Ultrasound and its combined application in the improvement of microbial and physicochemical quality of fruits and vegetables: A review

The eating safety and high quality of fruits and vegetables have always been concerned by consumers, so require a safe, non-toxic, environment-friendly technology for their preservation. The application of ultrasound is a potential technology in the preservation of fruits and vegetables. This paper describes the ultrasound mechanism for inactivating microorganisms, with the cavitation phenomena of ultrasound being considered as a main effect. Effect of ultrasound on microorganisms of fruits and vegetables was discussed. Ultrasound alone and its combined treatments can be an effective method to inactivate the spoilage and pathogenic microorganisms on the surface of fruit and vegetables. Effect of ultrasound on physicochemical quality of fruits and vegetables was reviewed. Ultrasound and its combined treatments reduced mass loss, decreased color change, maintained firmness, enhanced and inhibited enzyme activity as well as preserving nutritional components such as total phenolic, total flavonoids, anthocyanin, and ascorbic acid.     

Intensification of the Plant Products Drying Process by Improving Solar Dryer Design

The article presents the rationale for production of dried fruits and vegetables using a solar drying unit. To intensify the drying process, convection of drying agent flow in the proposed drying chamber is studied using Navier–Stokes equations. Numerical methods are used for solving equations describing the process of convective heat transfer. As a result, graphical interpretations of isolines of drying agent flow are obtained and location of passive zones in the dryer chamber are identified. Uniformity of the temperature zones in the chamber is ensured by supplying additional drying agent into the passive zones. Temperature values at various levels of the drying chamber are experimentally obtained. Results for drying cut-up mass of vegetables and fruits are presented.     

Power ultrasound as a pretreatment to convective drying of mulberry (Morus alba L.) leaves: Impact on drying kinetics and selected quality properties

The effect of ultrasound pretreatment prior to convective drying on drying kinetics and selected quality properties of mulberry leaves was investigated in this study. Ultrasound pretreatment was carried out at 25.2–117.6 W/L for 5–15 min in a continuous mode. After sonication, mulberry leaves were dried in a hot-air convective dryer at 60 °C. The results revealed that ultrasound pretreatment not only affected the weight of mulberry leaves, it also enhanced the convective drying kinetics and reduced total energy consumption. The drying kinetics was modeled using a diffusion model considering external resistance and effective diffusion coefficient D_e and mass transfer coefficient h_m were identified. Both D_e and h_m during convective drying increased with the increase of acoustic energy density (AED) and ultrasound duration. However, D_e and h_m increased slowly at high AED levels. Furthermore, ultrasound pretreatment had a more profound influence on internal mass transfer resistance than on external mass transfer resistance during drying according to Sherwood numbers. Regarding the quality properties, the color, antioxidant activity and contents of several bioactive compounds of dried mulberry leaves pretreated by ultrasound at 63.0 W/L for 10 min were similar to that of mulberry leaves without any pretreatments. Overall, ultrasound pretreatment is effective to shorten the subsequent drying time of mulberry leaves without damaging the quality of final product.     

The effect of high power airborne ultrasound and microwaves on convective drying effectiveness and quality of green pepper

The effectiveness of hybrid drying based on convective drying with application of ultrasound and microwave enhancement is the main subject of the studies. The drying kinetics, energy consumption as well as the quality aspect of green pepper is analysed. It was shown that hybrid drying methods shorten significantly the drying time, reduce the energy consumption and affect positively the quality factors. Each of the analysed aspects depend on combination of the convective-ultrasound-microwave drying programs. Besides, based on the drying model elaborated earlier by one of the authors, the effects of ultrasound on convective drying assessed by such phenomena as “heating effect”, “vibration effect” and “synergistic effect” are presented.     

Ultrasound- and microwave-assisted convective drying of carrots – Process kinetics and product’s quality analysis

The aim of these studies was to investigate the influence of airborne ultrasound-assisted convective drying and microwave-assisted convective drying, as well as their combination, on process kinetics, total color change, water activity, content of carotenoids, polyphenols and antioxidant activity of carrots (Daucus carota L.). The global model of drying kinetics based on coupled ordinary differential equations was used to describe the moisture and material temperature profiles during drying. Application of ultrasound and microwave in convective drying reduced drying time in the range of 9–81%, but the shortest drying time was observed for simultaneous action of convection, ultrasound and microwave. The results of qualitative analysis showed a product improvement due to ultrasound as compared to convective drying and microwave-convective drying. The proposed mathematical model of drying kinetics successfully simulated real drying processes. The proposed mathematical model of drying kinetics successfully simulated real hybrid drying processes.     

