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

Vallespir, Francisca, Rodríguez, Óscar, Eim, Valeria S., Rosselló, Carmen, Simal, Susana
Journal of food engineering 2018 v.239 pp. 83-91
apples, beets, color, drying, eggplants, freezing, liquids, mass transfer, microstructure, models, nitrogen, porosity, temperature
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.