Innovation in potato storage environment 

by Garry Isaacs  

Published in Potato Grower Magazine

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: 

  1. Peracetic acid breaks down at temperatures above 60 degrees Celsius (140 degrees Fahrenheit).
  2. Hydrogen peroxide dissociates at temperatures above 280°C (536°F).
  3. 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:

  1. Is it possible to remove the pathogens from the storage air? 
  2. Can it be done economically? 
  3. 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. 


by Garry Isaacs  

Published in Potato Grower Magazine

Silver scurf, although not especially harmful to consumers, is the most notorious cosmetic disease of potatoes. Its appearance is more prominent on red, yellow and other colored varieties because the silver color of the lesions blends better with the brown color of Russet varieties even though it may be just as prominent. Experts have recognized that silver scurf spreads to healthy tubers through the air. Stopping the spread of silver scurf in storage would save $millions in fresh potato losses to growers and producers and improve the quality and image of potatoes in the market place. Potatoes are one of the worlds most healthy and nourishing foods. Increasing the appeal  of potatoes to consumers by eliminating silver scurf would naturally increase the sale and consumption of potatoes world wide.

Silver scurf has been found by researchers in soil and also on seed potatoes. It is not readily seen in early stages, before the population is high enough to show visual evidence of its presence without extensive testing. It can therefore be inadvertently passed on seed potatoes to the commercial grower where it can inoculate healthy tubers and severely reduce the value of the crop.

Potato storage must be totally enclosed to preserve the temperature and humidity of the storage atmosphere. Ventilation with outdoor air is used for cooling and venting of airborne contaminants but introduction of outdoor air is not available whenever outdoor air is either too hot or too cold since temperature control is essential for the preservation of potatoes. Several types of enclosures have been utilized for the preservation of these precious and perishable food products. Techniques have been established to maintain a proper temperature and humidity within storage facilities in an attempt for optimum preservation throughout the storage season, among them are high capacity fans, humidifiers and refrigeration devices but until 2011, nothing had been specifically done to clean the air. Airborne pathogens had not been previously addressed as being significant to potato preservation because no practical method was available for intercepting them. 

A serious consideration of the way harmful organisms are spread in the air is beginning to be made in depth by pathologists and storage experts. Pathogens, particularly fungal spore such as silver scurf, fusarium and others are spread by movement in the air. As they sporulate, the spore are released from the host and go airborne, spreading to and infecting other tubers in the storage pile. This caused growers and producers to look to chemicals for killing pathogens after they have already infected the potatoes. However, it has been demonstrated in tests done by Colgan and Johnson in 1998 that a post harvest sterilant for apples and pears, peracetic acid that is commonly used in potato treatment, caused increased damage from fungi. It was concluded that this was probably because the chemical also killed microbials on the surfaces of the potatoes that were antagonistic to the fungi. This is likely to be true of silver scurf and other fungi that are notorious destroyers of potatoes.

Airborne fungal spore and bacteria are in atmospheres everywhere and can occur in potato storage air in huge numbers, especially whenever potatoes are being moved in or out of storage. Fungi are a more prominent destroyer of potatoes than bacteria. Silver scurf, blackdot, fusarium, pythium, early blight and late blight are all caused by fungals. Bacteria, such as Erwinia (soft rot) and ring rot, that affect potatoes are as small as 0.8 of a micrometer. The fungal spore that are most destructive to potatoes are much larger than bacteria, ranging from 4 to 60 micrometers. Silver scurf spore are elongated and are produced on microscopic, stringlike stems. (figure 1). The spore will remain fairly still as long as the humidity is kept at 95% or more in the surrounding air. It would be ideal to keep the humidity high in the storage to prevent shrink without condensation and wetting the potatoes, plenum floors, walls and other surfaces. The organism requires water to survive and grow, so excess moisture on potatoes and other surfaces will contribute to the development and spread of the disease. The spore are freed from their stems by disturbances such as the physical handling of the potatoes or by forced movement of surrounding air, particularly when the relative humidity is below 85%. Like weed seeds, silver scurf and other fungi propagate by spreading spore through the air to a new host. Silver scurf is unique in that it will freely establish itself on healthy potato skins, unlike fusarium, pythium and others that require an invasion point caused by cracks, peels or injuries.

