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

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.