## 4 Conclusions  
Considering obtained results, hybrid drying is an interesting alternative to traditional drying methods. Undoubtedly, an important aspect, in addition to the appropriate selection of the drying method, is also the optimization and modeling of the process, thanks to which it is possible not only to reduce operating costs but also limit energy consumption and ensure obtaining a product with the most desirable quality.  
The present research shows that it is possible to optimize the microwave-convective drying of apples with pulsed electric field pretreatment. PEF, by increasing the permeability of the cell membrane, led to a significant reduction in drying time (8-28%). No relationship was found between the amount of energy supplied during PEF pretreatment and the reduction of drying time; therefore, it is worth considering gentler conditions of treatment with an electric field to avoid overtreatment of the material. As it results from this study, the lowest amount of energy supplied during PEF processing (1 kJ/kg) allowed to obtain dried apples with the highest antioxidant activity (both EC\({}_{50}\) ABTS and EC\({}_{50}\) DPPH) and the highest total phenolic content among all PEF-treated and dried materials. Additionally, after drying these apples in the highest microwave power (300 W), the materials with the lowest total colour difference were obtained. This may be due to the partial inhibition of enzymes by PEF (e.g., polyphenol oxidase), which reduced the browning of the samples and limited the risk of oxidation of the bioactive ingredients. The amount of microwave energy supplied to the dried material can be increased to intensify the evaporation of water. At the same time, too high microwave power may cause partial burning of the dried samples. Increasing the microwave power from 100 to 200 W and from 200 to 300 W reduced the time of microwave-convective drying of apples by 45% in both instances. The use of the highest analysed microwave power (300 W) led to obtaining materials with the highest dry matter content, which means that the largest amount of water evaporated from them. Moreover, the highest microwave power, by significantly reducing the drying time and thus limiting the risk of thermal degradation of sensitive nutrients, allowed us to obtain dried apples with the highest antioxidant activity and TPC. Additionally, considering the water activity and the reconstitution properties of the dried apples (hygroscopicity and rehydration), the combination of the energy of 3.437 kJ/kg and microwave power of 300 W turned out to be the most optimal.  
Conceptualization, A.W., D.W.-R., A.M. and O.P.; methodology, A.W. and M.D.; software, A.M.; formal analysis, A.M., D.W.-R., O.P. and A.W.; investigation, A.M.; resources, A.W.; data curation, A.M. and M.D.; writing--original draft preparation, A.M.; writing--review and editing, A.W., D.W.-R. and A.M.; visualization, A.M.; supervision, A.W. and D.W.-R.; project administration, A.W.; funding acquisition, A.W. All authors have read and agreed to the published version of the manuscript.  
This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 817683.  
Not applicable.  
Data Availability Statement: The data presented in this study are available on request from the corresponding author.