## 1 Introduction
The application of sonication improved the rehydration properties of peppers, reduced the degree of degradation of natural pigments, and resulted in less deformation of the dried material [11]. Apples represent a substantial proportion of global fruit production, with an increasing trend in their consumption worldwide [12]. For instance, according to the Eurostat data, apples accounted for around 43% of all fruit produced in Europe in 2024, while 10 years earlier, it was only around 26%. Moreover, Poland, followed by Italy and France, has been the largest producer of apples in Europe for more than 10 years [13]. Nowadays, apples are utilized in diverse forms, including fresh fruits, juice, dried derivatives, and various processed products. In dried form, they can be used as a valuable ingredient of ready-to-eat foods (snacks, breakfast cereals, other functional foods, etc.). However, being an important source of bioactive compounds with high antioxidant activity, apples can be particularly vulnerable to quality loss through processing [14]. Today's consumers exhibit a growing demand for high-quality food products that retain fresh-like attributes,such as flavor, texture, and color, while prioritizing the preservation of nutritional value. Additionally, there is an increasing expectation for products with a stable or extended shelf life. Recently, there has been an increased focus on the health advantages of eating apples, prompting studies into how various processing methods influence the characteristics of apple products. The objective is to reduce quality loss and maintain their beneficial properties [15]. Specifically, there have been extensive investigations into the effects of ultrasound used before [16; 17; 18; 19; 20; 21; 22] or during drying [14; 15; 21; 23; 24; 25; 26; 27] on the quality of dried apples. The aim of this study was to investigate the effect of sonication used as a preliminary treatment and during the drying process (hybrid drying) on the course of the process and the properties of organic apples of the 'Gloster' variety. The novelty of this research is linked to the combined method where ultrasonic treatment with an ultrasound-assisted drying method was applied, and the response surface methodology (RSM) was used for the optimization of the whole processing (US pre-treatment with US-drying). Usually, the study is conducted separately for evaluation of the effect of ultrasonic treatment and then the effect of the chosen parameters of the sonication (based on the best obtained properties of plant material after US treatment) on the drying process conducted with different parameters. Such a procedure is not always correct. Some results show that after the drying process, the other US-pre-treated sample might obtain better results than at the beginning, which can be related to chemical reactions and changes occurring during the whole process of obtaining the final product. So, in this study, the whole processing was taken into account, and optimization was performed for the US pre-treatment together with the US-supported drying. The scope of this research included the analysis of the drying kinetics and selected chemical (dry matter content, vitamin C, total polyphenol content, total flavonoid content, antioxidant activity, sugar content, functional groups evaluated with FTIR analysis) and physical properties (water activity, total color change, rehydration properties, hygroscopic properties, texture, structure of the tissue evaluated using SEM and microtomography) of the dried material obtained as a result of the combination of the initial sonication time within 10, 20, or 30 min and the ultrasonic waves applied during drying with a power of 120, 160, or 200 W.