Enhancing carrot convective drying by combining ethanol and ultrasound as pre-treatments: Effect on product structure,quality, energy consumption,drying and rehydration kinetics

Ultrasound was combined with ethanol to improve different aspects of carrot convective drying, evaluating both processing and product quality. The ultrasound in water treatment resulted in cellular swelling and small impact on texture. Differently, the ultrasound in ethanol and ethanol treatments modified both carrot microstructure (cell wall modifications of parenchymatic tissue) and macrostructure (shrinkage and resistance to perforation). Pre-treatments with ultrasound in ethanol and ethanol improved the drying kinetics, reducing the processing time (~50%) and the energy consumption (42–62%). These pre-treatments also enhanced rehydration, whose initial rate and water retention were higher than the control. In addition, the carotenoid content was preserved after drying, for all the treatments. Any impact on shrinkage was observed. A mechanistic discussion, based on structural modification (microstructure and macrostructure) and physical properties of water and ethanol, was provided. As conclusion, this work not only described positive aspects of combining the technologies of ultrasound and ethanol as pre-treatments to convective drying, but also proposed mechanisms to explain the phenomena.     

Osmotic dehydration and convective drying of kiwifruit (Actinidia deliciosa) – The influence of ultrasound on process kinetics and product quality

The aim of this study was to assess the impact of ultrasound on two subsequent processes – initial osmotic dehydration and convective drying of kiwifruit (Actinidia deliciosa). The effect of ultrasound (at a frequency of 25 kHz) was assessed both in terms of process kinetics and product quality. During the study, three different osmotic agents were used – erythritol, sorbitol, and sucrose – in an aqueous solution at a concentration of 50% (w/w). The essential kinetic parameters were analyzed – water loss and increase of dry matter during osmotic dehydration, and evolution of moisture content and temperature of samples during convective drying (drying curves). Product quality was assessed on the basis of color, water activity, and content of relevant bioactive components – polyphenols and carotenoids. It was found that the application of ultrasound during osmotic dehydration resulted in higher water loss and solid gain. This surely results from the phenomena occurring during the propagation of the elastic waves in the liquid medium (mainly related to cavitation) and enhancement of the mass transfer. The use of ultrasound during convective drying also had a positive effect on the kinetics of this process. In most cases, during the ultrasonically assisted drying operations, a significant increase in drying rate was observed, leading to a reduction in drying time. This may be due to the intensification of both heat and mass transfer as a result of the mechanical and thermal effects of ultrasound. The assessment of product quality showed that the use of sugar alcohols was a good alternative to sucrose, and ultrasound-assisted convective drying increased the retention of valuable carotenoids and polyphenols. Moreover, in all dried kiwifruit slices, water activity was below 0.6, which can guarantee the microbiological stability of the tested samples.     

Application of ultrasonic technology in postharvested fruits and vegetables storage: A review

It has been an important research topic and a serious applicable issue to extend storage time of fruits and vegetables using advanced scientific and effective technology. Among various approaches, ultrasound has been regarded as one of the most pollution-free and effective technical means to significantly improve the preservation of fruits and vegetables. This paper summarizes the application of ultrasonic technology in fruits and vegetables storage in recent years, including removal of pesticide residues and cleaning, sterilization, enzyme inactivation, effect on physico-chemical indexes. Additionally, we also discussed limitations and negative effects of ultrasonic treatment on fruits and vegetables such as damages to tissues and cells. Furthermore, a proper application of ultrasonic technology has been proven to effectively extend the storage period of postharvest fruits and vegetables and maintain the quality. Moreover, the combination of ultrasound and other conventional preservation technologies can further improve the preservation in a coordinate manner and even have a broader application prospect.     

Application of ultrasound technology in the drying of food products

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.     

