Food Science and TechnologyVolume 35, Issue 3 p. 58-61 FeaturesFree Access Managing microbes First published: 16 September 2021 https://doi.org/10.1002/fsat.3503_15.xAboutSectionsPDF ToolsExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Nick Meakin of Aqualution Systems discusses the management of microbial pathogens and spoilage organisms in vegetable, salad, herb and fruit processing using a new hypochlorous acid decontamination process. Background Globally, the production of fruits, vegetables, salads and herbs is increasing rapidly, driven by rising middle-class populations, an upsurge in disposable income, rapid urbanisation, changing consumer lifestyles and growing awareness of the importance of a healthy diet. The decontamination of prepared produce is a vital part of producing safe ready-to-eat and minimally processed food and is also designed to prevent cross contamination and improve shelf life. Whilst decontamination processes in isolation do not guarantee that produce is totally free from pathogens and foreign bodies, they are validated to reduce the residual loadings to below an infective dose level. A number of decontamination processes are approved by UK retailers and adopted globally by the supply chain, each with its pros and cons. The most common methods are based on immersion in agitated water, often with a biocidal processing aid. Risk assessments are typically undertaken on the factory and produce in question to determine the right decontamination protocol for maximum protection. Factors taken into consideration include ease of produce decontamination and the opportunity for pathogen and spoilage organism contamination as well as the risk posed to operators from handling biocidal chemicals. The most popular and standard decontamination processes use ‘chlorine-based’ systems, with the term ‘chlorine’ most typically used to describe the biocidal products generated from the acidification of sodium hypochlorite. Innovation within the retailer-approved range of decontamination processes was limited until chlorate residues on fruits and vegetables became an issue in 2014, when sodium chlorate was voluntarily withdrawn from the EU market as a plant protection product because the cost of compliance with a new EU regulation was never going to be recovered from the size of the potential market. As a result, the established chlorate maximum residue limits (MRL) were also withdrawn and a limit of 0.01mg/kg came into force by default, highlighting one of the biggest drawbacks of the conventional acidified sodium hypochlorite-based decontamination process, namely the consequential chlorate residues on the produce. The European Food Safety Authority (EFSA) has continued to evaluate levels of chlorate in drinking water and on foods and after years of consultation and evaluation, the European Commission published new regulations on chlorate residues in 2020, with the maximum allowed level of chlorate residue being specified for the majority of fruits and vegetables (Table 1). Table 1. Examples of specified chlorate maximum residue limits (MRL) for fruits and vegetables (Commission Regulation (EU) 2020/749) Product Chlorate MRL (mg/kg) Fruit Stone/pome/citrus/berries/grapes 0.05 Olives 0.7 Other edible peel e.g. figs 0.3 Inedible peel e.g. kiwi 0.3 Vegetables/salad/herbs Leafy veg e.g. spinach/herbs 0.7 Broccoli 0.4 Beans/Peas/Lentils with and without pods 0.35 Other roots and beets 0.15 Tomatoes 0.1 Aubergines 0.4 Inedible peel e.g. pumpkin 0.08 Edible peel e.g. cucumber 0.2 Chlorine disinfection in water-based systems However the ‘chlorine’ solution is generated, whether by dissolving chlorine gas in water, diluting liquid bleach (sodium hypochlorite), or dissolving powdered bleach (calcium hypochlorite), the chemistry is a function of the pH. The resulting solution will be a mixture of chlorine, hypochlorous acid and hypochlorite (Figure 1). Figure 1Open in figure viewerPowerPoint The effect of pH on chlorine species Hypochlorous acid (HOCl) is one of the most effective known biocides. This weak acid is the first chemical produced by the mammalian immune system to kill invasive organisms and fight infection and is highly effective at relatively low concentrations. HOCl does not dissociate in water whereas sodium hypochlorite does, using the OCL- ion as the biocidal active. Depending on the organic load present, HOCl is up to 100 times more effective as a biocide than the same concentration of OCl-; 1ppm of HOCl will kill as many microbes as 100ppm of OCL-. HOCl is also very fast acting compared to most biocidal actives, requiring by far the shortest contact times to achieve a 99.9999% kill of pathogens, with a contact time of