Abstract:
Abstract: Okras (Abelmoschus esculentus (L.) Moench) belong to the Malvaceae family. It is proved that okra can act as a tonic for both man and woman and enable them to increase their vitality and vigour. Due to high content of vitamin A and C, calcium, dietary fiber and low content of saturated fat, okra products, which are consumed in dried, frozen, fresh forms and also available in canned and pickled forms, are popular around the world. Like others vegetables, okra is susceptible to rapid deterioration because of its high moisture content. Various physiological and morphological changes occur after harvest, which make these okras unacceptable for consumption, and thus it is necessary to remove the moisture of okras. Hot air drying is the most widely used and inexpensive drying technique for okra. Nevertheless, long-time drying at relatively high temperature during the falling rate period often leads to the overheating of dried products, and undesirable thermal degradation of the finished products may occur. Hence, it is necessary to apply innovative techniques on hot air drying to solve these drawbacks, which can increase drying rate and reduce quality loss. One of the methods that may be recommended is drying with tempering cycles. Tempering allows moisture diffusion from the interior to the external surface of the sample kernels to decrease the moisture gradients. Hence, the material structure can be well protected during drying process, and with the uniform distribution of moisture, the drying rate of next stage will be higher. While in a vacuum environment, the oxygen (O2) content is in a low level, which can reduce many undesirable oxidation reaction and slow down the physiological activity of materials. Moreover, many researchers have reported the vacuum tempering method can obviously reduce the tempering time compared to traditional natural tempering. Ventilation tempering is carried out under the condition of decreasing temperature, the moisture can be uniformly distributed not only by moisture gradient but also by temperature gradient. Consequently, combining the vacuum tempering method with ventilation tempering can reduce tempering time more significantly along with obtaining good product quality. As above introduction, applying the combined tempering method on hot air drying may overcome some limitations of traditional hot air drying process, which can be called as vacuum ventilation tempering combined with hot air drying (MVVT-HAD). To achieve high quality products, vacuum ventilation tempering was applied on okras hot air drying, and the change trends of drying and quality characteristics of okras were investigated in this study; the effect of tempering process on the drying mechanism of okras was analyzed by using the Weibull distribution function. The drying and quality kinetics model, which was applicable in okras MVVT-HAD process, was constructed by using a nonlinear regression analysis; and the weighted comprehensive evaluation of okras MVVT-HAD process was also carried out based on total drying time, total energy consumption, rehydration ratio, hue angle, and total nutrient retention ratio. Results showed that, tempering treatment could improve the drying rate of okras hot air drying, and with longer tempering time the effect would be more obvious; Weibull distribution function could accurately describe the change tendency of water ratio with drying time of okras during MVVT-HAD process; the scale parameter of okras was reduced with the prolongation of the tempering time length; different tempering time length had the same effect on drying moisture diffusion mechanism in okras hot air drying; taking total drying time, total energy consumption, rehydration ratio, hue angle and total nutrient retention ratio into account, 60 min tempering time length was proposed as the favorable condition for okras hot air drying. The study shows that MVVT treatment can enhance the drying rate and drying quality of okras during HAD stage, and our paper provides a theoretical basis for the application of MVVT-HAD on industrial production of high quality dried products of okras.