Convective Drying of Apple Enhanced with Microwaves and Ultrasound--Process Kinetics, Energy Consumption, and Product Quality Approach  
[PERSON]  
1  
[PERSON]  
2  
[PERSON]  
1 Division of Process Engineering, Institute of Chemical Technology and Engineering, Poznan University of Technology, ul. Berdychovo 4, 60-965 Poznan, Poland; [EMAIL_ADDRESS]  
2  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci)]([https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci))  
Footnote 3: [[https://www.mdpi.com/j](https://www.mdpi.com/j)]([https://www.mdpi.com/j](https://www.mdpi.com/j))  
shorten process time, and use low temperatures. Reducing energy consumption obviously reduces the costs of the process and, at the same time, has ecological significance. Reducing the process time directly affects the cost of the process and, additionally, reduces the loss of nutritional value of the fruit. Lowering the temperature usually lengthens the process time but may have a positive effect on the product quality. All of these strategies together can improve the process from an economic point of view. These include osmotic dehydration, freeze drying, or microwave drying [1]. This last technique seems to be particularly promising. Microwave energy is delivered directly to the moisture inside the material being dried. For this reason, drying takes place quickly and with little heat loss [3]. The hybrid technique, combining convection and microwave drying, is the most commonly used. As research shows [4], the use of microwave energy allows the drying time of fruits and vegetables to be shortened by as much as 90%. Thanks to this, there is huge potential for the use of microwave drying in industry. The use of microwaves increases investment costs, but in the long run, it leads to a significant reduction in operating costs, increasing the economic efficiency of the process. However, continuous microwave drying may cause overheating and deterioration of the quality of the dried material. This can be overcome, for example, by intermittent microwave-convection drying [5]. Another disadvantage of using microwaves is uneven heating due to the standing wave pattern. This disadvantage can be overcome by moving the material to be dried inside the drying chamber [6]. Another hybrid drying technique is convection-ultrasonic drying. In this case, ultrasound practically does not serve as an additional source of energy but improves heat and mass transfer [7; 8]. This drying method has been used to dry various fruits and vegetables, such as strawberries [9], green peppers [10], persimmon [11], potatoes [12] and carrots [13]. Convection drying, aided by airborne ultrasound, was also used to dry apples [14; 15; 16; 17]. In all cases, the use of ultrasound resulted in a shortening of the drying time and a certain improvement in the quality of the dry material compared to convection drying. According to [PERSON] et al. [18], application of ultrasound as a pre-treatment operation for drying carrot slices led to a higher sensory appreciation of the product color. The use of ultrasound requires a new approach for designers because it is not yet used in industrial drying. However, research to date shows that conducting the process using ultrasound can lead to improved process economics. In this work, it was decided to investigate a hybrid drying technique combining three methods of energy supply: convection, microwave, and ultrasonic. The tests were performed in a drum dryer, which allowed us to avoid the influence of non-uniform distribution of microwaves and ultrasound in the drying chamber. Apple was chosen as the dried material. This is because apples are one of the most widely cultivated fruits in the world. In the 2021/2022 season, global apple production amounted to 81.8 million tons.