Potato and other vegetable storage facilities are largely controlled by ventilation systems using outdoor air as a cleansing and oxidizing medium. However, there is a problem with using outdoor air that has not heretofore been adequately addressed: the bacteria and mold spore content of outdoor air.
Berkley Laboratories in Berkeley, Calif., has recently done research on the bacteria and microorganism content of outdoor air. This research had been conducted using a DNA method of identification instead of the usual surface colony count. This has enabled the lab to identify over 1,800 species of bacteria contained in ordinary outdoor air around two Texas cities.
The significance of these tests to the farming industry is that regardless of the location, outdoor air contains a great deal of bacteria, especially at harvest time in potato-growing areas.
When crops are ripe and ready to be harvested, the bacteria and mold spore count is highest because they have been growing and multiplying throughout the growing season. Since they are microscopic and invisible to the human eye, they have been ignored and their presence has not been adequately recognized.
Storage facilities are usually bathed profusely with outdoor air in preparation for the harvested products to be put to bed for the season. This practice introduces every form of bacteria and mold contained in the ambient air to be available for multiplication on the potatoes injured at harvest. Many of the bacteria are not harmful; however, those that are harmful are guaranteed to be present, along with the rest, thanks to this profuse ventilation method. Hence, multiplication of harmful microorganisms can begin, eventually promoting pathogenic activity such as soft rot, silver scurf and others.
When rot begins to develop in an area in a potato pile, the bacteria or fungi that promote the deterioration of the potatoes goes airborne, circulating by ventilation fans from place to place in the storage unit and becoming available to all of the potatoes in the storage facility. Hot spots begin to grow and produce more and more bacteria that is released into the air as they perform their designated duty to destroy potatoes. Unless checked, this bacteria is equipped to destroy an entire potato crop within a short period of time. As growers are fully aware, profits can be eaten away with just a little deterioration.
Fungi are great destroyers of potatoes. Fungals such as silver scurf, black dot,
Fusarium, pink rot and Rhizoctonia can have profound effects on the profit line if left unchecked. When these harmful organisms are left on the potatoes, as any marketer knows, it makes them unacceptable or at least harder to sell and less attractive to the consumer.
To limit the multiplication of bacterial rot, chemicals containing peracetic (peroxyacetic) acid and hydrogen peroxide have been commonly applied to the stored products. When chemicals are introduced to the storage area in a fog, they are not distributed efficiently because of the repelling affect of negative oxygen on the surfaces of water molecules in the ambient air. Pressure differentials play a big part in preventing even and thorough application of chemicals to all areas of the potato pile. If chemicals are introduced in droplets larger than 10 microns in diameter into the ventilation system by a fogger, they are restricted to a point where they are not as effective as desired for sanitation. Gravity will pull the larger droplets to the floor instead of distributing them through the potato pile. Where there is no control of pressure differentials, distribution is restricted.
In order to make the droplets small enough for better suspension in the air, thermal fogging is used by some applicators, using heat to create steam to provide smaller droplets that will stay airborne. The smaller droplets created by thermal fogging will distribute better through the potato pile, but additional facts should be considered as to the thermal effect on the chemicals being fogged:
- Peracetic acid breaks down at temperatures above 60 degrees Celsius (140 degrees Fahrenheit).
- Hydrogen peroxide dissociates at temperatures above 280°C (536°F).
- Thermal foggers use air heated to 500º to 600ºC (932° to 1,100°F).
Some have claimed that these chemicals are not in contact with the heat in a thermal fogger long enough to destroy them, but studies have shown that a residence time of less than one-half second will destroy peracetic acid. At any rate, a grower should ask himself, “Why inject chemicals with heat if there is even a chance that they will be limited in effectiveness if it is not necessary?”
Studies have found that as a postharvest sterilant for apples and pears, peracetic (peroxyacetic) acid caused increased damage from fungi This was possibly because it killed microbials on the surface that were antagonistic to the pathogens.
Peracetic acid, the main active ingredient in many popular and approved disinfectant chemicals, actually caused increased damage from fungal rots. Could this also be true in potatoes, causing an increased infestation of silver scurf and other fungal agents such as Fusarium, black dot, early blight, pink rot, powdery scab or Rhizoctonia? Where is the data to support that this is not happening?
So, how can these problems be fixed? If the air could be scrubbed and the microorganisms removed prior to entering the storage area, the possibility of bacterial and fungal deterioration could be significantly reduced. This begs a few questions:
- Is it possible to remove the pathogens from the storage air?
- Can it be done economically?
- Will it pay off in product sales and profits?
The answers to these questions and more can now be a resounding “yes” with the products such as the Dynamic Multi-Venturi (DMV) blower/scrubber/applicator, also known as the Humigator. Storage environments can now be totally controlled by a humigation system, including
- air transfer and circulation,
- constant removal of 99.65 percent of airborne microorganisms, and
- constant humidity maintenance without wetting walls and floors.
These functions can occur using far less air or water and less energy. To solve this problem the Humigator draws air from the area of the storage room, thereby creating a vacuum that will naturally be filled by the cleaned air delivered into the plenum. The air is continuously vacuumed into the inlet of the Humigator, where the airborne pathogens are captured into the scrubber fluid (water) and the cleaned air is discharged into the storage area and made available to fill the vacuum created by the suction. If clean, humid air is vacuumed through the pile by removing the pathogens and regulating and controlling the pressure differentials in the pile, disease reduction is assured and infestation of healthy potatoes can be eliminated.
How Humigation Works
Airborne particulates including bacteria, fungal spores and objectionable odors enter the Humigator, a rotary device consisting of a water atomizing impeller housed in a unique enclosure that incorporates a patented, advanced geometric design. Fluid is injected into the airstream and the fan/impeller blades impact and accelerate the air and water droplets to create a dense aerosol of suspended fluid droplets. The aerosol is passed through several low-pressure venturi zones, causing them to associate in either solution or a compact colloid with bacteria and fungal particles to be collected by the water stream in a dynamic mixing process and collection chamber. No filter is required.
Humigation constantly provides a net pressure increase instead of the significant pressure drop associated with venturi and electro-static scrubbers, substantially lowering operating costs.