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

• Published in: Food and bioprocess technology Vol. 11; no. 1; pp. 72 - 83
• Main Authors: Vallespir, Francisca, Cárcel, Juan A., Marra, Francesco, Eim, Valeria S., Simal, Susana
• Format: Journal Article
• Language: English
• Published: New York Springer US 2018; Springer Nature B.V
• Subjects: Agriculture; Antioxidants; Bioactive compounds; Biological activity; Biotechnology; Chemistry; Chemistry and Materials Science; Chemistry/Food Science; Computer simulation; Convective drying; Diffusion coefficient; Drying; Food matrix; Food Science; Freezing; Mass transfer; Microstructure; Original Paper; Pretreatment; Sugar beets; Ultrasonic imaging; Ultrasound; Beetroot; Diffusional model; Ultrasound; Quality; Freezing
• ISSN: 1935-5130; 1935-5149
• DOI: 10.1007/s11947-017-1999-8

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/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.