Freezing pre-treatments on the intensification of the drying process of vegetables with different structures

The effect of different freezing pre-treatments on the drying kinetics (50 °C and 1 m/s), and quality of vegetables with different structures such as beetroot, apple and eggplant has been studied. Samples cubes of 0.01 m edge were frozen at temperatures of −20 °C, −80 °C, or by immersion in liquid nitrogen (−196 °C). Then, frozen samples were dried at 50 °C and 1.0 m/s. Freezing pre-treatments promoted a significant (p < 0.05) increment of the drying rate, leading a reduction of the drying time up to 17, 27, and 34% in beetroot, apple and eggplant, respectively. A diffusion model was proposed to identify both the effective diffusion (De) and the external mass transfer (hm) coefficients during convective drying. The identified De in untreated (non-frozen samples) beetroot, apple and eggplant was of 4.2 ± 0.1 × 10−10, 4.7 ± 0.1 × 10−10 and 5.1 ± 0.3 × 10−10 m2/s, respectively. This coefficient was significantly higher in treated samples. Increments ranged from 18 to 31%, from 42 to 64%, and from 18 to 72% in beetroot, apple and eggplant, respectively and in all cases the higher figure was observed when samples were frozen at −20 °C. The identified hm was of 7.0 ± 0.5 × 10−4, 4.2 ± 0.2 × 10−4 and 2.3 ± 0.2 × 10−4 kg water/(m2 s) for beetroot, apple and eggplant drying, respectively. Regarding quality parameters, colour change and microstructure were deeply affected by both the freezing pre-treatment and the drying process. The extension of this effect varied accordingly to the porosity of the sample. The eggplant colour and microstructure, with a higher porosity, was the most affected, particularly by freezing pre-treatment at −20 °C.