just 0.55 seconds1 to kill E coli for example. Hypochlorous acid is able to kill all pathogens on the hierarchical scale of susceptibility (Figure 2) at relatively low concentrations. Spores can be inactivated at 150ppm with contact times of between one minute (Bacillus subtilis) and five minutes (Clostridium difficile). By comparison, a 1000ppm hypochlorite solution is required to kill Clostridium difficile spores in five minutes. Figure 2Open in figure viewerPowerPoint Susceptibility of different organisms to disinfectants Disadvantages of using sodium hypochlorite for decontamination For sodium hypochlorite-based produce decontamination, typically a solution of around 100ppm is used; this has a pH of approximately 7.8 and comprises 30ppm HOCl and 70ppm OCl-2. Citric acid is then added to further reduce the pH to usually 7.5, at which point the mixture contains 50ppm HOCl and 50ppm OCl-2. For many years this has delivered a robust and reproducible decontamination protocol for ready-to-eat and minimally processed produce. However, the loss of the established sodium chlorate MRL forced a re-assessment of the likelihood of biocide residues or disinfection by-products being created and remaining on the produce. Hypochlorous acid (HOCl) is one of the most effective known biocides. Contrary to common belief, sodium chlorate is not generated as a disinfection by-product in sodium hypochlorite-based disinfection systems. Sodium hypochlorite is not a very stable molecule and sodium chlorate is a degradation product. As the sodium hypochlorite decays, some oxygen gases off (explaining why a drum can expand in storage) and the sodium hypochlorite is converted into sodium chlorate and salt (sodium chloride). The kinetics of the reactions are complex but were investigated by Lister et al. in the 1950’s3, who identified the following reactions: 2 NaOCl → NaCI + NaC 1 O 2 NaOCI + NaC 1 O 2 → NaCl + NaCIO 3 NaOCI → NaCl + 1 2 O 2 Consequently, when making up the 100ppm hypochlorite solution with a two-three-month-old drum of hypochlorite, potentially 10% or more of the solution could be chlorate resulting in residues on food of up to 1-2mg/kg. Hypochlorous acid offers an alternative To address the problems associated with using hypochlorite for produce decontamination, Aqualution Systems, a science-led Scottish company spun out of a pharmaceutical company in 2009, has developed technology to deliver a pure hypochlorous acid solution for decontamination in healthcare and food processing. The new process, originally developed for Marks & Spencer and validated by Campden BRI, enables the well-established benefits of chlorine-based disinfection systems to be delivered and yet overcomes many of the residue and toxicity problems associated with sodium hypochlorite. All biocides used in the EU must comply with the requirements of the EU biocidal product regulations (EU528/2012) and the supplier must be able to provide an audit trail to demonstrate that the active(s) and/or precursors used in the formulation of the biocide have been sourced from a supplier on the article 95 list of approved substance suppliers for the correct product type. Aqualution's new hypochlorous acid decontamination systems are compliant with the above regulation, with Aqualution having submitted the now approved active substance dossiers authorising the use of hypochlorous acid for biocidal purposes in the EU (and UK post Brexit). The European Commission's SUSCLEAN (Sustainable Cleaning and Disinfection in Fresh-Cut Food Industries 2014) project has also validated the hypochlorous acid-based produce decontamination system and found it to be the only new technology evaluated that was both microbially effective and economically viable. The study also demonstrated that a test unit installed in a salad processing plant in Portugal, in addition to delivering on-site biocide production and reduction in water and energy consumption, also showed the hypochlorous acid to be less corrosive than acidified hypochlorite and as effective at 30% of the free chlorine concentration of an acidified hypochlorite wash4. Hypochlorous acid decontamination process design The Aqualution process (Figure 3) uses in-situ hypochlorous acid generators (Figure 4) to produce the biocide, which is automatically dosed into the flume water, where the produce is decontaminated via a combination of both jacuzzi agitation and hypochlorous acid treatment. Typically, 20-40ppm (measured as free chlorine) in the wash water is adequate. This can be further reduced once there is full confidence in the stability of the system controls; Aqualution is currently working with one partner to reduce hypochlorous acid in the flume to 