Ultrasonic washing has been widely applied to the postharvest storage of fruits and vegetables as a residue-free physical washing technology, which plays an important role in improving shelf-life, safety, and nutritional value. Phenolics are a large group of phytochemicals widespread in fruits and vegetables, and they have been considered potential protective factors against some diseases because of potent antioxidative properties. Previous studies have shown that ultrasonic washing can increase the phenolic content of fruits and vegetables immediately or during storage through the induction of plant stress responses, which is of great significance for improving the functional and nutritional value of fruits and vegetables. However, the mechanisms of ultrasound as an elicitor to improve the phenolic content remain controversial. Therefore, this review summarizes the applications of ultrasonic washing to increase the phenolic content in fruits and vegetables. Meanwhile, the corresponding physiological stress response mechanisms of the phenolic accumulation in terms of immediate stress responses (i.e., higher extractability of phenolics) and late stress responses (i.e., metabolism of phenolics) are expounded. Moreover, a hypothetical model is proposed to explain phenolic biosynthesis triggered by signaling molecules produced under ultrasound stress, including primary signal (i.e., extracellular adenosine triphosphate) and secondary signals (e.g., reactive oxygen species, Ca2+ , NO, jasmonates, and ethylene). Additionally, the techno-economic feasibility of ultrasonic washing technology is also discussed. Further, challenges and trends for further development of ultrasonic washing as an abiotic elicitor applied to the postharvest storage of fruits and vegetables are presented.

[This corrects the article DOI: 10.3389/fnut.2022.1006440.].

Ultrasonic washing has been proved to be an abiotic elicitor to induce the accumulation of phenolics in some fruit and vegetables. However, the feasibility of ultrasonic washing on the accumulation of phenolics in fresh-cut red cabbages has not yet been reported. Therefore, the effects of ultrasonic washing on the phenolics and related phenolic metabolism enzymes of fresh-cut red cabbages, as well as quality and microbial safety during cold storage, were investigated. Firstly, the single-factor tests were used to optimize the ultrasonic processing parameters, including frequency mode, frequency amplitude, power density, frequency cycle time, and ultrasonic washing. Then the activities of the enzymes related to phenolic metabolisms after optimal ultrasound treatment were investigated, including phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), and peroxidase (POD). Additionally, the quality and microbial safety of fresh-cut red cabbages stored at 4°C under the optimal ultrasound treatment were evaluated. The results showed that the content of soluble phenolics (SPs) in fresh-cut red cabbages increased significantly during storage under the optimal conditions (28 ± 2 kHz, 60 W/L, 400 ms, and 20 min) compared with the control (P < 0.05). The PAL activity was activated and the PPO and POD activities were inhibited after ultrasonic washing, which contributed to the increase in the content of SPs. Meanwhile, the storage quality and microbial safety of fresh-cut red cabbages were improved. Ultrasonic washing reduced the weight loss and respiration rate and improved the color and texture characteristics. Additionally, the fresh-cut red cabbages after ultrasonic washing showed more retention of ascorbic acid (AA), total soluble proteins (TSPs), total soluble sugars (TSSs), and total soluble solids (SSs) compared with the control. Finally, ultrasonic washing effectively inhibited the growth of bacteria, molds and yeasts, which is beneficial to the extension of the shelf-life of fresh-cut red cabbages. Therefore, ultrasonic washing can be used as a tool to increase the content of SPs in fresh-cut red cabbages while retaining quality attributes and microbial safety.

BACKGROUND: Pesticide residues in fruits and vegetables threaten food safety. Cleaning before eating is a usual way to remove pesticide residues, so it is very important to find the most efficient cleaning conditions for public health. However, many previous cleaning studies only focused on a single variable which required a large amount of time manpower and material resources. Plackett-Burman design (PBD) and response surface methodology can avoid the earlier-mentioned problems and have potential in studying the influence and interaction of multiple factors. In this study, the effect of five factors on the removal of triadimefon and boscalid from rape by ultrasonic washing was evaluated through PBD: pH of water, ultrasonic cleaning time, water temperature, initial residual concentration and volume of water.
RESULTS: Temperature had a significant effect on the rate of triadimefon removal while the other four factors impacted boscalid removal greatly. A higher temperature was better for the removal rate of triadimefon. Under alkaline environment, when initial residual concentration and cleaning time increased with decreasing water volume, the removal rate of boscalid increased. Furthermore, the interactions among factors were obtained. The regression coefficients of fitting equations about triadimefon and boscalid were 0.9657 and 0.9738, respectively.
CONCLUSIONS: Changing pH of water, cleaning time, water volume and temperature during the washing process of rape through PBD designed experiments represents a valid strategy for improving the removal rate of two pesticides residue. This study provides a reference for ultrasonic cleaning conditions by a sink dishwasher, which has a positive effect on food safety. © 2021 Society of Chemical Industry.

In this study, the ultrasonic dishwasher was used to remove five pesticides known to be frequently used on rape and grape. As compared with normal water washing, washing with the ultrasonic dishwasher was demonstrated to be more effective for pesticides removal, achieving removal rates between 14.7% and 59.8% on rape, and between 72.1% and 100% on grape. However, there were significant differences in order of the removal rate of five pesticides on rape and grape. From the adsorption experiments and analysis via Freundlich equation, the adsorption index (n, 0.551-1.056 on rape and 0.362-1.478 on grape) and adsorption coefficient (KF, 10-2.47-10-1.65 and 10-3.64-10-1.56 (mg·dm-2)/(mg·L-1)-n on rape and grape) were obtained. Taken together with the observation of the matrix surface by scanning electron microscopy and the evaluation of the physicochemical properties of pesticides, the different pesticides removal may be related to surface structure of the matrix.

Ultrasonic technology is a promising tool for washing oily sludge to recover oil. In this study, the influence of ultrasonic conditions on the removal of different oil components from oily sludge was investigated to optimize ultrasonic washing technology. Among the three frequencies (25, 50 and 100 kHz) applied, ultrasonic washing at 25 kHz exhibited the best performance in terms of oil extraction rate and efficiency. An ultrasonic intensity of 0.33 W/cm2 was necessary to overcome the energy threshold for oil washing. Application of a standing wave or dual frequencies did not improve the oil removal performance perceptibly. The optimum conditions for the removal of oil from oily sludge were an ultrasonic frequency of 25 kHz, intensity of 0.33 W/cm2 and sludge/water ratio of 1/2 (in volume). Thin-layer chromatographic flame ionization detection showed that ultrasonication could overcome high energy thresholds, resulting in an increase in the removal of asphaltenes and resins, with the removal of asphaltenes particularly affected by frequency. This study could provide valuable information for the application of ultrasonic technology in oily sludge treatment.