## 1 Introduction  
One of the world's greatest challenges is the preservation of food. Drying is among the most common methods for preserving fruits and vegetables, effectively reducing moisture content and water activity--key factors in limiting natural decomposition processes. Drying foods limits bacterial and fungal decomposition, resulting in product stabilization [1]. However, most drying techniques involve prolonged heat application, which often compromises product quality and value. Factors like drying method, temperature, humidity, air flow rate, and additional heat sources such as microwave (MW), infrared (IF), or ultrasound (US) influence the extent of quality changes. Convective drying, a widely used method involving hot air, often requires high temperatures and extended durations, which trigger chemical and biochemical reactions that alter color, flavor, aroma, and nutritional properties [2, 3, 4, 5]. Drying can also cause excessive shrinkage and/or shape deformation [6, 7, 8], making the product less appealing to consumers.  
Another significant issue with drying is its high energy consumption. The energy needed to provide sufficient heat for evaporation makes drying one of the most energy-intensive unit operations. It is estimated that drying processes account for up to 25% of total industrial energy consumption [9, 10]. Given the environmental and energy concerns, such as fossil fuel depletion and the need to reduce greenhouse gas emissions [11], reducing energy consumption in all industrial sectors, including those utilizing drying processes, is paramount. Therefore, ensuring good product quality while minimizing energy consumption remains a key challenge in drying operations.  
One possibility for improving drying processes is the use of pretreatment. Examples of pretreatment methods include osmosis [11, 12, 13, 14], ultrasound [15, 16], or pulsed electric field [12; 17]. Another way to enhance drying processes is to use hybrid drying, which combines two or more techniques in a single process [18]. To accelerate the process, several methods are used in conjunction to supply additional energy to the process during drying. Such acceleration can result in reduced energy consumption and improved product quality. Plant materials are often dried using convection with the addition of microwaves [15; 19; 20; 21], infrared [15; 22; 23], or ultrasound [24; 25; 20; 26]. In most cases, the materials being dried in these processes are stationary, with exceptions being rotary convection-microwave and convection-infrared drying [15; 19; 22].  
The aim of this work was to investigate the effect of the independent use of microwaves and ultrasound, as well as the simultaneous use of both energy sources, on rotary convective drying. Carrot root was chosen as the research material due to its high degree of hardness and inhomogeneous structure, making it particularly challenging to dry [27]. In total, four drying schedules were experimentally tested to determine the most effective process: convective-hot air drying (CV), convective drying assisted by ultrasound (CVUS), convective drying assisted by microwaves (CVMW), and convective drying assisted by both microwaves and ultrasound (CVMWUS). Each schedule was assessed in terms of drying kinetics, product quality, and energy consumption. The use of ultrasound increased the drying rate by 13%, microwaves by 112%, and both sources combined by as much as 140%. All processes resulted in a significant reduction in water activity, with the greatest reduction observed in the CVUS process. Ultrasound also led to a decrease in the color difference index (\(dE\)), while microwaves significantly increased the value of this index. Additionally, the use of microwaves significantly reduced energy consumption, while ultrasound resulted in a slight increase in energy consumption.