Basics of Ozone Applications for Postharvest Treatment of Vegetables

Basics of Ozone Applications for Postharvest Treatment of Vegetables

T.V Suslow, PhD, Department of VegetableCrops, UC Davis

In the search for effective disinfectant treatments for fresh vegetables and fruits, the postharvest handling industry often operates within areas of regulatory uncertainty. Some produce handlers and processors use ozone for water sanitation, cold room air treatment, and other postharvest applications. For applications to whole or peeled produce, handlers and processors are relying on the self-determination that ozone has achieved Generally Recognized As Safe (GRAS) status as a food processing aide. Recent expert advisory panel recommendations have made this determin- ation, but to date, the U.S. Food and Drug Administration (FDA) has not released an official determination on these materials. Unlike chlorine gas, calcium hypochlorite, and sodium hypochlorite, no postharvest uses of ozone in contact with produce are currently registered by the U.S. Environmental Protection Agency (EPA) or California Department of Pesticide Registration (DPR). 

Ozone(O3).  
Ozone is a strong, naturally-occurring oxidizing agent with a long history of safe use in disinfection of municipal water, process water, bottled drinking water, and swimming pools. More recent applications include treatment of wastewater, dairy and swine effluent, cooling towers, hospital water systems and equipment, aquariums and aquaculture, water theme parks, and public and in-home spas.  

In clean, potable water free of organic debris and soil particulates, ozone is a highly effective sanitizer at concentrations of 0.5 to 2 ppm. Ozone is almost insoluble in water (0.00003g/100mL at 20oC [68oF] and effective dispersal is essential for antimicrobial activity. Ozone's disinfectant activity is unaffected at a water pH from 6 to 8.5. Ozone is highly corrosive to equipment and lethal to humans with prolonged exposure at concentrations above 4 ppm. Ozone is readily detectable by human smell at 0.01 to 0.04 ppm. OSHA limits of exposure specify a 0.1ppm threshold for continuous exposure during an 8hr period and 0.3ppm for a 15 minute period. At 1 ppm ozone has a pungent disagreeable odor and is irritating to eyes and throat. The need for off-gas containment in an open process line would have to be carefully evaluated for each planned use but current experience would not forecast a serious problem for line workers. Effective but safe concentrations are difficult to maintain in process water, because automated detection systems have not been highly reliable. Past research is often difficult to evaluate and reproduce due to uncertainty of reported concentrations of delivered ozone in the experimental or commercial system. Newer electrode probes that measure oxidation reduction potential (ORP) of the water or colorimetric kits are being used to monitor ozone concentrations more accurately but problems in practical application still exist. Ozone is also highly unstable in water and decomposes to oxygen in a very short time (less than half the activity remains after 20 minutes). In process water with suspended soil and organic matter, the half-life of ozone activity may be less than one minute. Lower water temperatures extend the half-life of ozone. Maintain- ing effective concentrations for microbial disinfection by using remote ozone generation and injection into a centralized water system, as is done with chlorine and chlorine dioxide, has proved difficult. With increased practical use in postharvest handling of fresh vegetables and fruits these obstacles will very likely be overcome. In some appli- cations, a reduced (lower than if used as the sole oxidizing agent) amount of hypochlorite or other more stable disinfectant is added to water to provide a residual effect downstream of the primary ozone injection. 

How is ozone formed? 

Ozone is formed by a high energy input splitting the O2 (oxygen) molecule. Single O rapidly combines with available O2 to form the very reactive O3. 

In nature, ozone is formed by UV irradiation (185nm)from the sun and during lightning discharge. Commercially, UV-based generators pass ambient air (20% O2) across an UV light source, typically less than 210nm. These systems have a lower cost but also have a more limited output than corona discharge systems. Corona discharge generators pass dry O2 enriched air across a high electric voltage (>5,000 V) or corona; similar to a spark plug. Excess O3 not dispersed in water must be captured and destroyed to prevent corrosion and personal injury. On method of destruction is by UV light at a longer wavelength, 254nm, combined with the use of a catalytic agent. 

How does water quality impact effectiveness?  

Dissolved and suspended organic and inorganic substances react quickly with ozone and interfer with a desired antimicrobial action. Similar to chlorine, water quality has an important impact on "ozone demand" and stability in water. In particular, dissolved iron, manganese, copper, nickel, hydrogen sulfide, and ammonia will increase the concentration and contact time needed for maximum lethality to microorganisms. Complexes of suspended organics and inorganics are believed to provide a protective effect for microbes against the action of ozone. High suspended solids (or insufficient contact time in flumes or drench tanks) are often cited as the responsible factor for lower than expected reductions in viable microbial counts from treated water, often no more than a 10 fold (1 log) decrease. In filtered systems, a 3-4 log reduction may be expected. 

Well water will generally have lower organic and higher inorganic loads than surface water. Recirculating process water will have a higher microbial load, higher suspended organic solids, and, potentially, pesticide residues and other organic chemicals. Because of ozone's reactivity with organics, its use may actually assist filtration devices in clarifying recirculating process or cooling water. 

How is ozone applied to water? 

The ozone generator supply line interfaces with the process water supply or return line at a Venturi-type injection dispersor unit. Adequate mixing and sensitive process monitoring are essential for uniform treatment with the low concentrations applied to water for postharvest uses. 

How does Ozone compare to Chlorine? 

