Exploring Impact of Ultrasound on Food Drying

Introduction

One of the earliest techniques for preserving food, which has been modernized by technological advancements in the past century is dehydration. The primary objectives of this method are to reduce water content and prolong the shelf life of food products. Dehydration is defined as the controlled application of heat to evaporate a substantial portion of the water content within a food item. As water activity diminishes, both the activity of microorganisms and enzymes decreases. Fresh fruits and vegetables, owing to their high moisture content can rapidly deteriorate in quality if not dehydrated promptly. Additionally, certain food items have a limited harvest window, necessitating the use of highly efficient dehydration methods to preserve a substantial quantity of the harvested produce. The removal of moisture prevents the growth of microorganisms, resulting in a lighter product that occupies less space in packaging, ultimately leading to cost savings in terms of shipping and storage. (Kaur & Singh, 2014; Tiwari, 2016)

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Drying facilities encompass a broad spectrum, ranging from simple sun- or hot-air drying setups to advanced, high-capacity units like spray or freeze-dryers. However, various drawbacks are associated with these dehydration techniques. For instance, the utilization of hot-air drying is acknowledged for its relatively elevated energy consumption and the likelihood of compromising the ultimate product’s quality. The application of microwave drying may lead to issues like non-uniform drying or overheating, whereas freeze and hybrid drying methods often entail substantial costs and expenditure. (Huang et al., 2020; Zalpouri et al., 2022; Zhang & Abatzoglou, 2020)

To mitigate these drawbacks, pre-treatment measures are frequently employed before the actual drying process. Pre-treatment before the drying process is a thoroughly explored field, which has led to the development of various methods. Thermal pre-treatment has the capability to eliminate microorganisms, soften the texture and enhance the drying rate. However, it can also result in undesirable changes in product quality.

Non-thermal methodologies such as pulsed electrical fields, ultrasound and many more offer promising solutions to address the limitations mentioned earlier. Ultrasound treatment applied to food items before the drying process has garnered considerable attention in recent years, showcasing significant potential for substantially reducing drying times. In contemporary times, ultrasound pre-treatment has been employed with various agro-food products such as apple, blackberry, celery, carrot, kiwifruit, pineapple, olives, potato, sweet potatoes, strawberries, saffron, etc. The main aim is to accelerate the drying process and improve the overall quality attributes of the end products. Moreover, recent progress has resulted in the formulation of efficient procedures for incorporating ultrasound as a pre-treatment step in the drying process. (Pandiselvam et al., 2023)

Ultrasound

Ultrasound, characterized as mechanical waves with frequencies ranging from 20 kHz to 1 MHz offers several approaches when applied to food drying. These include ultrasound pre-treatment, airborne ultrasound-assisted drying and contacting ultrasound-assisted drying. Ultrasound pre-treatment generally involves the application of ultrasound waves to an aqueous medium, like distilled water using an ultrasonic bath or probe. These acoustic waves cause compression and expansion of the food samples, a phenomenon akin to a sponge effect. This action creates microscopic channels within the sample, allowing intracellular liquids to seep into the surrounding environment. When sufficient ultrasound power is reached, the attractive forces among liquid molecules can be exceeded by the rarefaction cycle, leading to the formation of cavitation bubbles (Fig. 1). These bubbles gradually expand until reaching a critical size, at which point they become unstable and violently collapse. The resulting breakdown causes swift and substantial alterations in local pressure and temperature, causing the disintegration of water molecules into extremely reactive free radicals such as H+ and OH-. Certain chemical reactions can be accelerated by these free radicals and other molecules can be modified by them, including their reaction with readily oxidizable food molecules. (Kalsi et al., 2023; Leong et al., 2011)

Ultrasound-Assisted Drying Mechanism

Ultrasound-assisted drying revolves around studying how airborne ultrasound affects the drying process. Under high-intensity airborne ultrasound exposure, fluctuations in pressure occur at the interfaces between gas and liquid. During the negative pressure phase within this pressure cycle, moisture moves away from the sample and does not return during the subsequent positive pressure phase, thereby expediting moisture evaporation. Furthermore, an oscillating velocity effect is induced by acoustic energy during convective drying, further accelerating the drying procedure. High-intensity airborne ultrasound generates micro-streaming at these interfaces, reducing the diffusion boundary layer and improving water diffusion. Consequently, airborne ultrasound technology proves effective at expediting drying. (Fernandes & Rodrigues, 2023)

However, there are limitations in using airborne ultrasound, primarily because acoustic energy undergoes significant attenuation in a gaseous medium and there is an acoustic impedance mismatch between air and the system being used. To address these issues, researchers have explored direct contact between ultrasound equipment and food materials. This approach eliminates the problem of energy loss by ensuring that ultrasonic energy directly interacts with the food items. When the vibrating system directly contacts the food material, excellent acoustic impedance matching enables ultrasonic energy to penetrate deep into the product. This leads to a swift series of oscillations between contractions and expansions, resembling the action of repeatedly compressing and releasing a sponge. This eases the drying process by establishing tiny pathways for water to migrate. Furthermore, intense ultrasound prompts the occurrence of cavitation phenomena, which assist in dislodging tightly bound moisture from the products. In general, contact ultrasound leads to more significant improvements in drying rates compared to airborne ultrasound. However, due to the requirement of direct emitter-product interaction, putting this method into practice on an industrial scale presents difficulties. (Huang et al., 2020)

