The aim of this study was to investigate the effect of application of pulsed electric field (PEF) and different freezing methods (fast, slow and vacuum freezing) on the drying kinetics as well as selected physical properties of freeze-dried apple. The apples were subjected to PEF treatment with range of pulses from 0 to 160 and the intake energy from 0 to 1327 kJ·g-1. Apples with and without PEF treatment were frozen with different rates and the freeze-dried. The water content, water activity and colour attributes of freeze-dried apples were investigated. Regression analysis and fitting procedures showed that among six different models, the Midilli et al. model the best described the drying curves of all dried samples. The highest value of the parameter L* = 71.54 was obtained for freeze-dried sample prepared without PEF pre-treatment and fast frozen. Application of PEF pre-treatment resulted in increase in browning index of freeze-dried apples (BI). The studies confirmed the positive effect of PEF on the freeze drying rate only in the case of the slow or fast freezing of the material after the application of low-energy PEF treatment. However, the increase in drying rate was also observed after application of slow and vacuum freezing of the material without PEF pre-treatment. These technologies can be recommended for optimization of the freeze drying process of apples. The statement that the freeze drying process with application of appropriately selected PEF processing parameters causing only partial destruction of cell membranes can be considered as an innovative contribution to the development of science about the possibilities of PEF application.
Keywords: colour; drying curves; fast and slow freezing; freeze drying; pulsed electric field; vacuum freezing; water activity.