Ultrasound Pretreatment Applications in the Drying of Agricultural Products

Background

Fruits and vegetables contain fiber, minerals, and vitamins, which are important sources of essential dietary nutrients. Due to the high moisture content of fruits and vegetables, it is classified as highly perishable products. The way to keep the product's main nutritional values high is to keep the product fresh. However, there may be difficulties in keeping the product fresh due to the unfavorable to be experienced during the storage and distribution chain of the product (Sagar & Kumar, 2010). In the food industry, pasteurization, sterilization, freezing, storage in controlled atmosphere and drying are used as food preservation methods. Drying, which promises a longer shelf life, is an important food preservation method. Drying is the process of removing the water contained in the product and thereby reducing the water activity. Furthermore, it is a process which increases the dry matter content in which the wholesomeness and physical appearance are preserved without damaging the tissue due to the reduction of the water content in the food. The purposes of the drying of agricultural products can be listed as follows (Castro et al., 2018):

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  Providing inhibition of growth of microorganisms

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  Preventing deterioration reactions by reducing water activity

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  The reduction of transport and storage costs due to product weight and volume decrease.

In the literature, there are more than 200 drying types used for different purposes. Furthermore, the drying properties (air velocity, product retention time, pressure, relative humidity) vary according to the material and the drying method. It is reported that the total energy requirement of the drying operation in the food sector in developed countries consumes 10% to 15%. Therefore, energy is one of the important parameters of drying, which is the traditional method in product preservation. The conventional method is still the most widely used method in the drying industry, although new drying technologies have emerged. One of the most important reasons for this is the ease of installation and use of the dryer (Onwude et al., 2016). The pretreatment process is being worked on to reduce energy costs and to improve product quality. The pretreatment of agricultural products before drying has been investigated, and many methods have been developed. These methods have been shown in Figure 1 by Deng et al. (2019).

Figure 1.

Pretreatment of food materials

The objectives of the pretreatment can be listed as follows:

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  To facilitate the discharge of water contained in the product by affecting the cell membrane.

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  Prevent energy consumption loss by increasing the drying rate

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  To enhance retention of the nutrients (fat, protein, carbohydrate, minor and macro minerals) compounds

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  Inhibiting enzymes from preventing deterioration during drying and storage

Ultrasound Approach

The application of ultrasound pretreatment to agricultural products before drying has emerged as an important popular topic for researchers as it greatly reduces the overall processing time of the drying process. This technology has started to be researched since the 1970s. These studies include the use of ultrasound in meat technology and the effect of ultrasound on the diffusion rate (Howkins, 1969; Sajas & Gorbatow 1978). Ultrasound is the sound frequency range above 16 kHz, which is the threshold of the human ear. It is classified in two different ways as the high and low frequency in the food industry. While low energy high frequency (megahertz) ultrasound application is used for food quality monitoring, high energy low frequency (kilohertz) ultrasound application has changed the structure of food materials and has an important role in the drying process (Mothibe et al., 2011).