Effect on Drying Kinetics

Significant enhancements in the drying kinetics of various food products have been achieved through the incorporation of ultrasound technology into the drying process. For instance, when considering apples, research has unveiled noteworthy reductions in drying time, reaching up to 57%, accompanied by substantial decreases in energy consumption of up to 54% (Sabarez et al., 2012). Similarly, the application of ultrasound-assisted drying has proven advantageous for carrots, with drying times trimmed by as much as 37.5%. The benefits extend to other food items like cassava and apples, where drying time reductions of up to 70% have been achieved alongside notable improvements in key parameters such as effective diffusivity and mass transfer coefficient. (Huang et al., 2020; Kowalski & Mierzwa, 2015; Szadzińska et al., 2016)

However, it’s worth noting that in some cases such as the study conducted by Kowalski et al., (2017), while convective drying may exhibit longer drying times than ultrasound-assisted drying, the latter can still be more energy-efficient. This is due to the additional energy required for ultrasound generation and the energy consumed by the experimental system, including devices that raise air temperature and flow velocity, contributing to the overall energy consumption.

Effect on Product Quality

1. Colour:

An increase in L* value is typically seen as a favourable change, whereas variations in a* or b* value are regarded negatively, as they indicate excessive browning. It’s important to note that total colour difference during the drying process is inevitable and while ultrasound pre-treatment and other methods can mitigate it to some extent, complete prevention is unattainable. When employing ultrasound-assisted drying techniques, the total colour difference generally remains below threshold. Direct contact ultrasonic drying stands out among the other ultrasound techniques for its notable effectiveness in minimizing apple browning. This is likely attributed to its heightened ability to inactivate the enzyme responsible for browning, since the fruit is continuously exposed to ultrasound during the course of drying. (Fernandes & Rodrigues, 2023; Huang et al., 2020)

2. Bioactive compound:

Ultrasound-assisted drying techniques, including methods such as infrared drying, air-drying and contact air-drying have demonstrated the ability to produce dried fruits with elevated polyphenolic compound, compared to traditional air drying. This augmentation in polyphenolic compound primarily stems from the shorter exposure of polyphenolic compounds to elevated or moderate temperatures, typically within the range of 40 to 70°C employed during the drying process. Many polyphenolic compounds are heat-sensitive and tend to degrade when exposed to prolonged elevated temperatures.

Consequently, the shortened drying periods characteristic of ultrasound-assisted methods lead to diminished phenolic degradation in contrast to conventional air drying. Another rationale behind the increased phenolic content in fruits dried using ultrasound-assisted methods relates to the heightened bioavailability of phenolic compounds post-ultrasound treatment. The energy produced by ultrasound disrupts the bond between phenolics and cell membranes, leading to the release of polyphenolic compounds that were previously bound to these membranes, both on their surface and within the lipid layer. This increased bioavailability contributes to the elevated polyphenolic compound observed in the dried fruits. (Fernandes & Rodrigues, 2023)

3. Texture:

Integrating ultrasound into the drying process offers an avenue for enhancing the textural attributes of the product. Primarily, it has a pronounced effect on the product’s hardness. Moreover, ultrasound-assisted drying demonstrates improvements in other textural qualities such as chewiness, gumminess and brittleness. This enhancement in texture is rooted in the mechanism that involves the formation of microscopic channels. These channels emerge as a consequence of the compression and expansion cycles induced by ultrasound. As a result, there is a decrease in hardness due to the subsequent loss of turgor pressure, detachment of protoplasm from the cell wall, disruption of cellular structure and mild denaturation of proteins. (Aslam et al., 2021; Pandiselvam et al., 2023)

Conclusion

The extensive research has been devoted to advancing ultrasound-assisted drying systems and airborne ultrasonic transducers. However, when it comes to integrating ultrasound-assisted drying into convective dryers, the most straightforward approach is to utilize airborne ultrasonic waves. Nevertheless, the present state of transducer technology faces constraints in generating high-power airborne ultrasound on a large scale, which presents a significant challenge to this application.

The adoption of ultrasound-assisted drying has demonstrated numerous advantages, including an accelerated drying rate, reduced energy consumption and improved food quality through the minimization of exposure to excessive heat. This has led to a surge in interest in ultrasound technology, due to expanding research efforts in this field and the development of industrial-scale equipment. This growing interest encompasses both the characterization and quality assessment of food and ingredients, as well as the stabilization of various food products. These combined advantages encourage sustainable production and consumption practices. Ultrasound-assisted dried fruits can be stored by consumers for longer duration, thus substantially reducing food wastage and promoting sustainability within the food industry.

References:

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13. Zalpouri, R., Singh, M., Kaur, P., & Singh, S. (2022). Refractance window drying–a revisit on energy consumption and quality of dried bio-origin products. Food Engineering Reviews, 14, 257–270. https://doi.org/10.1007/s12393-022-09313-3

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