Effects of freezing treatments before convective drying on quality parameters: Vegetables with different microstructures

Effects of freezing (at - 20, - 80 and -196 degrees C) before convective drying (at 50 degrees C and a flow rate of 1 m/s) on microstructure, texture, total polyphenol content (TPC) and antioxidant activity (AA) of vegetables with different porosity, beetroot (low), apple (medium) and eggplant (high), have been studied. Drying time of frozen samples was 11-32% shorter (p < 0.05) than untreated samples. The highest drying time reductions were obtained in eggplant (22 +/- 10%) and the lowest in beetroot (14 +/- 3%). Changes in microstructure after freezing were remarkable at - 20 and - 80 degrees C, but minor after freezing by liquid nitrogen immersion in beetroot and apple; and not significant (p > 0.05) in eggplant. The elastic modulus decreased 96 +/- 2% and the stress-strain curves were significantly lower (p < 0.05) with no rupture point in the three vegetables after all freezing treatments. Both TPC and AA of frozen samples were significantly lower (p < 0.05) than those of untreated samples before and after drying, except in beetroot frozen by immersion in liquid nitrogen, which was not significantly different (p > 0.05) to untreated sample. Freezing by immersion in liquid nitrogen promoted the lowest TPC and AA reductions in apple and eggplant. In conclusion, freezing pre-treatment before drying affected differently depending on both, the freezing rate and the original microstructure of the vegetable.