Overview of Ozone Use at Snokist Growers
Ozone is used within a fruit packing warehouse as part of an overall sanitation program. Ozone is a very powerful oxidizer, similar to chlorine but about 200X stronger. This gives it the ability to break down and kill bacteria, mold and fungus spores. Ozone can be generated through the use of ultraviolet (UV) light or electricity. The latter method, called corona discharge, is the most common way to generate ozone in large quantities. Ozone, as a water disinfectant treatment, has been in use since 1901 and is used worldwide in many municipal water treatment systems. Among its advantages are: a half-life of only 15 minutes; it breaks down to oxygen; and it is viewed as a device by both the federal government (EPA) and Washington State (WSDA) which means no label is required. Among its disadvantages are: a half-life of only 15 minutes; it needs thorough mixing to be effective; and a high oxidation potential. Ozone 'eats' many common materials, like rubber and mild steel, which tends to push you into stainless steel, expensive stuff.
At Snokist we first started experimenting with ozone in 1989, looking for alternatives to Benlate, Botran, and other postharvest fungicides. At the same time, we were putting in hydrocoolers at the end of our cherry lines and wondered if we could use the long contact times (3-5 minutes in the cooler) to help. We set up half of one packingline to use ozone as the fungistat in one hydrocooler and used chlorine in the other half. We did this for an entire season and sent mixed loads all over the United States and Taiwan and Hong Kong. We could send good, sound, cold cherries anywhere and get good quality on arrival, even when sent by boat to southeast Asia. The keys to making it work are no decay going into the box and no reinfection through the dump tanks or hydrocoolers. This is where the ozone or chlorine comes in and we felt that ozone was the better of the two systems. We have since expanded the ozone system to our Grandview plant and it has been the only postharvest treatment on our cherry packingline for the past couple of seasons.
We brought this equipment to our apple and pear lines, fruit that is much dirtier than cherries. It has taken a while to size the filter correctly. This is critical as the more dirt there is, the less effective the ozone will be. Essentially the ozone is tied up by the dirt and not enough is left to attack the pathogens in the dump tanks effectively. The other problem we encountered concerned pear floats. No matter which you use, it makes the water thicker and thus requires more effort to process. It tends to make the equipment less automatic, though it will still function properly. Most of the time the equipment can be set and essentially walked away from, but with the pear float the entire system should be periodically monitored to make sure all systems are working. Also, lignin sulphonate tends to have some organic material already in it; thus the load is somewhat increased when that pear float is used. For this reason, as well as others, we chose sodium silicate as our pear float. We also use a small amount of chlorine (5-10 ppm) in the dump tank due to the lack of residual when using ozone and the amount of pathogens coming in with the fruit. We do not have the same problem when we run apples. We've had good success for the past two seasons and are looking to expand the use of ozone to our presize when we lose the use of SOPP on apples.
We have also implemented regular testing of our dump tanks for pathogens (mold/fungus/bacteria). Dr. Spotts, OSU Hood River, helped develop a simple procedure for us (in conjunction with Jim Loofburrow, TechnOzone, Inc.). We sample our dump tanks, plate out the sample, incubate it, and then count the colonies. Rather than try to separate out the different types of colonies, we work on just total counts. If our tank counts are below a threshold level, the likelihood of infection due to spores in the dump tank is not worth worrying about.
Here are some facts to consider in deciding whether or not ozone would work in your warehouse.
- This is a water treatment system. It will not kill
pathogens already on the fruit. It will help with the
problem of reinfection from the dump tank.
- The equipment you use must filter out dirt
- Anticipate problems when using pear float.
Seriously consider adding chlorine (5-10 ppm).
- Size the ozone system to turn over dump tank
contents frequently. Ours turn dump tanks over about every
20 minutes. All the water from the dump tank is treated 3
times per hour.
- Have at least one device that continuously
monitors the air around the device and the dump tank for
the presence of ozone. It is lethal! Connect the monitoring
device to a shut-down device on the ozone generator.
- This is only one part of your warehouse sanitation. By itself ozone will not cure, prevent, or provide a residual knockdown to any pathogen. But if integrated properly, ozone will help provide excellent warehouse sanitation.
Tree Fruit Postharvest Journal 4(1):14-15