Investigations done by Philip B. Hamm, Dennis A. Johnson, Jeff S. Miller, Nora L. Olsen, and Phillip Nolte, representing three Universities in the Northwest United States, Washington State University, Oregon State University and The University of Idaho, have identified silver scurf to be a major contributor toward the reduction of potato value, cosmetic appearance being the biggest problem, causing a severe loss in profit to the potato grower.Probably more important from a cosmetic standpoint are the secondary lesions that result from infections that develop while the tubers are in storage. Secondary lesions may occur anywhere on the tuber and can be so numerous as to cover virtually the entire tuber surface. In storage, infected tubers lose water at a higher rate than healthy tubers due to the pathogens (silver scurf) disruption of the periderm.Olsen, Miller, Nolte, University of Idaho CIS 1131.

Reed Searle, a grower of red and yellow varieties in Shelley, ID, experienced heavy losses to silver scurf and black dot disease for several years before Humigators were installed on five of his storage bins in 2012. The diseases have not shown themselves in shipping from any of the five cellars where they were installed during the two years since installation but was still prevalent in the cellars that did not have the machines. He has since declared that the machines paid for themselves within the first year of installation. His experience has led to additional work being done toward verification of the benefits of the removal of airborne spore to prevent the spread of spore to previously uninfected tubers in storage. 

In May, 2015, a series of tests were done by Isaacs Hydropermutation Technologies (IHT) of Rigby, ID, in cooperation with Allen Floyd and his team at Harvest Fresh Produce in Othello, WA. on one of their storages where silver scurf had been previously identified. A Model #1225 Humigator, produced by IHT, was used to vacuum the air surrounding the potato pile in the storage facility and to filter the air with water as the filtering mechanism to intercept all types of microscopic particles, including the silver scurf spore into the water. The filtered, humid air was released into the plenum of the ventilation system. 

A number of samples were taken of the air, using a Zefon Z-Lite Linear Pump with integrated flow meter/regulator valve and spore traps, before entering the Humigator in the storage area and also in the plenum at the outlet of the Humigator. Water samples were taken periodically from the drain valve of the Humigator separation tank with pre-sanitized water sample bottles. The air sample spore traps were analyzed by Dixon Information, a laboratory in Salt Lake City, Utah that specialized in the analysis of fungi and mold samples taken from multiple sources. The water samples were analyzed by the Oregon State University laboratory in Hermiston, Oregon.

Air samples were taken periodically under varying conditions. Analysis of the samples produced the following data.

A count of the target spore in the air at the exit of the Humigator showed a reduction of 81% in a single pass. 

After 12 hours of continuous circulation through the Humigator, no spore were found in the samples, as shown by the graph below.

Water samples were heavily loaded with all kinds of both organic and inorganic particles, including heavy debris and  a large number of the target spore.

These tests have shown that silver scurf is airborne in significant numbers in storage and that it can be intercepted from the air by Humigation. Intercepting airborne silver scurf in storage will eliminate the spread of the pathogens to healthy tubers through the air. Humigation technology can significantly increase profit to growers and producers, improve the appearance of potatoes in the market place and increase sales of potatoes overall as a result of making potatoes more appealing to grocery shoppers and consumers. More information is available on the IHT website at


by Garry Isaacs

Published in Potato Grower Magazine

Shrink and pressure bruise are directly related since dehydration makes potatoes soft, allowing for indentation from the weight of potatoes in a pile. Shrink is weight loss and pressure bruise is a symptom  and the result of shrink. Shrink loss can be heavy, as much as 15% or more of the potato crop, which directly reduces the bottom line because potatoes are sold by weight. Any amount of shrink can reduce profit proportionally to a grower, and in too many cases, can even represent the entire annual profit of a potato growing season.

Shrink begins in the field during the growing season, making it essential for proper and efficient irrigation practices but it manifests itself most in storage. If potatoes are placed in storage in a flabby, dehydrated condition, they have already lost weight and are subject to pressure bruise immediately when placed into the pile. Reducing the height of a pile or using basket or box containers has proven to reduce pressure bruise by reducing the weight of potatoes stored high in the pile on those that are below. However, pressure bruise can also be avoided by keeping potatoes hydrated before placing them into storage. Growers have learned that irrigating the field prior to digging helps keep the potatoes firm when harvested.

