Comparison between airborne ultrasound and contact ultrasound to intensify air drying of blackberry: Heat and mass transfer simulation, energy consumption and quality evaluation

Yang Tao,Dandan Li,Wai Siong Chai,Pau Loke Show,Xuhai Yang,Sivakumar Manickam,Guangjie Xie,Yongbin Han

doi:10.1016/j.ultsonch.2020.105410

Abstract

This study aimed at investigating the performances of air drying of blackberries assisted by airborne ultrasound and contact ultrasound. The drying experiments were conducted in a self-designed dryer coupled with a 20-kHz ultrasound probe. A numerical model for unsteady heat and mass transfer considering temperature dependent diffusivity, shrinkage pattern and input ultrasonic energies were applied to explore the drying mechanism, while the energy consumption and quality were analyzed experimentally. Generally, both airborne ultrasound and contact ultrasound accelerated the drying process, reduced the energy consumption and enhanced the retentions of blackberry anthocyanins and organic acids in comparison to air drying alone. At the same input ultrasound intensity level, blackberries received more ultrasound energies under contact sonication (0.299 W) than airborne sonication (0.245 W), thus avoiding the attenuation of ultrasonic energies by air. The modeling results revealed that contact ultrasound was more capable than airborne ultrasound to intensify the inner moisture diffusion and heat conduction, as well as surface exchange of heat and moisture with air. During air drying, contact ultrasound treatment eliminated the gradients of temperature and moisture inside blackberry easier than airborne ultrasound, leading to more homogenous distributions. Moreover, the total energy consumption under air drying with contact ultrasound assistance was 27.0% lower than that with airborne ultrasound assistance. Besides, blackberries dehydrated by contact ultrasound contained more anthocyanins and organic acids than those dried by airborne ultrasound, implying a higher quality. Overall, direct contact sonication can well benefit blackberry drying in both energy and quality aspects.

Highlights

Dehydration is an old but effective technology for the preservation of perishable foods like fruits and vegetables
Drying kinetic curve and temperature history of blackberry are plotted in Figs. 2 and 3, respectively
Moisture content in blackberry during contact ultrasound assisted drying declined the fastest, followed by airborne ultrasound drying and air drying alone. These results were in line with many studies on ultrasound drying re ported in the literature [7,9]

Summary

Introduction

Dehydration is an old but effective technology for the preservation of perishable foods like fruits and vegetables. The deficiencies of convective air drying are obvious, including high energy consumption, deterioration of food quality, loss of food nutrition, and relatively low drying rate [1,2]. A consensus on ultrasound-assisted food drying has be achieved that mechanical vibration and cavitation phenomenon gener ated by ultrasound can speed up the drying process, reduce energy consumption and enhance the quality of dehydrated foods [3,4,5]. Due to these merits, ultrasound has a big potential to be applied in food

Objectives

Methods

Results