Effect Of Efficient Microwave Pulse-spouted Drying On The Quality Of Typical Vegetables And Its Regulation

Fruits and vegetables are the major source of essential dietary nutrients such as vitamins,minerals and dietary fibers or physiologically active components. The consumption of vegetables plays a significant role in modern balanced diet. Fruits and vegetables are highly perishable due to high moisture content and the characteristics of highly seasonal and regional harvest, which severely limit the development of fruits and vegetables industry. Drying is a crucial solution to these problems, however, high energy consumption and low quality after drying are the major drawbacks of the traditional drying methods. Microwave drying, as an emerging drying technology, has been widely applied to the field of further processing of fruits and vegetables due to its unique heating characteristics and mechanisms of heating. The application of efficient microwave pulse-spouted drying technology in process of typical vegetables was investigated in this study. Besides, the drying uniformity and its dynamic model were investigated.Firstly, carrot was chosen as raw material to study the effects of microwave pulse-spouted drying method(MPSD) on the drying characteristics and product quality. Based on MPSD technology, stage-drying arrangements were designed focusing of drying characteristics of MPSD, then the quality and characteristics of dried and rehydrated carrot cubes, obtained from stage-drying methods were investigated. The results showed that the transformation point of two-stage drying occurred when moisture content reached 1.7 g/g(d.b.) with 45 min of corresponding drying time. The moisture content of 5.0 g/g(d.b.) was considered as the other transformation point for three-stage drying. During the middle and later stages of drying, at the same level of moisture content, the drying rate of stage-drying arrangements was higher than MPSD. Compared to fresh carrots, the color parameters L* and a* were significantly(p<0.05) affected by all the drying methods tested, it was also observed that rehydration significantly(p<0.05) influenced two quality indicators, a* and b*, while L*was slightly affected. Due to the high penetrability and high efficiency, materials dried by MPSD and stage-drying arrangements all preserved higher level of rehydration ability,carotenoids and chlorophyll contents. In addition, flavor components of carrot cubes dried by MPSD and stage-drying arrangements were analyzed by electronic nose. The major volatiles identified in fresh carrot were terpenoids. However, aldehydes and aromatic compounds were also identified at the end of drying.Secondly, okra was chosen as a raw material to study the effects of pulse-spouted microwave vacuum drying(PSMVD) method on the drying characteristics and product quality. This research mainly investigated the effects of vacuum freeze drying(VFD),microwave vacuum drying(MVD) and PSMVD on the drying characteristics and quality ofokra and evaluated the advantage of PSMVD method. Based on the drying time that materials required to reach the equilibrium moisture content, PSMVD was the fastest with 90 min,followed by VFD(720 min). Besides that, the surface temperature distribution of materials dried by PSMVD was more uniform, and the area of higher temperature was smaller throughout the entire drying process, compared to MVD. When compared with MVD, the materials dried by PSMVD showed better quality such as color(a*-11.32, h-0.46), texture(hardness 1.68 kg), content of chlorophyll, vitamin C and sensory score(7.3), because of the drying uniformity, a higher shrinkage rate(21.22%) was observed as well. In addition, the major volatiles identified in fresh okra were alcohols, aldehydes, acids and alkenes. However,the major volatiles identified in dehydrated materials were esters and alcohols.Thirdly, this study investigated the effects of pulse frequency, microwave power and pre-treatment on the drying characteristics and quality of burdock slices dried by PSMVD under the pressure range between 12 to 21 k Pa. Results showed that with the pulse frequency3.3 r/min, microwave power 19.8 W/g and frozen pretreatment, burdock slices presented good color, rich flavor and crispiness, and the maximum fracturability was 2.4 kg.Last but not least, the quality uniformity of materials dried by MPSD and PSMVD was studied. Different mathematical models were used to describe the drying process and rehydration process of materials. The results showed that pulse-spouted technology could significantly improve the uniformity of microwave drying. It was found from the thermal images that the surface temperature distribution of materials dried by MPSD and PSMVD were uniform because of the vigorous pneumatic agitation, which avoided the phenomenon of local over-heating. Mathematical analysis data indicated that the moisture content and color uniformity of okra dried by PSMVD were higher than MVD, the standard deviations were±0.0098, and ±0.0093, respectively. In terms of MPSD, either in two or three stages, the moisture content of dehydrated cubes and size of rehydrated cubes both showed high level of uniformity. In addition, Midilli Model was the most suitable one to describe the drying process of carrot cubes dried by MPSD. For PSMVD, both Midilli Model and Two-term Model could describe the drying process. Based on the experiment data, Exponential model was the most suitable for describing the rehydration process. Higher drying rate was observed when the value of Deff increased. The Deff of carrot dried by MPSD was3.3006×10-7~7.4264×10-7 m2/s, and the Deff of okra dried by PSMVD was4.3284×10-7~6.4659×10-7 m2/s.