Combining ultrasound,vacuum and/or ethanol as pretreatments to the convective drying of celery slices

This work studied three emerging approaches to improve the convective drying (50 °C, 0.8 m/s) of celery. Celery slices of 2 mm thick were pretreated for 5 min using ultrasound (32 W/L, 40 kHz), vacuum (75 kPa vacuum pressure) and ethanol (99.8% v/v, as drying accelerator) applied individually or in combination. To evaluate individual effects of ultrasound and vacuum, the treatments were also performed with distilled water or air medium, respectively. Moreover, the cavitational level was characterized in each condition. Drying kinetics was evaluated tending into account the drying time required by each treatment and the Page’s model parameters. In addition, microstructural effects and shrinkage were evaluated. As results, ethanol combined with ultrasound significantly improved drying kinetics reducing drying time by around 38%. However, vacuum pretreatment did not affect drying kinetics even in combination with ethanol and/or ultrasound. Microstructural evaluation did not evidence cell disruption, suggesting changes in intercellular spaces, pores and/or cell wall permeability. The use of ethanol and vacuum showed a greater effect on shrinkage after pretreatment and after drying, respectively. In conclusion, at the studied conditions, the drying acceleration by vacuum and ultrasound is lower compared to the effect produced using ethanol.     

Individual and interactive effect of ultrasound pre-treatment on drying kinetics and biochemical qualities of food: A critical review

One of the earliest and most prevalent processing methods to increase the shelf-life of foods is drying. In recent years, there has been an increased demand to improve product quality while lowering processing times, expenses, and energy usage in the drying process. Pre-treatments are therefore effectively used before drying to enhance heat and mass transfer, increase drying efficiency, and lessen degradation of final product quality. When food is dried, changes are expected in its taste, color, texture, and physical, chemical, and microbial properties. This has led to the need for research and development into the creation of new and effective pre-treatment technologies including high-pressure processing, pulsed electric field, ultraviolet irradiation, and ultrasound. Sound waves that have a frequency >20 kHz, which is above the upper limit of the audible frequency range, are referred to as “ultrasound”. Ultrasonication (US) is a non-thermal technology, that has mechanical, cavitational, and sponge effects on food materials. Ultrasound pre-treatment enhances the drying characteristics by producing microchannels in the food tissue, facilitating internal moisture diffusion in the finished product, and lowering the barrier to water migration. The goal of ultrasound pre-treatment is to save processing time, conserve energy, and enhance the quality, safety, and shelf-life of food products. This study presents a comprehensive overview of the fundamentals of ultrasound, its mechanism, and how the individual effects of ultrasonic pre-treatment and the interactive effects of ultrasound-assisted technologies affect the drying kinetics, bioactive components, color, textural, and sensory qualities of food. The difficulties that can arise when using ultrasound technology as a drying pretreatment approach, such as inadequate management of heat, the employment of ultrasound at a limited frequency, and the generation of free radicals, have also been explained.     

Ultrasound applications in drying of fruits from a sustainable development goals perspective

This review focuses on the many contributions of ultrasound technologies for fruit drying toward the United Nations Sustainable Development Goals (SDG). Along this review, several aspects attained from the application of ultrasound technologies are correlated with the SDGs. The main ultrasonic technologies applied for fruit drying, such as ultrasonic bath, probe ultrasound, air-borne ultrasound air-drying, and ultrasound-assisted contact air-drying, are presented. An in-depth discussion on ultrasound contributions, its advantages, disadvantages, and limitations are made. The effects of ultrasound on water diffusivity in several fruits are presented by correlating this effect with drying time and cost of energy. Ultrasound-assisted fruit drying, like other food processing technologies, directly impacts Zero Hunger, but ultrasound technologies contribute to much more than delivering long shelf-life food. This technology can be used to produce healthy foods and provide well-being, which will be discussed by correlating the effects of ultrasound-assisted air-drying with the concentration of nutritional compounds. Ultrasound-assisted fruit drying reduces wastewater toxicity and energy consumption and improves productivity, potentially improving workplaces and salaries. A walk through the technology is presented from Zero Hunger to No Poverty.     

