Study On The Microwave Vacuum Drying Technology Of Okra

Okra is a vegetable popular with the general public due to its unique taste, flavor and the high nutritional and hygienic value. However, fresh okras are highly perishable and deterioration starts within a day after harvest, which can cause loss of nutritional constituents. Therefore, dehydration becomes an important means for better processing and storage. Microwave vacuum drying has become a hot topic in research due to its combination of the advantages from both microwave and vacuum drying that results in energy conservation and high efficiency. In our study, microwave vacuum drying technology was applied in the okra drying. Characteristics and effects of drying parameters on the main qualities of okra were studied and parameters were optimized. Finally, tempering technique was applied in microwave vacuum drying and the parameters were optimized. The results were compared with those of other drying methods. The main research points and results are as follows.1. We studied the dehydration characteristics of okra under microwave vacuum drying. The results suggested three phases constitute the dehydration process, which include the increasing rate period, the constant rate period and the falling rate period. Furthermore, the dehydration rate increased with increasing microwave power, vacuum degree and load capacity.2. We investigated the effects of various drying parameters (microwave power, vacuum degree, and load mass) on product quality, of which the indexes include the rehydration ratio and the content of polysaccharide, flavonoid, and chlorophyll. The results indicated that pretreating okra with fumigation before microwave-vacuum drying yielded the best quality of product. As microwave power increased, rehydration ratio also increased. The content of polysaccharide, flavonoid and chlorophyll increased at first and then decreased. As vacuum degree increased, a rise was observed in the rehydration ratio, polysaccharide and chlorophyll contents, while the flavonoid content increased at first and then decreased. As load capacity increased, we found an increase in the rehydration ratio and the content of polysaccharide, flavonoid, and chlorophyll.3. We optimized the parameters of microwave vacuum drying with Box-Behnken experimental design to study the effect of different drying parameters (microwave power, vacuum degree and load mass) on the polysaccharide and the flavonoid content of okra. The optimal parameters were determined as:microwave power 3.15kW, vacuum degree 82.72kW, and load capacity 283.30g.4. We combined microwave vacuum drying with tempering for okra drying and studied the effects of tempering conditions on drying time, and the content of polysaccharide and flavonoid. The effects of initial moisture content and tempering time on product quality (drying time, the content of polysaccharide and flavonoid) were also studied. The parameters of vacuum microwave-tempering drying were optimized using miscellaneous design with a quadratic regression model built with Response Surface Methodology. The optimal parameters were determined as: tempering time 180min and initial moisture content 100%.5. We studied the four drying methods, including hot air drying, microwave drying, microwave vacuum drying, and vacuum microwave-tempering drying. We evaluated the effects of the four methods on rehydration ratio and the content of polysaccharide, flavonoid, and chlorophyll of dried okra samples. The results showed that vacuum microwave-tempering drying received a better quality than the other methods and demonstrated promising potential for industrial application.