Lotus root (Nelumbo nucifera Gaertn.) is cultivated extensively in China and is also an aquatic vegetable favored by consumers all over the world. It is abundant in potassium, iron, thiamin, riboflavin, pyridoxine, vitamin C, polyphenol, and other nutrients that can facilitate blood circulation, invigorate the spleen and boost appetite, and strengthen the immune system (Wang et al., 2021). Among these nutrients, phenolics, as one of the major secondary metabolites of lotus root, are also widely distributed in other fruits, vegetables, and grains, which can be categorized into flavonoids, phenolic acids, and tannins (Hu, Yang, Wu, Li, & Zhan, 2014). The phenolic composition of fruits and vegetables varies considerably with varieties, organizational structures, stages of maturity, and storage conditions (Acosta-Estrada, Gutierrez-Uribe, & Serna-Saldivar, 2014). Additionally, the phenolic metabolism of plants can be influenced by external effects such as physical and chemical stimuli, as well as mechanical damage, which affects the integrity and functionality of cell membranes, leading to the counteraction by the antioxidant system formed by the original phenolics in plants, along with other antioxidant substances and enzymes. On the other hand, plants also respond to external stimuli by initiating a series of metabolic pathways that cause the production of monomeric phenols and polymerized phenols (Hu, Sarengaowa, & Feng, 2022; Zhao, Zhang, & Zhang, 2017).

Fresh-cut lotus roots are a convenient product that requires minimal pre-cooking processing, so it has gained the favor of consumers (Zhang, Yu, Xiao, Wang, & Tian, 2013). However, the mechanical damage of lotus roots during processing triggers a series of intricate physiological changes in primary and secondary metabolism, including enzymatic browning, histolysis, and the development of off-flavors. These changes not only impact the quality and acceptability among consumers but also impose limitations on the shelf-life of fresh-cut lotus roots. The enzymatic oxidation of phenolics, specifically the chelation of natural phenolic compounds with copper ions in the presence of polyphenol oxidase (PPO) and peroxidase (POD), leading to the formation of quinone compounds, has been identified as the primary factor responsible for the browning of lotus root slices (Min et al., 2017). This phenomenon has emerged as a significant obstacle hindering the progress of the fresh-cut fruits and vegetables industry. The conventional thermal washing technique can deactivate microbial growth and browning, as well as passivate enzyme activity to extend the shelf life of fresh-cut fruits and vegetables (Li et al., 2018). However, it should be noted that excessively high temperatures may cause tissue damage and nutrient loss in fresh products (Guo et al., 2022). Nowadays, there has been a deepening awareness of the importance of applying moderate process technologies to derive products that are substantially more akin to their natural state while ensuring microbiological safety (Abdulstar, Altemimi, & Al-Hilphy, 2023).

Thermosonication (TS), also referred to as ultrasound-assisted heat treatment or acoustic heat treatment, is a technique in which acoustic energy and heat can interact simultaneously on cellular structures to facilitate the deactivation of pathogenic, spoilage microorganisms and endogenous enzymes, thereby maintaining the nutritional and organoleptic properties of fruits and vegetables (Abdulstar et al., 2023; Urango, Strieder, Silva, & Meireles, 2022). Microbiological loadings reflect the freshness of freshly cut fruit and vegetables, which is commonly used as a basic indicator of their shelf life. Rois and Deog-Hwan (2015) concluded that TS (40 °C, 3 min, 400 W/L) combined with electrolyzed water (5 mg/L) was more effective on the inactivation of pathogens and deteriorating microorganisms (>3 logs CFU/g), demonstrating the potential as a decontamination process in the fresh-cut industry. A study by Sun et al. (2022) had mentioned that no Escherichia coli or molds were detected in the thermosonication (TS) treatment groups. In comparison, Escherichia coli and molds in the fresh orange juice (the control group) were 4.17 log CFU/mL and 3.5 log CFU/mL, indicating that TS was able to inactivate Escherichia coli and molds effectively. After TS treatments, the TBC and yeasts decreased by 75.95% and 52.84% compared to the control group. The results showed that TS treatment could effectively reduce the microbial load, thereby extending the shelf life of the product.

Thermosonication, as a physical induction mechanism, utilizes ultrasound to generate cavitation bubbles that release a massive amount of energy upon collapse, which effectively disintegrates water molecules into highly reactive •OH radicals, thus inducing oxidative stress on plants. In addition, these free radicals combine with hydrogen atoms to form hydrogen peroxide, which exhibits potent oxidation properties and effectively eliminates bacteria in fruits and vegetables (Yeoh & Ali, 2017). Xin, Zhang, Yang, and Adhikari (2015) demonstrated that a TS protocol utilizing an ultrasonic intensity of 11.94 W/cm2 at 85 °C for 60 s was the optimal protocol for blanching argy wormwood leaves. This treatment inactivated 92.7% of POD activity while retaining 96.7% of the total chlorophyll. Pinheiro, Rui, Goncalves, and Silva (2019) found that TS treatment delayed color change and increased the total phenolic content of tomatoes during storage by up to 29% compared to untreated tomatoes, thereby improving overall tomato quality. Therefore, phenolic accumulation and changes in related enzyme activities during thermosonication played a conclusive role in the antioxidant properties and quality characteristics of the products. In the TS washing process, many parameters are decisively correlated with the ultrasonic intensity, especially the ultrasound frequency, which plays a crucial role in cavitation bubbles formation and their subsequent energy release, thereby affecting the quality of the ultrasonic products (Sango, Abela, McElhatton, & Valdramidis, 2014; Seydi, Harun, Gökçe, & Mehmet, 2019).

Different from the common mono-frequency probe-type and bath-type ultrasound devices, a new multi-frequency ultrasound device furnished with three ultrasonic generators was designed by our research group for TS washing in simultaneously or sequentially mono-, dual- and tri-frequency. In this study, fresh-cut lotus roots (FCLs) were selected as the sample to investigate the effects of TS at different frequencies on phenols and storage quality features. Specifically, the potential mechanisms of TS in the induction of total phenolic content and antioxidant capacity of FCLs were explored from the aspects of free phenols, bound phenols, and monomer phenols. Meanwhile, the storage quality and structural characteristics of lotus root slices treated with TS washing at different frequencies were evaluated. The specific results can also be generalized to the TS washing of other fruits and vegetables, providing valuable information for the design and further elaboration of ultrasound washing devices.