Revealing the influence of ultrasonic processing on the kinetic parameters of convective and combined drying of raw apple materials

Abstract

The object of research is the process of convective and combined drying of apple raw materials treated before dehydration in an ultrasonic bath. The use of pretreatment solves the issue of intensifying the dehydration process and maintaining the quality of the product. Sonication for 5 min reduces the dehydration process by 13.7 %, and at 10 min processing – by 27.8 % compared to traditional convective drying. With increasing processing time to 20 minutes, the dehydration rate deteriorates. Sonication of raw materials for 5 minutes under combined heating does not intensify the dehydration process, and within 10 and 20 minutes it reduces the efficiency of moisture removal. When processed for 10 minutes, the time to reach the final moisture content is increased by 17.2 %, and at 20 min – by 23.4 % compared to control samples. Regardless of the processing time, there is a decrease in the maximum temperature of the samples in the combined drying process. When processed for 5 minutes, the maximum temperature of the samples decreased by 4.3 %, and with 10 and 20-minute processing – by 8.6 % and 12 % compared to the temperature of samples without sonication. The results are explained by the "sponge effect" caused by ultrasonic vibrations and the phenomenon of cavitation that occurs in the liquid during the action of the ultrasonic field. The peculiarity of the pre-sonication before drying is the possibility to intensify the convective dehydration process without increasing the heat carrier's temperature. The use of this type of processing in a combined energy supply will solve the problem of preserving the quality of the finished product by reducing the maximum temperature of the raw material. The research reported here could be a prerequisite for practial design of an energy-efficient electrical system for drying fruit and vegetable raw materials.