Joan M. King and Terry Walker
Ozone is a substance best known in two divergent ways. It is both beneficial— as in the ozone layer protecting the Earth from the sun’s harmful ultraviolet rays—or detrimental when ground-level concentrations become excessive, particularly on hot, humid days.
Because ozone is one of the strongest oxidants known, it can be used to irreversibly degrade harmful microbes and toxic compounds. Ozone normally exists as a gas, which is more soluble in cold water than warm water. Ozone contains three atoms of oxygen whereas, in the air we breathe, oxygen contains only two atoms. Ozone is more reactive because of the extra oxygen atom and can donate an oxygen atom to other substances to oxidize them, leaving the remaining two oxygen atoms to form regular oxygen found in air. This fact is important because there is no residue left from the ozone itself.
Ozone can be produced commercially either through electrolysis or corona discharge. In corona discharge, air or pure oxygen is fed into a unit that converts the oxygen to ozone using high voltage. This procedure has disadvantages that include high capital and operating costs, generation of toxic molecules containing both nitrogen and oxygen if air is used, possible toxic contamination by the electrode material and, most important, low ozone concentrations of 2.5 to 7.5 weight percent.
The second method involves the use of water in an electrolytic cell. Oxygen in the water is converted to ozone by passage through positively charged and negatively charged surfaces. Concentrations of ozone in this method can exceed 20 weight percent. Municipal water can be used, and the reactants and products are safe. Only water and oxygen are left. This process is less costly than the corona discharge process because it uses less electricity and no special gases must be purchased. A commercial process for producing ozone with an electrolytic cell has been patented by Lynntech Inc. of College Station, Texas.
Although ozone has been produced artificially since the beginning of the 20th century and used in Europe for several decades, the Food and Drug Administration only recently approved (1997) ozonation for use in the U.S. food processing industry.
Ozone is becoming a widely used replacement for chlorine-based chemicals for water quality and for sanitation in food processing, especially in the meat industry. Studies have shown that ozone is a viable alternative to chlorine for bactericidal effects. For example, poultry carcasses chilled with water containing 3.0 to 4.5 parts per million ozone scored lower in microbial counts than those chilled in non-ozonated water. There was no significant color change or off flavor in the ozone-treated product compared with controls.
Recently, ozone has been used in aquaculture for control of bacteria, to disinfect and for water quality. Ozone generators are used in the United States, Russia and Japan to make ozonated processing water for cleaning fish. Ozonation has shown potential gains in catfish shelf life, ice quality production and more efficient operation of water chillers in fish processing plants.
At the LSU Agricultural Center, several studies have been initiated using ozonation. One involves degradation of off-flavor compounds in catfish fillets. Another is the decontamination of aflatoxin in corn grain samples. To help with these two research projects, an ozonation processing unit is being built in the Department of Biological and Agricultural Engineering in a joint project with the Department of Food Science. Figure 1 shows how this unit works. It involves the use of water in an electrolytic cell. A series of tubes and valves routes the generated ozone to the food product and to ozone detectors for determining inlet and effluent ozone concentrations. For catfish fillets, the food will be in the treatment tank on a series of trays. Corn contaminated with aflatoxin will be ozonated in bins.
The unit built for contacting the food product such as catfish fillets and contaminated grains will be capable of treating the products with either gaseous or dissolved ozone. The materials used for construction must be resistant to highly corrosive ozone and are limited primarily to silica-glass, 316 stainless steel and Teflon. All ozone streams from the system will be collected in a thermal destruction unit containing a manganese dioxide catalyst that converts all residual ozone back to oxygen before being emitted to the atmosphere.
(This article was published in the spring 2000 issue of Louisiana Agriculture.)