10ppm. Full automated QC monitoring of the free chlorine and pH can be added to the system with remote access if required. Figure 3Open in figure viewerPowerPoint Diagram of the hypochlorous acid decontamination process Figure 4Open in figure viewerPowerPoint Aqualution hypochlorous generators Aqualution's hypochlorous acid systems are operational in a number of countries providing decontamination for a wide range of produce including beans, onions, cherries, raspberries, strawberries, broccoli, pak choi and herbs. The company is about to install its first system in the UK, which will be targeted at herb decontamination. The technology can be retro fitted to an existing decontamination system and is fully scalable. The largest system designed so far targeted a reduction in water consumption of >1m litres of water per day. Design of each bespoke installation is based on the existing factory, the produce, the water source and any pre-wash processes, to optimise product quality and safety, along with commercial benefit. For example, whilst raspberries require dry fogging with hypochlorous acid prior to packing to ensure virus decontamination, cherries need to be cooled rapidly in a hydrocooler after harvesting otherwise they may split. HOCl is pumped into the hydrocooler so that the cherries are decontaminated and cooled at the same time Recycling water through the jacuzzi flume systems (Figures 5 and 6) enhances jacuzzi action, physically removing more micro-organisms from the product into the water, where they are easier to kill as they are no longer protected by biofilms, allowing free chlorine levels to be further reduced. The use of a vortex flow multi-media filter with a sieve size of 0.4μ to filter the water coming out of the flume allows water to be recycled back into the flume to wash the next batch of product. Figure 5Open in figure viewerPowerPoint Beans in the jacuzzi flume Figure 6Open in figure viewerPowerPoint Baby leaf salad flume system Advantages of using hypochlorous acid to decontaminate fresh produce 1 Pathogen kill rate Validation and comparison trials have demonstrated that in most cases, hypochlorous decontamination systems deliver an improved decontamination of the product by comparison with the conventional hypochlorite process, reducing the risk of food poisoning and supporting shelf life. The recycling process converts the conventional static bath batch process into a continuous process and, in addition to the jacuzzi jet action, as water flows through the flume at 10,000 litres per hour, this further enhances the physical removal of microorganisms from the produce further reducing the concentration of chemical required for decontamination (Table 2). Table 2. The effect of flow rate on hypochlorous consumption and log reduction Flow Rate Free available chlorine consumption Log reduction (litres/h) (ppm) Static 37 1.1 1,500 18 1.5 10,000 3 2 2 Solving the chlorate problem A pure hypochlorous acid solution does not degrade to sodium chlorate and typically the level of chlorate (a co-formulant) at the point of manufacture is less than 1% of that found in a fresh batch of hypochlorite. Hypochlorous acid has the added advantage of on-site, on demand production of the solution, so that at the point of use it is always less than 24 hours old. At the recommended dosing rates, chlorate residue levels have consistently been demonstrated to be below the level of detection. This results in three immediate benefits. Firstly, there are no chlorate (or other disinfection by-product) issues. Secondly, unlike almost any other retailer-approved produce decontamination biocides, the lack of chlorate or disinfection by-products of concern permits the produce wash water to be recycled leading to significant reductions in water and energy consumption. Tirdly, because salt is the only precursor used to generate the hypochlorous acid, the free chlorine concentration can be significantly reduced as the 50% of the hypochlorite that is not converted to hypochlorous acid at pH 7.5 in an acidified hypochlorite system is not required. A trial was carried out with diced beans (typically worst case product for chlorate retention) washed with various concentrations of hypochlorous acid (measured as free chlorine) in order to force chlorate into the water and onto the product. At the recommended wash concentration of 20ppm, chlorate was below the level of detection and even at 160ppm in the flume, the chlorate level was below the MRL of 0.35mg/kg (Table 3). Table 3. Chlorate levels in wash water using different concentrations of hypochlorous acid Sample type Chlorate (mg/kg) Perchlorate (mg/kg) 160ppm wash water 0.840 < 0.01 55ppm wash water 0.220 < 0.01 