A novel sub-pilot-scale direct-contact ultrasonic dehydration technology for sustainable production of distillers dried grains (DDG)

DDG is a major source of protein, calcium, phosphorus, and sulfur is arguably the most important byproduct of the bioethanol industry with increasing demand over the past few years. Reducing energy consumption in the DDG production process and energy recovery from DDG is vital for sustainable bioethanol productions. In this paper, a novel direct-contact multi-frequency, multimode, and modulated (MMM) ultrasonic dryer (US) was developed for the first time and has been applied in dehydration of wet distillers’ grain (WDG). Ultrasonic drying (US) was combined with a convective airflow (HA) at different temperatures of 25 (room temperature), 50 and 70 °C to evaluate the impact of US, HA, and US + HA on drying kinetics, activation energy, chemical compositions, microstructure, and color of DDG. Semi-empirical kinetic models were developed and evaluating drying performances showed that the application of ultrasound significantly enhanced the drying rate and decreased the drying time (by 46%), especially at low drying temperatures. The activation energy for moisture removal in the presence of ultrasound was about 50% of that without ultrasound. The final dried distillers' grains product processed by ultrasonic drying had a brighter color, a higher available protein, a higher digestible protein (the lowest acid detergent insoluble crude protein), and a better surface profile with no compromise on minerals and fiber contents.     

Ultrasonic drying of foodstuff in a fluidized bed: Parametric study

The application of high power ultrasound for dehydration of porous materials may be very effective in processes in which heat-sensitive materials such as foodstuffs have to be treated. In fact, high-intensity ultrasonic vibrations are capable of increasing heat and mass transfer processes in materials. The application of ultrasonic energy can be made alone or in combination with other kind of energy such as hot-air. In this case, ultrasound helps in reducing temperature or treatment time. The aim of this work is to study the effect of air flow rate, ultrasonic power and mass loading on hot-air drying assisted by a new power ultrasonic system. The drying chamber is an aluminium vibrating cylinder, which is able to create a high intensity ultrasonic field in the gas medium. To that purpose the chamber is driven at its centre by a power ultrasonic vibrator at 21.8 kHz. Drying kinetics of carrot cubes and lemon peel cylinders were carried out at 40 degrees C for different air velocities, with and without ultrasound. The results show that the effect of ultrasound on drying rate is affected by air flow rate, ultrasonic power and mass loading. In fact, at high air velocities the acoustic field inside the chamber is disturbed and the effect of ultrasound on drying kinetics diminishes.     

Evaluation on the air-borne ultrasound-assisted hot air convection thin-layer drying performance of municipal sewage sludge

The thin-layer drying behavior of the municipal sewage sludge in a laboratory-scale hot air forced convective dryer assisted with air-borne ultrasound was investigated in between 70 and 130 °C hot air temperatures. The drying kinetics in the convective process alone were compared to that for ultrasound-assist process at three ultrasound powers (30, 90, 150 W). The average drying rates within whole drying temperature range at ultrasound powers of 30, 90 and 150 W increased by about 22.6%, 27.8% and 32.2% compared with the convective drying alone (without ultrasound). As the temperature increasing from 70 °C to 130 °C, there were maximum increasing ratios for the effective moisture diffusivities of the sewage sludge in both falling rate periods at ultrasonic power of 30 W in comparison with other two high powers. In between the ultrasound powers of 0 and 30 W, the effect of the power on the drying rate was significant, while its effect was not obvious over 30 W. Therefore, the low ultrasonic power can be just set in the drying process. The values of the apparent activation energy in the first falling rate period were down from 13.52 to 12.78 kJ mol⁻¹, and from 17.21 to 15.10 kJ mol⁻¹ for the second falling rate period with increasing the ultrasonic power from 30 to 150 W. The values of the apparent activation energy in two falling rate periods with the ultrasound-assist were less than that for the hot air convective drying alone.     

Measurement of microstructural changes promoted by ultrasound application on plant materials with different porosity

This research investigated the effects of ultrasound application (192 ± 6 W/L) on the microstructure of vegetables/fruits with different porosities, cell sizes and patterns (eggplants, beetroots, and apples), submitted to an immersion treatment in different liquids: distilled water, citric acid (1% w/v), and the vegetable/fruit juice, at 25 °C during 5 min. The ultrasound application did not significantly (p > 0.05) affect the size of the cells of the most porous material (eggplant) compared to the samples immersed without ultrasound assistance. The apple samples (with a middle-high porosity and the largest cells) were the most affected by ultrasound application. The median cell areas of samples treated with ultrasound in water and apple juice were 26 and 20% larger than those of samples treated without ultrasound, mainly because of cell wall disruption which caused the cells to merge into bigger clusters, but no effect was observed with the citric acid. Ultrasound application significantly (p < 0.05) increased the median cell area of the less porous raw matter (beetroot) only when the treatment was carried out in the vegetable juice (cells were 26% larger after treatment assisted with ultrasound than without it). Thus, the effects of ultrasound differ in materials with initially different characteristics.     

