Potatoes are sold by weight. Weight loss is directly related to profit and it can represent as much as a 15 percent reduction or more of the bottom line. Pressure bruise, largely the result of dehydration of tubers, also decreases the value of potatoes. Experts have discovered that diseases such as silver scurf can also contribute to dehydration, shrink and weight loss. Silver scurf and other fungi grow rapidly on wet surfaces:
“Silver scurf may have a transient effect on potato growth and tuber yield” (Mooi, 1968; Denner et al., 1997).
“The light brown lesions (silver scurf) increase the permeability of the tuber skin, causing shrinkage/water loss and therefore weight loss” (Hunger & McIntyre, 1979; Read & Hide, 1984).
Relative humidity is the amount of water contained in air relative to the air temperature. It is measured by checking the difference between dry bulb and wet bulb temperatures of the air. Water content of potatoes in storage is directly related to the water content of the storage air. Dry air causes potato weight loss because the moisture is drawn out of potatoes toward equilibrium with the water content of the storage air. Potato storage air should constantly be maintained between 95 and 99 percent relative humidity at all times if possible to limit pressure bruise and to keep potatoes from shrinking by dehydration.
“If weight loss is compared over six months of storage at various relative humidity levels, potatoes stored at 90 percent relative humidity could lose 9 percent in weight, or nearly twice as much weight as those stored at 95 percent relative humidity,” says University of Idaho Extension potato storage specialist Nora Olsen. “Given a storage capacity of 100,000 hundredweight and a value of $5 per hundredweight, the building maintained at 90 percent humidity would return $22,000 less than the storage controlled at 95 percent relative humidity. The impact of maintaining the proper relative humidity cannot be overstated.”
Relative Humidity is Invisible to the Eye
It is important for storage managers to understand that at 64 degrees Fahrenheit, the air will only hold about 15 grams of water per cubic meter of air space (0.42 grams of water per cubic foot of air). As the temperature drops, the air will hold less and less water. At 42 degrees, the atmosphere only holds about 5.5 grams of water per cubic meter of air. Air at 53 degrees will contain almost twice as much water (10.4 grams per cubic meter) than air at 42 degrees (5.5 grams per cubic meter).
For instance, a storage building with an area of 60 by 150 feet and 18 feet high contains 162,000 cubic feet—about 4,576 cubic meters—of air space. A gallon of water weighs about 3,785 grams. Total water saturation of the air space of this empty building at 42 degrees would be achieved with only a total of about 6.65 gallons of water.
If the air space remaining after a building is full of potatoes is 25 percent, the total amount of water that the air space will hold at 42 degrees is less than 2 gallons. If more water could be added to the air space, it would leave the air and things would begin to get wet. Cold surfaces on building walls or beams will also lower humidity by condensation.
This causes the drip lines commonly seen below the beams of a storage bin. Whenever condensation happens, the air in the storage is giving up water and is becoming drier. The colder the air, the drier the air, and the more potatoes will shrink.
Atomizer-produced water droplets large enough to be seen with the naked eye are of no use to humidify the air except for the small percentage that evaporates by temperature differential. They may look very small to the eye but in order to be seen, they must be larger than 10 to 20 microns in diameter. Droplets this big stay suspended for a time and then fall to the floor or attach themselves to tubers, the ceiling and the walls. Standing water, wet tubers and other surfaces invite the growth of diseases such as silver scurf, black dot and rotting.
A Perfect Atmosphere
Potatoes are refrigerated to prevent the multiplication of harmful pathogens. Swamp cooler (wetted fabric)-type refrigeration systems produce a great deal of excess water. Wet fabric, floors and standing water provide a breeding ground for pathogens.
When the temperature rises above 41 degrees, pathogenic organisms begin to grow and multiply. Even at just 45 degrees, bacteria grow 10 times as fast as they would at 41 degrees.
A technology called humigation, introduced by Isaacs Hydropermutation Technologies in the last few years, removes pathogens from the air as it is circulated in the storage facility. Humid air is produced by a Humigator after pathogen removal and is dispensed in small enough water particles to be invisible to the eye and to remain airborne. With continuous operation, humigation for potato storages has proven to maintain humidity at 95 to 99 percent relative humidity, just below the dew point, without saturating the plenum or any of the potatoes.
Excess Water Contributes to Disease and Shrink
A good and simple gauge for checking humidity in a storage bin is to blow your breath through it. If you can see your breath at any temperature—even though you don’t see wet potatoes or surfaces—the RH of the air is between about 95 and 99 percent, right where you want it to be. To be able to do this throughout the storage season without wetting the plenum is particularly desirable, since dry air shrinks potatoes and destructive pathogens need wet surfaces to grow. Good humidity control throughout the storage season contributes greatly to substantial profit for the grower.