22ppm wash water 0.060 < 0.01 Diced Beans (160ppm) 0.030 < 0.01 Diced Beans (55ppm) 0.014 < 0.01 Diced Beans (22ppm) ND < 0.01 Control Unwashed (Kenya) ND < 0.01 3 Continuous flow systems and reduced water usage Unlike sodium hypochlorite systems that require the water in the flume to be discarded several times a day due to hypochlorite consumption, decay and the risk of chlorate build up, the wash water may be recycled many times in the hypochlorous acid process because chlorates are not present at a level of concern. An additional benefit is that the produce wash process is transformed from a batch process to a continuous flow process, yielding productivity benefits in current installations of 25-30%. Aqualution's first installation in a vegetable packhouse in Kenya (Figure 7) has been operational since 2013 and consistently consumes 100,000 litres of water per week to process approximately four tonnes of produce per hour for 16 hours per day, six days per week. As the previous process consumed over 500,000 litres of water a week, the new installation has reduced water use and effluent generation by 400,000 litres a week. Across all Aqualution's global operations, water consumption has been reduced by an average of over 80%. Figure 7Open in figure viewerPowerPoint Aqualution system being built in Kenya (1,000 litre flume, 60kg beans, 2 min wash time) 4 Reduction in chemical use Existing decontamination systems typically acidify sodium hypochlorite to a pH of about 7.5, which converts approximately 50% of the sodium hypochlorite to hypochlorous acid to reduce the numbers of contaminating microorganisms on the product. The hypochlorous acid technology produces 99.6% pure hypochlorous acid allowing the free chlorine concentration to be halved. Pumping water through the system at 10,000 litres per hour to filter, recycle and re-dose it with hypochlorous acid further enhances the effectiveness of the jacuzzi flume and the biocide, permitting a further reduction in the free chlorine level while maintaining the desired log reductions. Trial results demonstrated that it was possible to achieve equivalent log reductions in microbial numbers to those obtained using hypochlorite at 90ppm at a significantly lower concentration of hypochlorous acid (29ppm)4. The reduction in chemicals also offers health benefits to employees. Because the process operates at lower concentrations of free chlorine and there is no hypochlorite in the flumes, the strong ‘bleach’ smell prevalent in the conventional hypochlorite process is eliminated. 5 Reduced electricity usage Because the water can be recycled in the hypochlorous acid process via the continuous flow system, there is a significant reduction in the volume of chilled water needed to operate the process. Once the water is cold, the system requires little power to keep it cooled as it does not pick up much heat flowing around the system (typically only 1°C). This can reduce electricity consumption for powering the chillers by about 85%. 6 Improved shelf life During the SUSCLEAN trial in Portugal, it was shown that washing with the lower concentration of HOCl had no impact on shelf life or rate of organism growth on the product4.User experience suggests improved shelf life of fresh produce. Conclusions Hypochlorous decontamination systems are fast becoming the gold-standard in produce decontamination across the world and are now approved protocols in the codes of practice of some of the leading UK supermarkets. Not only is the process proven to deliver effective decontamination from spoilage and pathogenic microbes, but it has additional benefits in terms of reducing use of water, chemicals and electricity, as well as extending shelf-life. Nick Meakin, CEO, Aqualution Systems Ltd, Duns, Scotland Email nick.meakin@aqualution.co.uk Web aqualution.co.uk/ References 1Albrich, J.M., Hurst, J.K. 1982. Oxidation inactivation of E. coli by hypochlorous acid. Federation of European Biochemical Societies Letters 144: 157- 161Google Scholar 2Bowman, G., Mealy, R. 2007. The fundamentals of chlorine chemistry and disinfection. Wisconsin State Laboratory of Hygiene Google Scholar 3Lister, M.W. 1956. Decomposition of sodium hypochclorite: the uncatalyzed reaction. Canadian Journal of Chemistry, https://doi.org/10.1139/v56-068Google Scholar 4Machada et al. 2016. Disinfection with neutral electrolyzed oxidizing water to reduce microbial load and to prevent biofilm regrowth in the processing of fresh-cut vegetables. University of Porto, Food and Bioproducts Processing; and Susclean WP3 and WP4 feedback presentation, Campden BRI, Dec 2014 Google Scholar Volume35, Issue3September 2021Pages 58-61 FiguresReferencesRelatedInformation