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

摘要

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.