Improvement of Mass Transfer by Freezing Pre-treatment and Ultrasound Application on the Convective Drying of Beetroot (Beta vulgaris L.)

Abstract

The effects of freezing pre-treatment and ultrasound application during drying on microstructure, drying curves, and bioactive compounds of beetroot have been evaluated. Raw and previously frozen (at − 20 °C) beetroots were convectively dried (40 °C and 1 m/s) with and without ultrasound application using two acoustic densities (16.4 and 26.7 kW/m3), and a diffusional model was proposed to simulate the drying curves. Freezing pre-treatment and ultrasound application caused significant disruptions in the beetroot microstructure and reduced the drying time, enhancing the mass transfer. The external mass transfer coefficient significantly (p < 0.05) increased by 28–49% when ultrasound was applied; moreover, the effective diffusion coefficient significantly (p < 0.05) increased by 60–73% and 204–211%, respectively, due to the ultrasound application on the drying of raw and pre-frozen samples. Freezing caused significant (p < 0.05) increases in betalain and total polyphenol contents and antioxidant activity compared with the raw sample (16–57%), probably due to the release of free forms from the food matrix; meanwhile, drying had the opposite effect (8–54% decrease). Significantly (p < 0.05) higher decreases (32–81%) in bioactive compounds and antioxidant activity were observed when drying was assisted by ultrasound compared with dying without ultrasound. Therefore, freezing pre-treatment and ultrasound application enhanced mass transfer during drying. However, significant changes in quality parameters of the final product were observed.