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

[EN] 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 A degrees C) beetroots were convectively dried (40 A degrees C and 1 m/s) with and without ultrasound application using two acoustic densities (16.4 and 26.7 kW/m(3)), 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. The authors would like to acknowledge the INIA for the financial support (RTA2015-00060-C04-03 and RTA2015-00060-C04-02 projects) and the Spanish Government (MINECO) for the BES-2013-064131 fellowship. Vallespir, F.; Carcel Carrión, JA.; Marra, F.; Eim, VS.; Simal, S. (2018). Improvement of Mass Transfer by Freezing Pre-treatment and Ultrasound Application on the Convective Drying of Beetroot (Beta vulgaris L.). Food and Bioprocess Technology. 11(1):72-83. https://doi.org/10.1007/s11947-017-1999-8 Ando, Y., Maeda, Y., Mizutani, K., Wakatsuki, N., Hagiwara, S., & Nabetani, H. (2016). Impact of blanching and freeze-thaw pretreatment on drying rate of carrot roots in relation to changes in cell membrane function and cell wall structure. LWT - Food Science and Technology, 71, 40–46. AOAC. (2006). Moisture in dried fruits 934.06, 16th ed. Maryland: Association of Analytical Communities. Boukouvalas, C. J., K