Microwave-vacuum drying system (MIVAC). Progress report No. 4

This study was conducted to optimize the drying process of Polygonum cuspidatum slices using an orthogonal experimental design. The combined effects of pretreatment methods, vacuum pressure and temperature of inner material, drying kinetics, color value, and retention of the indicator compounds were investigated. Seven mathematical models on thin-layer drying were used to study and analyze the drying kinetics. Pretreatment method with blanching for 30 s at 100°C increased the intensity of the red color of P. cuspidatum slices compared with other pretreatment methods and fresh P. cuspidatum slices. P. cuspidatum slices dried at 60°C retained more indicator compounds. Furthermore, microwave pretreatment methods, followed by microwave vacuum for 200 mbar at 50°C, resulted in high concentration of indicator compounds, with short drying time and less energy. This optimized condition for microwave vacuum drying and pretreatment methods would be useful for processing P. cuspidatum. The Newton, Page, and Wang and Singh models slightly fitted the microwave vacuum drying system. The logarithmic, Henderson and Pabis, two-term, and Midilli et al. models can be used to scale up the microwave vacuum drying system to a commercial scale. The two-term and Midilli et al. models were the best fitting mathematical models for the no-pretreatment case at 600 mbar and 60°C.

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Effect of Intermittent Microwave Volumetric Heating on Dehydration, Energy Consumption, Antioxidant Substances, and Sensory Qualities of Litchi Fruit during Vacuum Drying

Molecules ◽

10.3390/molecules24234291 ◽

2019 ◽

Vol 24 (23) ◽

pp. 4291 ◽

Cited By ~ 2

Author(s):

Xiaohuang Cao ◽

Jianping Chen ◽

Md. Nahidul Islam ◽

Wanxiu Xu ◽

Saiyi Zhong

Keyword(s):

Energy Consumption ◽

Sensory Evaluation ◽

Vacuum Drying ◽

Volumetric Heating ◽

Microwave Vacuum Drying ◽

Sensory Qualities ◽

Drying System ◽

Litchi Fruit ◽

Heating Profiles

To examine the processing characteristics and high quality of an improved microwave vacuum drying system, litchi fruits were dried using intermittent microwave volumetric heating while microwave vacuum drying at 2 W/g was carried out for comparison; the intermittent microwave heating profiles were set as (1) 5 min drying-on, 5 min drying-off; (2) 5 min drying-on, 10 min drying-off; and (3) 5 min drying-on, 15 min drying-off. Energy consumption during drying was determined, and physicochemical properties such as moisture content, vitamin C, total phenolics, color, and sensory evaluation of dried products were assessed. In microwave vacuum drying, intermittent microwave volumetric heating was found to be energy-efficient (about 32 KJ/g to 45 KJ/g) and saved at least 31% of energy consumption compared with microwave vacuum drying as well as decreasing product browning. In addition, microwave volumetric heating had no substantial effects on sugar and protein contents, while antioxidants were affected significantly (p ≤ 0.05). Moreover, sensory evaluation showed that intermittent microwave-assisted vacuum drying (IMVD) increased the acceptance of the dried product compared with microwave vacuum drying (MVD).

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