Dehydration of apple slices using ohmic heating after vacuum impregnation pre-treatment

In the study, the effects of different pre-treatment and drying methods on the drying of apple slices were determined at three different temperatures (50, 60, and 70°C). In the first stage, the vacuum impregnation (VI) process was optimized to enhance the electrical conductivity of the slices to be pre-dried with an ohmic heating system. Subsequently, the slices were dried using the following methods: convective drying (CD), CD after VI pretreatment (VI+CD), CD after ohmic pre-drying at atmospheric pressure (OPD+CD), CD after ohmic pre-drying under vacuum pressure (VOPD+CD), AOPD+CD after VI pretreatment (VI+AOPD+CD), and VOPD+CD after VI pretreatment (VI+VOPD+CD). For the slices, whose drying behavior was analyzed using thin-layer drying models, the Page and Wang & Singh models provided a better fit at 50°C, whereas the Wang & Singh model showed better compatibility at the other two temperatures. The effective diffusion coefficient ranged from 1.187×10⁻⁸ to 4.264×10⁻⁸ m²/s, while the activation energy varied between 10.48–54.20 kJ/mol. The specific moisture extraction ratio was the lowest at 70°C in the CD method (0.20×10⁻² kg water/kWh), while the highest value was found at 50°C using the same drying method (3.50×10⁻² kg water/kWh). Specific energy consumption was the lowest at 50°C with the CD method (298.40 kWh/kg water) and the highest at 50°C with the OPD+CD method (446.80 kWh/kg water). Total energy consumption was the lowest in the OPD+CD method (0.96 kWh) and the highest in the VI+VOPD+CD method (1.77 kWh). The puncture hardness of the dried slices ranged from 17.42 to 37.58 N, while the total phenolic content varied between 201.85–83.27 mg GAE/100 g dry matter. The lowest glass transition temperature was observed at 70°C in the VI+CD method (-33.59°C), while the highest value was found at 70°C using the CD method (-11.57°C). These results indicated that the VI process led to a more pronounced structural relaxation in the product matrix, whereas the CD method preserved a more stable structure. Results demonstrated that ohmic pre-drying and VI applications had significant effects on the drying kinetics, energy consumption, functional properties, and microstructural stability of apple slices. Interestingly, low-temperature and combination pretreatments provided significant advantages in terms of product quality preservation and energy efficiency.