ORIGINAL PAPER Effect of Ultrasound Treatment Combined with Carbon Dots Coating on the Microbial and Physicochemical Quality of Fresh-Cut Cucumber Kai Fan 1,2 & Min Zhang 1,3 & Huizhi Chen 1 Received: 21 October 2019 /Accepted: 19 February 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020 Abstract Effect of ultrasound (US) treatment combined with carbon dots (CDs) coating on the microbial and physicochemical quality of fresh-cut cucumber was investigated. Cucumbers were dealt with ultrasound (226 W/cm 2 ), CDs coating with CDs concentration of 4.5%, and their combination and then packaged and stored for 15 days at 4 °C. Results exhibited that US treatment combined with CDs coating markedly inhibited total bacterial count to 5.18 log CFU g -1 , mold and yeast to 3.45 log CFU g -1 after 15 days of storage. US treatment combined with CDs coating also kept minimum respiration rate of 4.67 mg kg -1 h -1 CO 2 , weight loss of 8.54%, and malondialdehyde content of 2.24 μmol kg -1 and higher total soluble solids of 2.29 °Brix, firmness of 6.78 N, and ascorbic acid content of 0.0243 g kg -1 ; inhibited peroxidases activity to 139.83 U kg -1 s -1 and polyphenol oxidase activity to 137.17 U kg -1 s -1 ; preserved flavor and taste; and reduced the change of water status after 15 days of storage. These results illustrated that US treatment combined with CDs coating can effectively improve the microbial and physicochemical quality of fresh-cut cucumber. Keywords Ultrasound . Carbon dots coating . Quality . LF-NMR . Fresh-cut cucumber Introduction Fresh-cut products are in increasing demand, and the main focus is on the high quality and safety aspects of the products. Fresh-cut cucumber (Cucumis sativus L.) is welcomed in peo- ple’ s life due to rich nutrients such as vitamins, nicotinic acid, and mineral substance (Mohammadi et al. 2016). However, fresh-cut cucumber suffers from deterioration in quality dur- ing preservation (Wu et al., 2012; Meng et al., 2014). Fresh- cut cucumber has a short storage period owing to tissue dam- age, nutritional loss, and microbial infection in cutting and processing (An et al., 2006; Pinheiro et al., 2013). Thus, the fresh-keep for fresh-cut products is critical. Recently, the application of ultrasound (US) is increasing in food preservation. US is able to control the quality and microorganisms because of its cavitation effect (Sagong et al. 2011; Mothibe et al. 2011; Xin et al. 2013; Lagnika et al. 2013; Islam et al. 2014). Numerous researchers reported that US treatment is helpful to prolong the storage period of fresh products such as tomato (Pinheiro et al. 2015), kiwifruit (Vivek et al. 2016), and plum (Hashemi 2018). Yang et al. (2011) found that US and salicylic acid treatment delayed the decay and inhibited enzyme activity in peach fruit. Moreover, there were no obvious changes in quality. Combined treatment of US and oregano essential oil exhibited a synergetic effect on the inactivation of microorganisms (Millan-Sango et al. 2015). Gani et al. (2016) presented that US treatment exhibited low microbial load and good quality retention for strawberry, although several studies found that US treatment caused microbial inactivation and maintained the quality characteristic of fresh food. However, ultrasound treatment cannot completely control microbial and non- microbial spoilage at the storage period. Therefore, other pre- servative methods were needed to strengthen the preservative effect of US treatment. Chitosan (CH) as a safe coating material can form a mod- ified atmosphere to inhibit microorganisms and reduce * Min Zhang min@jiangnan.edu.cn 1 State Key Laboratory of Food Science and Technology, Jiangnan University, Wuxi 214122, Jiangsu Province, China 2 International Joint Laboratory on Food Safety, Jiangnan University, Wuxi, China 3 Jiangsu Province Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, Wuxi, China Food and Bioprocess Technology https://doi.org/10.1007/s11947-020-02424-x