the drying performance and quality of dried products. Freeze drying, microwave freeze drying and microwave vacuum drying were used as the control. Some parameters were also evaluated on the drying performances, energy consumption, rehydration ratio, shrinkage ratio, color, hardness, crispness, and microstructure of Chinese yam. The results showed that the MMD scheme Ⅰ, Ⅱ, and Ⅲ reached the critical temperature at 300, 240, and 210 min, respectively. The evaporation stage arrived when the material temperature reached the critical temperature, at which the conversion point moisture content on a dry basis for MMD schemes I, II, and III were 0.36, 0.59, and 0.79 g/g, respectively. The energy consumption of drying Chinese yam using MMD was reduced by 68%-70%, and 34%-38%, respectively, compared with the FD and MFD. The MMD drying rate of Chinese yam increased with the increase of microwave power level, whereas, the energy consumption decreased. There was an increase in the conversion point moisture content and shrinkage on a dry basis, whereas, the rehydration decreased. The hardness was much larger than before, leading to the broken cell structure of Chinese yam. MMD scheme Ⅰ (conversion point moisture content on a dry basis 0.36 g/g) entered the evaporation stage with the lowest moisture content. The internal pore channels of the material formed a stable skeleton in the sublimation drying stage. A porous structure was maintained in the evaporation drying stage, indicating better rehydration. The drying time of the MMD scheme Ⅰ was reduced by 31.3%, compared with the microwave freeze drying. The energy consumption was lowered by 68% and 34%, respectively, compared with the freeze drying and microwave freeze drying. The dried Chinese yam products were obtained with a uniform porous structure and superior quality. There was no significant difference in the rehydration capacity (2.44±0.04), shrinkage ratio (0.88±0.02), color, hardness (4.95±0.45) N, and crispness (2.48±0.51) N, compared with the microwave freeze drying. The shrinkage rate of MMD scheme Ⅲ was significantly lower than that of schemes Ⅰ and Ⅱ. This was because the higher the moisture content at the conversion point was, the more microwave energy absorbed. The greater deformation of material was formed in the subsequent drying, due to the combined heat and moisture migration. The outstanding honeycomb structure was obtained after freeze-drying Chinese yam, indicating the best rehydration and quality, and the lowest shrinkage but with a relatively high energy consumption. Microwave vacuum drying with low energy consumption shared the lowest rehydration, the most severe shrinkage and cell structure collapse. Multiphase microwave drying can be used to serve as the Chinese yam, according to the requirement of high product quality, efficiency and energy-saving drying. The microwave loading scheme can be adjusted for the multiphase microwave drying of Chinese yam. The moisture content can be controlled on a dry basis at the conversion point at a suitable level. The high-quality Chinese yam can be produced similarly to the quality FD products.

       

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