Control of pathogens.
The control of pathogens is generally reported in the concentration of ozone, C (in mg/L or ppm) per unit of contact time, T (in minutes), to reduce the proportion of colony forming units, CFU. The process is called a log reduction because the population reduction is logarithmic. A formula for expressing this is:
CT= K log (CFU start/CFU finish)
Where K is a constant for the food in question, the units of measurement used, and for a given temperature and humidity. For example, if 5 mg/L of ozone applied for one minute will reduce the CFU levels by a factor of 10, then 5 mg/L of ozone applied for an additional minute will reduce the remaining CFU levels by another factor of 10. In other words, the constant application of ozone will continuously reduce the CFU levels but will not eliminate them. Also, 5 mg/L of ozone applied for one minute has the same CFU killing effect as one mg/L of ozone applied for 5 minutes. The ozone treatment could be in air or wash water although the value of K will depend on whether the carrier medium is air or water.
Treatment in Air
Typical ozone concentrations reported for treating foods in an air environment, such as a cold storage room, are 2-7 ppm. Some researchers feel that bacteria may actually be stimulated in low ozone levels somewhere below 0.1 ppm. High ozone levels can damage fruit with black spotting, etc., and this concentration starts at 6-7 ppm for some fruits. Temperatures in the treatment areas are typically a few degrees above freezing. Treatment time for significant reduction of pathogen levels ranges from hours to days. Virus and bacteria are the most easily controlled pathogens. Fungus and its byproducts such as molds require higher concentrations of ozone for longer periods of time.
Ethylene and Ripening
Ripening of fruits and vegetables and the blooming of flowers will be delayed in an ozone atmosphere. This is because the ripening agent for all plant life is ethylene, a simple volatile organic compound (VOC) produced by all plant life, and the ethylene is oxidized to carbon dioxide and water when treated with sufficient ozone. After this reaction there will be less ethylene and less ozone. With less ethylene present, the ripening time is prolonged. Increases of about 50% are reported. The ozone concentration will be lower, however, near the fruit and vegetable surfaces emitting the ethylene and so the pathogen killing power will be less in those areas.
We recommend for general storage area monitoring that the ozone sensors be positioned at least 15 cm (6″) from fruits or vegetables with strong ethylene emissions because the ethylene can cause reduced and confusing readings.