Effects of cold plasma treatments on spot-inoculated Escherichia coli O157:H7 and quality of baby kale (Brassica oleracea) leaves

Abstract

Abstract The efficacy of non-equilibrium atmospheric pressure-pulsed dielectric barrier discharge (cold plasma)-treated mist in disinfecting baby kale leaves, and its effect on color values and cuticle composition were evaluated. UV-treated baby kale leaves were spot-inoculated (0.04-mL of 106 CFU/mL) with rifampicin-resistant strain E. coli O157:H7 ATCC 700728 incubated (0–4 °C) overnight and treated with plasma mist at 60, 120, 180, 240 or 300 s. Treated and untreated leaves were analyzed for change in color values (L*, a*, b*, chroma, hue, and browning index (BI)) using a Chroma meter. Functional groups (alcohols, esters, aldehydes, and ketones) were determined in the cuticle using FTIR. Color stability of treated leaves was also evaluated after refrigerated storage (4 °C) for 12 days. Levels of E. coli O157:H7 on the kale leaves were reduced below the detection limit of 5.5 × 103 CFU/mL after plasma treatment for 300 s with no significant change in color values. Visible change in color (browning or leaf damage) was observed after 600 s of plasma treatment. Color stability of plasma-treated leaves was enhanced during refrigerated storage (4 °C), indicated by a lower BI value of 34.4 ± 5.4 at 120-s plasma treatment compared to untreated leaves after 12 days of storage. Fourier transform infrared (FTIR) spectroscopy analysis concluded that plasma treatment of kale did not negatively affect functional groups in the cuticle. This study demonstrated that cold plasma mist has the potential to reduce E. coli on the surface of baby kale leaves with no significant change in the color values. Industrial relevance Kale leaves are consumed worldwide owing to their health-promoting and nutritional benefits. However, in the past two decades there has been an increase in foodborne diseases outbreaks caused by microorganisms associated with leafy vegetables. The food industry faces challenges in improving food safety while maintaining quality of the food. Chemicals in wash water or heat treatment (70–121 °C), commonly used methods for killing microorganisms, can have negative effect on the aesthetic (color) and nutritional quality of the product. Cold plasma treatment of fresh produce, an emerging food preservation technology, is an efficient, quick, and environmentally-friendly method of disinfecting the product at low temperature (30–40 °C). This study demonstrated that treatment of baby kale with cold plasma mist not only inactivated E. coli, but also enhanced its color stability during refrigerated storage. Hence, cold plasma mist has a potential application as an efficient disinfection technology, thus enhancing shelf-life of fresh leafy greens.