## 3 Results and Discussion
### Color Changes and Textural Properties  
Consumers tend to make their first assessment of a food product based on its appearance. Therefore, color is one of the characteristics that has a great impact on the acceptability of foods. The natural pigment compounds present in food that are mainly responsible for the color of food matrices can be degraded by both drying and various pre-treatment methods [53]. Figure 5a shows the impact of US pre-treatment time and US power applied during drying on the total color difference of hybrid-dried apples. Regardless of the time of US pre-treatment and the US power applied during drying, substantial color changes were recorded in relation to the fresh sample. As a result of the ultrasound pre-treatment and hybrid drying, the total color difference between fresh and processed apple changed from 15.71 \(\pm\) 0.74 (sample treated for 20 min with US pre-treatment and US-assisted drying with 160 W--T_20\(D\)160W) to 18.55 \(\pm\) 0.23 (sample treated for 20 min with US pre-treatment and US-assisted drying with 200 W--T_20\(D\)200W). This means that a standard observer will see apples of two different colors when seeing a fresh and a processed sample [54]. There were numerous possible factors that affected the color changes as a result of US treatment and hybrid drying, e.g., Maillard reaction, pigment degradation, enzymatic and non-enzymatic browning, and ascorbic acid oxidation [49; 55]. Ultrasound can induce diverse effects on the fruits and vegetables. Typically, US pre-treatment may contribute to high-quality dried products as a mild technique to facilitate the removal of water from plant tissue. For instance, [PERSON] et al. [49] compared the non-thermal ultrasound contact drying (US-CD) of apple slices with hot-air drying and concluded that the better color retention in the US-CD samples could be the result of the shorter drying time combined with the reduced temperature of drying. Furthermore, [PERSON] et al. [56] applied low-intensity ultrasound combined with heat (LIUH) pre-treatment prior to microwave vacuum drying and stated that it reduced the color change of dried materials. Additionally, the researchers observed that by using LIUH with a higher power intensity  
Figure 5: Effects of the US treatment time and the US power applied during drying on the (**a**) total color change (\(\Delta\)E) of hybrid dried apples and the (**b**) texture properties (F\({}_{\text{max}}\) recorded during a 50% deformation test) of hybrid dried apples. Blue dots on the figure shows research data. (0.4 W/cm\({}^{2}\)), a smaller color change could be reached than with the lower one (0.2 W/cm\({}^{2}\)). However, the physical destruction and membrane deformation of cells caused by the US application may also lead to the leakage of pigments from plant tissue. This may be the explanation why, in the study by [PERSON] and [PERSON] [48], a 10 min or 20 min US pre-treatment of quince followed by freeze drying led to a statistically smaller color change than a 30 min treatment. In our study, based on the Pareto chart (Figure S4a), it can be found that neither ultrasonic pre-treatment time nor the power of sonication applied during drying had any significant linear or quadratic effect on the color change of apple tissue. The textural properties of all the samples were evaluated in terms of the maximum force needed to deform the material by 50%. The fresh sample was characterized by a maximum force equal to 2.39 \(\pm\) 0.53 N. Generally, both ultrasonic pre-treatment and sonication applied during drying did not significantly alter the textural properties in comparison to fresh apple, except the sample sonicated for 10 min before drying and with an ultrasound of 200 W during drying (T_10\(D\)200W). [PERSON] et al. [44] reported similar observations for sonicated apples prior to convective drying regarding maximum cutting force. The authors noted no statistically significant difference between fresh apples and different sonication times (10 min and 20 min).