Ozone Postharvest Application Tips

Ozone has been used for many postharvest applications such as cold room air treatment and even water sanitation. But, what about using ozone for disinfecting fresh fruits and vegetables? Well, the subject of ozone processing has been a favorite topic of fruit and vegetable processors as well as shippers for over two decades. Read on to find out more.

Since ozone has achieved the status of GRAS - Generally Recognized as Safe, some processors and produce handlers are considering its application for both whole and peeled produce. To date though, the FDA still hasn't given their stamp of approval.

Neither the EPA - US Environmental Protection Agency, or the DPR - California Department of Pesticide Registration show any registered postharvest use of ozone. Other conventional treatments like sodium hypochlorite, chlorine gas, and calcium hypochlorite are recorded.

The EPA Office of Pesticide Programs has put out a very informative summary called Label Review Manual which goes over what a pesticide is, classifications, precautions, and directions for use.

How Ozone Works in Water Treatment

Ozone is a natural and robust disinfectant. For years, it has been used safely in water treatment for:

bottled drinking water
water processing
municipal water
swimming pools

More recently, this naturally-occurring oxidizing agent is being used for wastewater treatment and has been used in:

hospital equipment and water systems
dairy and swine effluent
cooling towers
aquariums and water theme parks
spas - both commercial and in-home

2. At What Levels Are Ozone Sanitation Effective?

Ozone sanitizes clean potable water very effectively. When water is safe to drink, use in food preparation, along with being free of soil particulates and organic debris - ozone works well.

Here are a few more facts about ozone:

At 0.00003g/100mL at 20 degrees C [68 degrees F], it's almost insoluble in water
The concentrations should range from 0.5 to 2 ppm.
Ozone is almost insoluble in water (0.00003g/100mL
For antimicrobial activity, there must be effective dispersal
When water pH is 6 to 8.5 ozone's disinfectant activity is unaffected

3. Ozone Cautions and Concentrations

Here are precautions to keep in mind

a prolonged concentration of above 4 ppm is lethal to humans
it can also damage equipment because it is highly corrosive
levels of 0.01 to 0.04 ppm is detectable by smell
OSHA has limits for continuous exposure during 8-hours at 0.1ppm
The OSHA limit for a 15 minute period is 0.3ppm
At 1 ppm levels, it smells, and it can irritate the throat and eyes.

For open process lines, there may or may not be a need for gas-off containment. Each situation would need to be evaluated. Currently, it is difficult to maintain safe and effective concentration levels because of the unreliability of automatic detection systems.

4. Historical Results Are Difficult to Evaluate

It's often difficult to evaluate past ozone studies because it is hard to reproduce the previous results. That is most likely because of variations in the ozone delivery system, whether it was commercial or experimental, and the concentration reported.

There is new technology like electrode probes and colorimetric analysis kits that more accurately monitor ozone concentration. Although these methods can measure (ORP) oxidation-reduction potential more accurately, they still are not foolproof.

5. Where Does Ozone Come From?

Ozone is formed in nature when lightning interacts with the UV irradiation of the sun. It can also be produced commercially with UV generators. These machines pass ambient air across a UV light source. There are different types of ozone machines with varying costs. For example:

UV-based generators move ambient air (typically 78% nitrogen and 21% oxygen) across a UV light source that is usually less than 210nm. This type of system costs less than a Corona discharge, but it also has less output.
Corona discharge generators are mainly used by industrial and commercial applications. There are unlimited variations of this type of ozone generator. It works like a spark plug. When dry oxygen (O2) passes over high voltage of more than 5,000 volts, ozone (O3) results. To prevent personal injury or corrosion, any excess ozone must be gathered and destroyed.

6. How Water Quality Impacts Ozone Effectiveness

The antimicrobial action of ozone works best in clean water. When water has suspended inorganic and organic substances, these particles react with ozone, and the desired antimicrobial result does not occur.

The quality of the water being used has a critical impact on the ozone's stability in water and ozone demand. Here are a few examples of dissolved minerals that will interfere with the production of ozone:

nickel
copper
manganese
iron

Additionally, if there is ammonia or hydrogen sulfide in the water, the process will take longer and be less effective.

If the drench tanks or flumes contain high suspended solids, there can be less than the expected reduction of microbials.

7. Which is More Effective - Ozone or Chlorine?

Voeks, Inc., believes ozone is the clear winner here with one-and-a-half times more oxidizing potential than chlorine. Plus, it works four-to-five times faster. Compared to chlorine, ozone is more effective on plant pathogens and bacterial cell walls.

Although ozone uses about five-times more energy than chlorine gas, it still uses about twenty-five times less than chlorine dioxide.

8. More Postharvest Uses For Ozone

For years, the efficacy of ozone on disease control postharvest has been a hot topic.

A few commercial growers have had success on the following crops:

potatoes
onions
cherries
carrots
apples

The modern farmer is showing more interest in using ozone for both air and water treatment. Some of the current uses are:

odor elimination
humidification disinfection
storage room fungal spore removal
onion superficial mold treatment

Certain plants such as carrots and grapes experienced better disease control. There have also been instances of plants natural defenses improving, and them becoming more disease resistance

Although more study is needed, the potential of ozone's disinfectant properties for fruit and vegetable crops postharvest looks bright.