High power airborne ultrasound assist in combined drying of raspberries

Abstract A new approach to fruit drying with the application of ultrasound to enhance convective and combined convective–microwave drying is presented. High-power ultrasound was used in the experimental studies, in which the acoustic energy significantly affected the drying of sensitive biological material like raspberries. The raspberries were first subjected to pure convective drying as a reference process and then to hybrid drying as a combination of convective, microwave and ultrasound drying methods. The combined hybrid processes significantly improved the drying kinetics as well as the energy utilization, however, not in each case given the acceptable product quality in comparison with the pure convective drying. The global model of drying was used to compare the theoretical drying kinetics with the experimental one. Moreover, the effectiveness of ultrasound-assisted convective drying was evaluated in terms of drying rate and such ultrasound phenomena as “heating effect”, “vibration effect” and “synergistic effect” were analyzed. Industrial relevance 1. Research objectives Product quality is a very important indicator that enables to assess the effectiveness of the drying process. For biological materials such as vegetables and fruits, that are characterized by instability and a delicate and sensitive internal structure to temperature, quality is a priority because it determines the functional value of these products. The traditional drying techniques, eg. convective drying, affect negatively on the final quality of dried biomaterials. Due to long exposure to a relatively high temperature and a high content of harmful oxygen in the drying agent, there is a change in the internal structure, deterioration of the sensory properties (color, aroma, texture) and changes in chemical composition (loss of bioactive components). One of the recommended modes to minimize these adverse changes is to carry out the drying process in non-stationary conditions. This method is based on the periodical changes in the process parameters such as temperature, humidity or flow rate. As a result, product quality is definitely better, at approximately the same drying time. Alternatively, drying may be carried out by hybrid method, providing the energy by a combination of several mechanisms, such as convection with microwave radiation, or with ultrasounds, which also leads to a significant improvement in quality of dried fruits and vegetables. The purpose of this article is a deep analysis of the effect of variable hybrid drying conditions, ie. cyclic supply of microwave and ultrasound energy during convective drying on a variety of properties of dried biomaterials. The research assumes that if drying results in changes of the biological material properties, a modification of the process parameters should minimize its negative effects and gain better quality products. The authors believe that a proper and skillful combination of different drying techniques will develop an optimal and effective drying method of thermo-labile materials, which retains their high quality, with reduced time and lower power consumption. Therefore, it is necessary to optimize the convective drying with microwave and ultrasound enhancement, in order to select the best process parameters for the established objective functions. 2. Research project impact The experimental results will contribute to the development of modern drying technologies of biological materials, characterized by, eg.: • much better and more controlled product quality (low level of processing), • much shorter process time resulting from the intensification of heat and mass processes due to additional sources of energy (microwaves and ultrasounds), • lower energy consumption resulting from the possibility of carrying out the drying process at much lower temperatures of the drying agent. The expected results of the drying tests relate to the development of research activities in this field and to link a cooperation between science and industry. The effective transfer of new solutions in the field of chemical engineering and technology between science and industry is a hope of improving the quality of dried products, as well as the rational use of raw biomaterials (to prevent their losses from reaching about 50% of produced foods). The proposed drying method will also have a positive value for consumers and contribute to the reduction in influence of that branch of industry on the environment. In the consideration of the increasing demands of product quality, the need for better control of unit processes and minimization of its energy intensity, this new technique would be extremely competitive with currently used, expensive and environmentally damaging drying methods.