Application of ultrasound technology in the drying of food products

Highlights

• Ultrasound pre-treatment before drying and ultrasound assisted drying were evaluated.

• Parametric effects, drying kinetics and food quality were discussed.

• Inconsistencies of the ultrasound effects were pointed out and analyzed in detail.

Abstract

This study presents a state-of-the-art overview on the application of ultrasound technology in the drying of food products, including the ultrasound pre-treatment and ultrasound assisted drying. The effect of main parameters and ultrasound technology on the drying kinetics and food quality were discussed. Inconsistencies were pointed out and analyzed in detail. Results showed that for ultrasound pre-treatment, the food products may lose or gain water and increase of ultrasonic parameters (sonication time, amplitude and ultrasound power) promoted the water loss or water gain. When ultrasound technology was applied prior to drying, an increase in drying kinetics was always observed, though some different results were also presented. For ultrasound assisted drying, the ultrasound power always gave a positive effect on the drying process, however, the magnitude of ultrasound improvement was largely dependent on the process variables, such as air velocity, air temperature, microwave power and vacuum pressure, etc. The application of ultrasound technology will somehow affect the food quality, including the physical and chemical ones. Generally, the ultrasound application can decrease the water activity, improve the product color and reduce the nutrient loss.

Introduction

Freshly harvested food products (e.g., fruits and vegetables) usually have high water contents. The water provides conditions to grow and reproduce microorganisms, leading to irreversible damage to food products. Thus it is necessary to reduce the moisture content and nutrient loss by using appropriate preservation methods. Drying is widely used to preserve food products. The removal of moisture can not only inhibit the microbial growth, but also decrease the total weight of the product, shorten the packaging space and thus minimize the packaging and transportation cost [1].

Sun drying is the most traditional and simple method to dry food products. However, sun drying depends entirely on natural environmental conditions. Besides, the slowness of the drying process, high labour requirement and the exposure to the surrounding environment during the drying process limit its application. With increased requirement for food nutrition and health among consumers, researchers are challenged to explore innovative ways to develop high-quality dried products [2]. Nowadays, most works are dealt with the relationships between drying factors (e.g., drying temperature and air velocity, etc.) and quality aspects (e.g. retention of nutritional content, colour change, rehydration capacity, texture and sensory quality, etc.). Using innovative technologies to improve and optimize existing techniques, the quality of dried products has improved to a large extent. Recently, various new drying methods such as hot-air, microwave, freeze drying, vacuum, and hybrid techniques (e.g. hot air-vacuum, hot air-microwave, microwave-freeze, microwave-vacuum, etc.) have invariably been adopted instead of sun drying [3], [4], [5], [6].

However, several disadvantages of these dehydration methods have been identified, such as the relatively large energy consumption and quality deterioration of the final product for hot-air drying, uneven drying or overheated for microwave drying and high cost expenditure for freeze and hybrid drying, etc. Thus, pre-treatment is often employed before drying. Pre-treatment prior to drying is a well-explored area, and many methods have been developed. The chemical and physical pre-treatments of fruits and vegetables have been reviewed by Deng et al. [7]. They found that although chemical pre-treatment can speed the drying process, it causes soluble nutrients losses and triggers food safety problems by chemical residuals. Thermal pre-treatment can destroy microorganisms, soft the texture and increase the drying rate. However, it induces undesirable quality of products.

On the other hand, non-thermal technologies (e.g., ultrasound and pulsed electrical field, etc.) can be a better alternative to overcome these drawbacks. Ultrasound pre-treatment of food products before drying has been a hotspot in recent years and has shown potential in greatly decreasing the drying time. Wiktor et al. [8] investigated the combined effect of pulsed electric field and ultrasound pre-treatments on the drying characteristics of carrots. They showed that the pre-treatments contributed to a shorter drying time by up to 40% and a higher retention of carotenoid content. Tao et al. [9] also applied ultrasound pre-treatment on the drying of mulberry leaves. The drying time and accordingly the energy consumption were significantly reduced.

The ultrasound technology was also used to assist the above drying methods. For example, the ultrasound assisted convective drying enables the drying temperature to be a low value and thus can be applied in drying heat-sensitive materials. Szadzinska et al. [10] investigated the drying characteristics of red beetroot by a hybrid drying technology (ultrasound + microwave + convective drying). The utilization of ultrasound not only reduced the total drying time and energy consumption but also enhanced the product quality. Tao et al. [11] developed a hot-air convective dryer coupled with a contacting ultrasound system to the dehydration of garlic slices. The drying process was significantly accelerated. The retention of organosulfur compounds was higher and the browning problem was also alleviated. Baslar et al. [12] compared three different drying methods (ultrasonic-vacuum drying (USV), vacuum drying (VD) and oven drying (OD)). The beef and chicken meats dried faster with USV than with VD and OD techniques.

The aim of the present study is to assess the ultrasound technology used in food drying including ultrasound pre-treatment and ultrasound assisted drying in the last decade. The effect of ultrasound technology on the drying process (such as drying rate, effective moisture diffusivity and energy consumption, etc.) and quality aspects of the dried products are reviewed in detail.