Ozone Food Rinse
Ozone in water is often described as an alternative to hypochlorite as a disinfectant or sanitizer, al- though they differ in many aspects (Table 1). Ozone solubility in water is low, its maximum solubility at 20°C(68°F) is 29.9 μg/ml; in practice, it is difficult to exceed 10 μg/ml, and many systems produce 5 μg/ml or less. Ozone in water above 1 μg/ml can liberate ozone into the air that exceeds safe levels (OSHA workplace maximum = 0.1 ppm). Significant advan- tages of ozone in water are that it decomposes quickly to oxygen, leaving no residues, and it has more potency against bacteria, cysts of protozoa, viruses, and fungal spores than hypochlorite (White 1992). Ozone was reported to have a mode of action to control a plant pathogen not based solely on its antimicrobial activity. Sarig et al (1996) reported ozone controlled Rhizopus stolonifer and induced resveratrol and pterostilbene phytoalexins in table grapes, and that these made the berries more resistant to subsequent infection. Ozone can oxidize many organic compounds, particularly those with phenolic rings or unsaturated bonds in their struc- ture (Razumovski and Zaikov 1984) and can have a role in reducing pesticide residues in process water (Nickols and Varas 1992) and mycotoxins in durable commodities (McKenzie, et al 1997).
Some packinghouse processes where ozone in water could be applied include:
1. Ozonation to santitize packing line process water. The water in tanks where fresh fruit are dumped or floated before cleaning, sorting, and packing operations is an important site for the accumulation of pathogens that infect fruit later in storage, shipping, or marketing. Examples are blue mold of apples and pears, caused by Penicillium expansum, and green mold of citrus, caused by Penicillium digitatum. Therefore, disinfection of this water is important, and usually is accomplished with hypochlorite. Ozone has been employed in flume water in apple and pear packinghouses, and some facilities have ozonated hydrocooler water. Pre-conditioning of the water (to reduce particulates, BOD, turbidity, etc.) before ozonation is needed in systems where water is recycled, and this can be difficult and expensive. A contact time of two minutes in 1.5 μg/ml ozone killed 95-100% of all eight fungi tested, and none survived 3 minutes of contact (Figure 1). Spores of these pathogens die quickly in ozonated water, but fruit, soil, and other debris in the water can reduce the ozone concentration completely or to ineffective low levels.
|Microbial potency||Kills plant pathogens and microbial saprophytes effectively. Some human- pathogenic, spore-forming protozoa resistant. Maximum allowable rates under regulatory control||Kills plant pathogens and microbial saprophytes effectively, including spore-forming protozoa. Maximum rate limited by ozone solubility, difficult to exceed about 10 μg/ml|
|Cost||Chemical cost low. Repeated delivery required, sometimes pH and concentration controller systems needed, minor maintenance and energy costs, chlorine storage issues||Variable: no chemical cost, but high initial capital cost for generator, usually needs filtration system when water re-used some are complex, modest maintenance and energy costs|
|Influence of pH||Efficacy diminishes as pH increases, above pH 8, pH adjustment may be needed. Chlorine gas released at very low pH (4 or less)||Potency not influenced very much by pH, but ozone decomposition increases at high pH|
|Disinfection by- products||Some regulatory concern, tri-halo compounds, particularly chloroform, of some human safety concern||Less regulatory concern, small increase in aldehydes, ketones, alcohols, and carboxylic acids created from organics, BrO3- from OBr-|
|Worker safety issues||Chloroamines can form and produce an irritating vapor, chlorine gas systems require on-site safety measures, OSHA (TWA) limit for chlorine gas: 1 μg/ml||Off-gas ozone from solutions an irritant and must be managed. MnO2 ozone destruction efficient and long-lived. OSHA (TWA) limit for ozone gas: 0.1 μg/ml|
|Persistence in water||Persists hours in clean water, reduced persistence to minutes in dirty water||Persists minutes in clean water, reduced persistence to seconds in dirty water|
|Use rates||Limited by regulation to 25 to 600 μg/ml, depending on application||Not limited by regulation, but Henry’s law limits theoretical maximum ozone in water to about 30 μg/ml at 20°C (68°F). Most ozone systems produce 5 μg/ml or less.|
|Use in warm water||Increases potency, some increase in vapors||Not practical, rapidly accelerates ozone decomposition, increases off- gassing, decreases ozone solubility|
|Influence on product quality||Little risk of injury at recommended rates. Some injury possible above 50 μg/ml on tree fruits. Off-flavors on some products at high rates||In brief water applications, risk of product injury low. Stem, calyx, and leaf tissue more sensitive than fruits. Risk of injury needs more evaluation.|
|Impact on water quality||Minor negative impact: water salt concentration increases somewhat, may interfere with fermentation used to reduce Biological Oxygen Demand, some pesticides inactivated, discharge water dechlorination may be required.||Mostly positive impact: does not increase salt in water, many pesticides decomposed, Biological/Chemical Oxygen Demand may be reduced, flocculation and biodegradability of many organic compounds enhanced, precipitates iron, removes color, odors|
|Corrosiveness||High, particularly iron and mild steel damaged||Higher, particularly rubber, some plastics, yellow metals, aluminum, iron, zinc, and mild steel corroded|