Ozone is reported to have 1.5 times the oxidizing potential of chlorine and 3,000 times the potential of hypochlorous acid (HOCl). Contact times for antimicrobial action are typically 4-5 times less than chlorine. Ozone rapidly attacks bacterial cell walls and is more effective against the thick-walled spores of plant pathogens and animal parasites than chlorine, at practical and safe concentrations. 

In comparison to the potential negative effects of residues and organic reaction products formed with chlorine applications, ozone has no reported deleterious by-products. Oxidized products and oxygen are the outcome of ozone action. 

Various estimates are available, but discounting capital costs for generators and dispersion system equipment, the relative costs are approximately as follows; 
 

Cl2

- $0.35 / lb

NaOCl

- $1.14 / lb

Ca(OCl)2

- $1.14 / lb granular

 

- $1.29 / lb tablet

O3

- $0.48 / lb

  
Total operating costs are reported to be $1,000 per lb. of O3. Ozone generation requires approximately 5 times more energy input than Cl2 but 25 times less than ClO2

Has ozone been tried for other postharvest uses?  

Ozone has been evaluated for postharvest disease control and other storage uses for many years. Some commercial use has occurred with a few commodities such as apples, cherries, carrots, onions, and potatoes. There is increasing interest and empirical activity in the evaluation of ozone for a diversity of water treatment and air treatment uses in postharvest quality management. Examples include ethylene degradation (within a confined reactor), odor elimination for mixed storage, disinfection of humidification systems (including retail super markets), fungal spore elimination in storage room aerosols, and treatment of superficial mold after long-distance shipping of onions. Both effective disease control and phyotoxicity of ozonated air on certain varieties was reported for table grapes and carrots (bleaching). Ozone treatment has been reported to induce natural plant defense response compounds thought to be involved in postharvest decay resistance.  

Additional research is needed to define the potential and limits of effective use of ozone for postharvest treatment of whole and minimally-processed vegetables and fruits.  

Currently, ozone is not registered by the California DPR as a postharvest treatment for direct contact with produce. The recent expert panel recommendation to the FDA supporting GRAS classification of ozone as a disinfectant for foods has opened the door for the produce industry to establish independent affirmation of safety when applied in a manner consistent with good manufacturing practices. When using ozone for produce contact, a copy of the public disclosure of the expert panel (Graham 1997) must be available on-site for an inspector requesting the authority upon which GRAS classification is presumed. 

The information contained within this bulletin should not be viewed as an authoritative source for current registration status or legal use recommendations of any product. For more information contact the California Department of Pesticide Registration Information Center at (916) 324-0399 


Relative Antimicrobial Disinfection Efficiency 

OCl < HOCl < ClO2 < O3 

Efficacy Impacted By pH 


HOCl
Strong

O3
Moderate

ClO2 
Low 

 

 Relative Corrosive Potential 

ClO2< HOCl < Cl2 < O3 


Recent Related or Background Articles  
 

    • Barth, M M; Zhou, C; Mercier, J; Payne, F A. 1995.Ozone storage effects on anthocyanin content and fungal growth in blackberries. Journal of Food Science 60:1286-1288. 
    • Beltran, F J; Encinar, J M; Gonzalez, J F. 1997 
    • Industrial wastewater advanced oxidation: Part 2. Ozone combined with hydrogen peroxide or UV radiation.Water Research, 31: 2415-2428. 
    • Botzenhart, K; Tarcson, G M; Ostruschka, M. 1993. Inactivation of bacteria and coliphages by ozone and chlorine dioxide in a continuous flow reactor. Water Science and Technology 27:363-370. 
    • Bullock, G L; Summerfelt, S T; Noble, A C; Weber, A L; Durant, M D; Hankins, J A.1997. Ozonation of a recirculating rainbow trout culture system: I. Effects on bacterial gill disease and heterotrophic bacteria. Aquaculture 158: 43-55. 
    • Graham, D. M. 1997. Use of ozone for food-processing. Food Technology. 51: 72-75 
    • Hunt, N K; Marinas, B J. 1997. Kinetics of Escherichia coli inactivation with ozone. Water Research 31:1355-1362. 
    • Joret, J C; Mennecart, V; Robert, C; Compagnon, B; Cervantes, P. 1997. Inactivation of indigenous bacteria in water by ozone and chlorine. Water Science and Technology 35: 8 
    • Liew, C L; Prange, R K. 1994. Effect of ozone and storage temperature on postharvest diseases and physiology of carrots (Daucus carota L.). Journal of the American Society for Horticultural Science 119: 563-567. 
    • Richardson, S.D., et.al..1998.Chemical by-products of chlorine and alternative disinfectants. Food Technology. 52:58-61 
    • Sarig, P; Zahavi, T; Zutkhi, Y; Yannai, S; Lisker, N; Ben-Arie, R. 1996. Ozone for control of post-harvest decay of table grapes caused by Rhizopus stolonifer.Physiological and Molecular Plant Pathology 48: 403-415. 
    • Watkins, B D; Hengemuehle, S M; Person, H L; Yokoyama, M T; Masten, S J.1997. Ozonation of swine manure wastes to control odors and reduce the concentrations of pathogens and toxic fermentation metabolites. Ozone Science & Engineering 19: 425-437. 

Partial Listing of Ozone Generator Providers: 

    • AgTech International 760.480.4488 
    • Cyclopss 801.972.9090 
    • Del Industries 800.676.1335 
    • Novazone 510.454.0303 
    • Oxion,Inc. 800.552.0617  
    • See also Postharvest Chlorination Basics, DANR Publication #8003