Ozone: "Develop an integrated system to allow it to provide a benefit"

We are riding another wave of keen interest in the potential for ozone-treated water to supplement or wholly substitute for current antimicrobials added to postharvest wash and cooling water. Similarly, gaseous ozone and ozone-fogging applications are triggering cautious interest for surface sanitization in pre-coolers and cold storage. The attraction to drop other chemistries, predominantly various chlorine-based formulations, in favor of ozone is clear:- Ozone is a powerful oxidizing agent- Ozone is FDA listed as Generally Recognized As Safe (GRAS)- Ozone is allowed as an “ingredient” under the USDA National Organic Program- Ozone lethality to viruses and parasites in contaminated water far exceeds chlorination- Ozone treatment enhances water reuse systems by micro-flocculation of suspended particulates- Ozone has been shown to degrade pesticide residues in reuse water, recirculated produce wash water systems, and on produce surfaces (in controlled lab studies)- Ozone creates negligible disinfection by-products (identified concern is if water is brominated as well)- Ozone breaks down to atmospheric oxygen Postharvest water ozonation, in particular, in both raw fresh and minimally-processed produce sector has increased over the past ten years. Ozone generation and delivery device suppliers (ozone generators) cite recognized safe and effective use in water treatment since the early 1800’s, with levels as low as 1 ppm (treatment efficacy at this dose limited to highly filtered systems). As an antimicrobial oxidizer, Ozone > Peroxyacetic Acid > Hydrogen Peroxide > Hypochlorous Acid > Chlorine Dioxide. However, decades of promise from bench-top studies and volumes of peer-reviewed papers have not resulted in broad and effective application of water ozonation in fresh produce packing as the sole antimicrobial additive to a postharvest packing process. Along with the impressive list of beneficial traits, there are equally apparent limitations to both effective and safe applications. Upfront, I want to share that I have conducted many lab, pilot-scale, and on-site tests with various ozone-based systems for over 25 years, most recently within the past six months. In my experience, the most straightforward and beneficial use of ozone in postharvest handling and packing is as a terminal rinse step and as the post-ultrafiltration treatment of re-circulated water in postharvest wash and fluming systems. Another commonly cited beneficial application is in cold storage or forced-air treatment with gaseous ozone or room fogging. The most cost effective applications to room ozonation, in my experience, are for bulk-stored product packed to order rather than pre-packed and palletized cartons. In long-term cold storage, whole system design including bin stacking, sensor deployment, and detailed airflow mapping to minimize dose gradients are critical for beneficial outcome within a lot and to prevent ozone injury to the product, especially during storage and distribution. Additionally, the cost of facility and equipment conversion or design to ozone-compatible materials and components must be considered. A tree fruit grower/shipper recently asked me, “Why can’t we make ozone work in our pack-line?” My simple answer was that you likely could if you develop an integrated system to allow it to provide a benefit. Don’t expect a ‘silver bullet’ outcome to microbial control objectives with ozone; don’t rely on a simple plug-and-play marketing scheme to work by merely installing an ozone generator and injection point. You have to define your expectations for where and how your operation will realize a value to product quality and environmental management. A key issue here is that the majority of peer-reviewed journal papers extolling the promise of both gaseous, fogging, and aqueous ozone treatment for quality, decay control, and food safety fail to provide a true practical context for efficacy expectations to the end-user. Without getting too deep into the weeds of technical issues and experimental methods, the microbial challenges using lab-grown cells are too likely to over-predict lethality in a commercial context. In the absence of a demonstrated performance in lethality to naturally occurring and environmentally adapted index microbes, expectation for claimed 99.99 or 99.999% kill of some target-inoculated pathogen is highly suspect. Similarly, model systems, which report outstanding pathogen kill potential, often have incompatible parameters for dose and product exposure duration or uniformity of contact which is highly unlikely to be matched in high-throughput handling systems. One of the common pitfalls is matching the ozone Ct exposure (Concentration x Time) curves for phytotoxicity (product injury) to microbial disinfection (log kill) of the naturally present index microbes mentioned above. Some commodities have good ozone exposure tolerance but our experience has been that a number of inherent product traits or mixed tissue type (such as tolerant fruit but highly sensitive green calyx or tender “caps-stem”) and the preharvest factors influencing susceptibility to injury fall well below the threshold for beneficial levels of pathogen control, whether postharvest decay spores or foodborne human pathogens. Most recently, I have had the opportunity to observe systems in a few locations with recent installations of ozonated wash-rinse systems for fruit handling, as either a low-pressure spray or waterfall application to maximize retention of dissolved ozone and minimize worker discomfort or injury from excessive off gassing. From some preliminary tests, the greatest benefit is realized for in-shift control of microbial build-up on produce contact and immediately adjacent non-contact surfaces. Some minor reduction of microbial levels, above the action of mechanical removal of soil and adhering particulates alone, is likely but highly dependent on product surface traits and lot-specific characteristics. Despite my tempering the enthusiasm I encounter among ozone disciples, I remain optimistic that as operations evaluate their postharvest quality and food safety goals and apply a full systems approach, integrating ozone will result in practical benefits for some commodities. By doing this and assessing applications in a scientifically valid but practical approach, I feel the best chance to extract the antimicrobial and oxidative benefits of ozone applications may be achieved.