The leakage of poorly constructed storage buildings can have a negative and costly affect on the preservation of potatoes. If you enter a storage when it is empty and can see light through cracks and holes in the walls and doors, these should be repaired or at least covered prior to filling with potatoes. Studies done in UK have pointed out that “It’s difficult to spot energy leaking from a store through badly-fitting doors, old louvres and the like, but it can cause no end of problems, including: High energy use Hot and cold spots where condensation can form on potatoes leading to the development of disease Reduced effectiveness of chlorpropham (CIPC) applications.” - See more at:

Respiration that occurs in the first six weeks of storage, commonly known as the sweat period can cause additional dehydration.Test results produced by North Dakota State University have shown that “respiration accounted for about one-tenth to one-half of the total weight loss, except when tubers were stored in 100 per cent relative humidity. Under this condition, there was an actual net gain, apparently from the tubers absorbing moisture from the surrounding atmosphere.”  Butchbaker, A. F.; Promersberger, W. J.; Nelson, D. C.

Air distribution of conditioned air is severely reduced or lost if the conditioned air is leaked or intentionally ventilated excessively from the storage throughout the storage season. Excessive ventilation is applied for cooling and as a precaution against the assumed accumulation of CO2 produced by stored potatoes. CO2 should be monitored and controlled as needed by systems specific for the purpose such as the system provided by IHT for the Humigation process and atmosphere control. Ventilation activated by CO2 reading instead of by temperature is a better method for controlling shrink and disease than applying excessive ventilation at random.

A process called Humigation, produced by Isaacs Hydropermutation Technologies of Pocatello, Idaho will rehydrate potatoes immediately after they are placed in a storage without producing excess water in the plenum or ventilation tubes. However, since pressure bruise occurs before the rehydration process is implemented, the flat spots caused by pressure will prevail and cannot be removed, even thought the potatoes are firm and solid and there is no weight loss. The value of potatoes with flat spots is reduced because of appearance and the perception of buyers and consumers who have have seen deterioration and brown coloring related to pressure bruise. Potatoes preserved by the Humigation method prevents the deterioration and browning but it cannot repair the flat spots.

Refrigeration immediately after harvesting lowers the temperature of the storage air. Cold air does not hold much moisture even when relative humidity is high. Cold, dry air shrinks potatoes and hinders rehydration, which also contributes to pressure bruise. It is dry air that shrinks potatoes. Cold air is also dry air. Preserving potatoes at higher temperatures is an enhancement of profits to growers, packers and processors because shrink and pressure bruise could be avoided and less starch is converted to sugars, making the cooking process and appearance better. The reason refrigeration has become habit is to reduce the spread and multiplication of disease related pathogens such as soft rot, silver scurf, black dot and fusarium. Reducing the population of these costly, airborne disease agents by means other than chemicals or refrigeration enhances the profits of potato crops in storage. All of these agents are airborne and prevalent in storage air even though they are invisible. Scrubbing storage air with Humigation and removing the disease causing agents prevents the spread of disease, thereby reducing the need for aggressive refrigeration. The value of storing potatoes above 50ºF is immeasurable but is certainly an advantage, as stated by Michael Thornton, Professor, Research Plant Physiologist at the University of Idaho Extension Service office in Parma, Idaho

Conscientious and advanced storage measures for the reduction of shrink and pressure bruise are sound management practices that are protective of the bottom line. A little effort and a small investment toward improvement to storage facilities and ventilation practices can be very profitable for potato growers, packers and processors.


by Garry Isaacs


Potatoes are sold by weight. Weight loss is directly related to profit and it can represent as much as a 15% reduction or more of the bottom line. Pressure bruise, largely the result of dehydration of the tubers also decreases the value of the potatoes. Experts have discovered that disease 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 (RH) 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 an equilibrium with the water content of the storage air. Potato storage air should constantly be maintained between 95% and 99% RH 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 RH levels, potatoes stored at 90% RH could lose 9% in weight, or nearly twice as much 

weight as those stored at 95% RH. Given a storage capacity of 100,000 cwt, and a value of $5.00 per cwt, the building maintained at 90% RH would return 

$22,000 less than the storage controlled at 95% RH. The impact of maintaining the proper RH cannot be overstated.” Nora Olsen, University of Idaho Extension, potato storage specialist


It is important for storage managers to understand that at 64º F, 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 42º F, the atmosphere only holds about 5.5 grams of water per cubic meter of air space (0.155 grams of water per cubic foot of air). Air at 53ºF will contain almost twice as much water (10.4 grams per cubic meter) than air at 42ºF (5.5 grams per cubic meter). 

