Ultrasound and cold atmospheric plasma disinfection technologies for the production of safe and high quality fresh produce

Abstract

Non-thermal technologies have drawn the attention of both researchers and the food industry in recent years. Notable interest has been shown by the fresh produce industry in particular. The fresh fruit and vegetable industry is constantly looking for new processes in order to produce fresh produce which is safe with respect to the microbial levels and the retention their quality properties. In addition, consumers' concerns about the use of chemical products such as chlorine and its derivatives in the disinfection of fresh produce have highlighted the need for the development of new methods for fresh produce production. Therefore, non-thermal technologies, which have been reported to be effective in inactivating the microorganisms and are able to retain the organoleptic properties of the fresh produce can be applied by the fresh produce industry. Furthermore, these technologies have been also classified as environmentally friendly due to the fact that less energy and water consumption is required. Their application across different steps in the food chain (e.g., during processing and post-processing) can contribute to control better the production of safe fresh produce while resulting in less adverse environmental impacts. Ultrasound is a non-thermal technology able to inactivate microorganisms based on physical mechanisms. Moreover, the efficacy of this technology can be enhanced by its combination with another antimicrobial process. In this thesis It was found that the combination of ultrasound (20 kHz, 90 µ111, 200 W) and essential oils significantly increased the rates of reduction of Escherichia coli and Salmonella enterica inoculated on Romain lettuce leaves, especially when a critical concentration of 0.018% (v/v) of essential oil of thyme and essential oil of oregano was used. Additionally, the impact of ultrasound treatment on the lettuce leaves was assessed by SEM and histological cross sections, has shown no damage on the biological structure of the leaves. The efficacy of ultrasound was also evaluated in other fresh produce. E. coli and S. enterica inoculated on alfalfa and mung bean sprouts were treated by ultrasound and the combination of ultrasound with CI02. An additive effect in the reduction of Salmonella on inoculated alfalfa and mung bean sprouts for the combination of ultrasound and CI02 was found, resulting in microbial reductions of 1.93±0.42 and 2.06±0.23 log CFU/g, respectively. Application of the same treatments on E. coli inoculated alfalfa and mung bean sprouts also resulted in additive effects, i.e., 2.61±0.02 and 2.07±0.02 log CFU/g reductions, respectively. Similarly, an additive effect was observed in terms of total inactivation (accounting for both the presence of the bacteria on the surface of the sprouts and the treated water) both for Salmonella and E. coli in alfalfa (i.e., 1.56±0.40 and 1.90±0.33 log CFU/sample, respectively) and mung bean sprouts (i.e., 1. 78±0.10 and 2.05±0.02 log CFU/sample, respectively). Ultrasound requires the use of water as a mean to decontaminate fresh produce. Cold Atmospheric Plasma (CAP) is an alternative dry (i.e., it does not require the use of water) decontamination technology which is based on the generation of ionized air. This can inactivate bacteria through different mechanisms. However, bacteria have different repair and defend mechanisms. In the course of this thesis the main antimicrobial species generated by a CAP (operating at 230V, 50 Hz, carried gas air) were identified and the effect of these reactive species on the growth rate, µmax, of different E. coli mutants was determined. Furthermore, the effect of CAP on E. coli inoculated on butter head lettuce leaves was assessed and it was found that treatments of 60 and 90 s were able to reduce the bacteria by more than 1.5 log CFU/cm2 Safety of produce is as important as the safety of the water effluents produced during decontamination treatments. UV-C light has been widely applied for water disinfection. However, the presence of organic matter can decrease the antimicrobial efficacy of this process. In order to avoid this drawback, an ultrasound system, which has been reported to be effective in reducing conglomerates, was applied in combination with UV-C during the current research. Lettuce wash water treated by the combination of both systems resulted in the highest bacterial inactivation (i.e., 3.57±0.39 log CFU/ml), colour reduction (43.31 ±7.02%) and reduction of organic matter (29.86±5.39%) after 30 min of treatment. As a general conclusion, non-thermal technologies show a high potential for application in fresh produce decontamination in terms of their antimicrobial efficacy either in processing or post processing environments. These observations have been assessed by a range of microbial, structural and biochemical tests and support further investigations at industrial scale levels.