Ultrasound as pre-treatment for drying of pineapple

Dehydration of fruits is an alternative to reduce post-harvest loss of fruits and also a process to produce dried fruits, which can be directly consumed or become part of foodstuffs like cakes, pastries and many others. The effect of ultrasonic pre-treatment and ultrasound-assisted osmotic dehydration, before air-drying, on dehydration of pineapple (Ananas comosus) was investigated. This study allowed estimating the water diffusivity in the air-drying process for pineapples submitted to ultrasound. Results showed that the water diffusivity increased after application of ultrasound and that the overall drying time was reduced by 8% (over 1h of air-drying time). During the ultrasonic treatment in distilled water the pineapples lost sugar (23.2% in 30 min), so in this condition the ultrasonic pre-treatment can be an interesting process to produce dried fruits with low sugar content. Results showed that the water loss increased with increasing soluble solids content of the osmotic solution and that the ultrasound-assisted osmotic dehydration incorporated more sugar than conventional osmotic dehydration. The water effective diffusivity of the pineapples during the air-drying process was influenced by the pre-treatment, increasing the water effective diffusivity when ultrasound was applied.     

Influence of material structure on air-borne ultrasonic application in drying

This work aims to contribute to the understanding of how the properties of the material being dried affect air-borne ultrasonic application. To this end, the experimental drying kinetics (40 °C and 1 m/s) of cassava (Manihot esculenta) and apple (Malus domestica var. Granny Smith) were carried out applying different ultrasonic powers (0, 6, 12, 19, 25 and 31 kW/m³). Furthermore, the power ultrasound-assisted drying kinetics of different fruits and vegetables (potato, eggplant, carrot, orange and lemon peel) already reported in previous studies were also analyzed. The structural, textural and acoustic properties of all these products were assessed, and the drying kinetics modeled by means of the diffusion theory.A significant linear correlation (r > 0.95) was established between the identified effective diffusivity (D_W) and the applied ultrasonic power for the different products. The slope of this relationship (SDUP) was used as an index of the effectiveness of the ultrasonic application; thus the higher the SDUP, the more effective the ultrasound application. SDUP was well correlated (r ⩾ 0.95) with the porosity and hardness. In addition, SDUP was largely affected by the acoustic impedance of the material being dried, showing a similar pattern with the impedance than the transmission coefficient of the acoustic energy on the interface. Thus, soft and open-porous product structures exhibited a better transmission of acoustic energy and were more prone to the mechanical effects of ultrasound. However, materials with a hard and closed-compact structure were less affected by acoustic energy due to the fact that the significant impedance differences between the product and the air cause high energy losses on the interface.     

Effects of preliminary treatment by ultrasonic and convective air drying on the properties and oil absorption of potato chips

The initial water content was closely related to the oil absorption and properties of fried food. The effects of convective air drying (D) and ultrasound combined convective air drying (UD) pretreatment on the properties and oil absorption of potato chips have been investigated. The oil contents were 48.48 ± 1.42% and 39.78 ± 3.08% for control samples (without D and UD pretreatment) and ultrasound treated samples (without D pretreatment). When the mass loss of samples was reached the proportion of quality to without drying samples quality 80%, 50%, and 20%, the oil contents of D pretreated samples decreased by 12.67%, 28.24% and 62.07%, respectively, and the oil contents of UD pretreated samples decreased by 7.42%, 24.10% and 51.76% (compared to the ultrasound pretreated samples ), respectively. By applying ultrasound before frying, more cracks and pores were exhibited of fried potato chips. After drying process, potato chips exhibited less disruption of cell structure and less deformation of cell irregular. The hardness of the D and UD pretreated potato chips increased with the extension of drying. The FTIR analysis stated the formation of amylose-lipid complexes. This research could contribute to providing evidence for the development and application of the pretreatment strategies.