Non-thermal technologies for the disinfection of food and risk assessment for public health
Authors: Μπίρμπα Αγγελική 
Published: August 2021

Fruits and vegetables are considered as part of a healthy diet and lifestyle. However, concerns have arisen regarding the microbiological safety of Ready To Eat (RTE) produces due to a number of foodborne outbreaks associated with pathogens. Although strict practices for controlling the safety of RTE produce have been implemented in the fresh produce industry, the current commercial operations rely on a wash treatment with water or with an antimicrobial agent as the only step for reducing microbial populations on fresh produce. However, washing with common sanitizers has been demonstrated to achieve no more than 1-2 log10 reduction in pathogen populations. Recently, much research effort has been put into development to provide multiple-hurdle techniques which enhance produce safety. Thus, non-thermal technologies for the inactivation of microorganisms are of increasing interest to the food industry for the control of spoilage and safety, thus for assuring public health.In this study, the effects of non-thermal disinfection processes, Near UV-Visible light (NUV-Vis), Continuous Ultraviolet Light (UV 254 nm), High Intensity Light Pulses (HILP), Ultrasound (US), as well as conventional sodium hypochlorite (SH) disinfection solutions were used. The effect of the above technologies was tested against bacteria (Escherichia coli, Staphylococcus aureus, Salmonella Enteritidis and Listeria innocua) and viruses (Human Adenovirus). More precisely, the bacteria that were used were: E. coli K12, E. coli NCTC 9001 (representative microorganisms for the Enterohaemorrhagic foodborne pathogen E. coli O157:H7), S. aureus NCTC 6571, L. innocua NCTC 11288 (as a surrogate microorganism for the common foodborne pathogen L. monocytogenes), S. Enteritidis NCTC 6676 and HAdV (indicator virus selected as a surrogate of HAV and norovirus). The main scope of this work was to study the efficacy of three light technologies on liquid suspensions. Then, the effect of UV, US, SH and combined technologies were evaluated on their efficiency to disinfect inoculated romaine lettuce, strawberries and cherry tomatoes. Furthermore, the effect of the above technologies on quality (color) and physicochemical characteristics of the RTE produces was evaluated. The physicochemical characteristics tested were Total Antioxidant Capacity (TAC), Total Phenolic Content (TPC) and Ascorbic Acid (AA) concentration.This study demonstrates that the use of alternative non-thermal technologies is effective for inactivation of microorganisms in fresh RTE foods and could be used as an alternative to traditional chlorine immersions. However, the effect of UV and US on quality and nutritional quality retention of RTE foods should be considered before its use as a disinfection technique.As far as non-thermal light technologies are concerned, HIPL treatment inactivated both E. coli and L. innocua more rapidly and effectively than either continuous UV-C or NUV-vis treatments. With HILP at a distance of 2.5 cm from the lamp, E. coli and L. innocua populations were reduced by 3.07 and 3.77 log10 CFU/mL respectively after a 5 sec treatment time, and were shown to be below the limit of detection (<0.22 log10 CFU/mL) following 30 sec exposure to HILP (106.2 J/cm2). Treatment of lettuce with UV reduced significantly the population of E. coli, S.aureus, S. Enteritidis and L. innocua by 1.75, 1.21, 1.39 and 1.27 log10 CFU/g, respectively. Furthermore, more than a 2- log10 CFU/g reduction of E. coli, S. Enteritidis and L.innocua was achieved with US. In strawberries, UV treatment reduced bacteria only by 1–1.4 log10 CFU/g. The maximum reductions of microorganisms, observed in strawberries after treatment with US, were 3.04, 2.52, 5.24 and 6.12 log10 CFU/g for E. coli, S. aureus, S. Enteritidis and L. innocua, respectively. Finally, cherry tomatoes exhibited the best results when treated with non-thermal technologies. For instance, 3.16, 2.62, 3.29, 3.16 log10 CFU/g for E. coli, S. aureus, S. Enteritidis and L. innocua, respectively, were achieved when US treatment was used. UV treatment resulted in 2.39, 2.05, 2.62, 2.56 log10 CFU/g reduction of the above microorganisms. The combined technologies of alternative followed by conventional disinfection technologies resulted in 2-3.50 log10 CFU/g reduction for lettuce and strawberries. However, cherry tomatoes exhibited greater reductions (3.28-4.78 log10 CFU/g reduction). Finally, 1-2 log10 CFU/g log reduction was achieved for lettuce and strawberries when RTE foods were immersed in NaOCl 200ppm solutions, and greater reductions (3-4 log10 CFU/g) were achieved for cherry tomatoes.It was observed that HAdV was inactivated faster when chlorine treatment was used. However, UV non thermal technology found to be more effective for disinfection of HAdV compared to US, achieving a log10 reduction of 2.13, 1.25 and 0.92 for lettuce, strawberry and cherry tomatoes respectively when UV treatment for 30 minutes was implemented, whereas, US treatment for the same treatment period achieved a log10 reduction of 0.85, 0.53 and 0.36 log10 respectively. The sequential use of US and UV was found to be more effective and less time consuming, than when the treatments were used alone, indicating the existence of an additive effect. Treatment with UV and US, for time periods (up to 30 min) did not significantly (p > 0.05) change the color of RTE foods tested. Moreover, it was indicated that no significant differences (p>0.05) were observed as far as TAC is concerned when conventional treatments at different treatment times were used. However, when alternative disinfection treatments were used, an increase in TAC concentration was obvious from the first minutes of treatment. TPC concentration remained constant or was slightly decreased when RTE foods were immersed in NaOCl solutions. However, TPC increased significantly (p<0.05) in all RTE foods when UV and US alternative disinfection technologies were used. The Vit.C content of RTE foods did not exhibit any significant changes during different treatments. However, Vit.C was slightly decreased (p<0.05) when treatments of more than 30 minutes for US, UV and combinations of UV+US occurred.Furthermore, a computerized model was proposed based on critical points which are important during the production of lettuce. More precisely the development of a Decision Support System (DSS) using the theory of Fuzzy Cognitive Maps (FCMs), in order to diagnose the importance of critical control points (concepts) for the food safety and hygiene during the production of salad vegetables (lettuce), was implemented. The methodology described, extracts the knowledge from experts with different scientific background and exploits their experience on the process of lettuce production. The results of this study show that the present software tool can be explored and problems that can arise during the food production chain can be prevented.Generally, it was noted that the effect of each disinfection method is dependent upon the treatment time tested and the type of food. Treatment with UV and US reduced the numbers of selected inoculated bacteria on lettuce, strawberries and cherry tomatoes, which could be good alternatives to other traditional and commonly used technologies such as chlorine and hydrogen peroxide solutions. These results suggest that UV and US might be promising, non-thermal and environmental friendly disinfection technologies for fresh RTE produce industry. Taking everything into consideration, disinfection technologies play an important role in commercial practice in order to prevent the survival of pathogens and lower the risk of contamination thus assuring public health. However, nutritional and quality properties are essential as they can provide a protective role against the development and progression of many diseases and must be considered for the selection of disinfection process parameters.