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

摘要

The effect of different freezing pre-treatments on the drying kinetics (50 degrees 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 degrees C, -80 degrees C, or by immersion in liquid nitrogen (-196 degrees C). Then, frozen samples were dried at 50 degrees 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 (D-e) and the external mass transfer (h(m)) coefficients during convective drying. The identified D-e in untreated (non-frozen samples) beetroot, apple and eggplant was of 4.2 +/- 0.1 x 10(-10), 4.7 +/- 0.1 x 10(-10) and 5.1 +/- 0.3 x 10(-10) m (2)/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 degrees C. The identified h m was of 7.0 +/- 0.5 x 10(-4), 4.2 +/- 0.2 x 10(-4) and 2.3 +/- 0.2 x 10(-4) kg water/(m(2) 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 degrees C.