For instance, a storage building, 60 ft by 150 ft and 18 ft high contains 162,000 cubic feet or about 4,576 cubic meters of air space. A gallon of water weighs about 3785 grams. Total water saturation of the air space of this empty building at 42º Fahrenheit 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%, the total amount of water that the air space will hold at 42ºF is less than 2 gallons (1.66). 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.

Water droplets produced by atomizers that are 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 with gravity 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. 


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™ F pathogenic organisms begin to grow and multiply - even at 45™ F (just 4 degrees warmer), bacteria grow 10 times as fast as they would at 41™ F.

A technology, introduced by Isaacs Hydropermutation Technologies called Humigation, removes pathogens from the air as it is circulated in the storage facility. Humid air is produced by a Humigator™ after pathogen removal 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% RH, just below condensation (dewpoint), without saturating the plenum or any of the potatoes. 


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%, 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 highly to substantial profit for the grower.


by Garry Isaacs

The automatic monitoring and control systems typically set up to control temperature and humidity in potato storages have been programmed to fit the grower’s desires. Temperature is typically read into the system from thermocouple probes set strategically inside the storage area or the potato pile. 

Humigation®® from Isaacs Hydro Tech is being used on an increasing number of potato storage buildings (cellars) since 2012. I have visited these cellars periodically throughout the storage season and have been monitoring the operation of the Humigation® process monthly throughout that storage season. 

Part of my routine has been to check humidity in the storage area using a sling psychrometer. Both wet bulb and dry bulb temperature readings are taken and the difference between the two indicates the humidity or water content relative to the temperature of the air in the cellar.

There are 35.31 cubic feet in one cubic meter. 1 cubic meter of air at 30 degrees C can hold 30 grams of water As air cools it holds progressively less water, thus as 20 degrees C it is just under 20 grams per M3, and at 10 degrees C it's closer to 8 grams.

At 7.22 degrees celcius (45 F) 1 cubic meter of air holds only about 6.5 grams of water. That translates to about 0.18 gram of water per cubic foot of air. Any amount above that will vaporize and condense. That is why you can see your breath on a cold day.

Cold air is very dry. Dry air dehydrates potatoes, reduces weight and exacerbates pressure bruise. Once pressure bruise occurs, it is generally irreversible. That is why it is so important to keep the humidity as high as possible and the airborne pathogens removed during the healing period. Removing airborne pathogens in the process is a wise practice because that is when potatoes have most likely been injured by handling and are most subject to inoculation.

There is no reason to use more water than is necessary to keep the air saturated. Wetting floors and walls does not provide effective air humidification. It does however provides a place for pathogens (mold and bacteria) to grow and multiply.

Humigator uses whatever amount of water it takes to keep the humidity as high as possible without condensation at any temperature plus approximately 10 gallons per week (whatever is dumped ) during the healing period after digging and then reduced to about 10 gallons dumped monthly, after the initial air is clean and the potatoes have healed and are no longer being disturbed.

My periodical temperature readings have shown that temperatures are usually higher than growers think at the top of the pile. Since warm air rises, it would be expected that the temperature at the top would naturally be a little warmer than below. My readings have shown that from April through August, the temperature at the top of the pile is usually over 50º F in just about all of the cellars I have checked. 

The University of Idaho is considered to be the top experts in potato technology in our area. When I have discussed my findings with their pathologists and storage experts, they all agree that keeping the temperature above 50 degrees would improve the cooking quality of potatoes and reduce losses to sugar ends, shrink and pressure bruise. However, tradition has been that keeping the temperature down has been necessary in order to preserve food products from bacterial deterioration.

Humigation® is primarily an airborne pathogen eliminator. Simple reasoning indicates that if pathogens are being constantly removed from the storage atmosphere that the bacterial deterioration rate would be reduced to at least some degree at any temperature. 

There are potatoes being kept in storage with Humigation® through the summer with high integrity. Part of this positive result is caused by keeping the cellar and plenum dry while keeping the humidity high. The potatoes kept under Humigation® have all maintained high integrity and appearance even at temperatures as high as 65º F. 

Russet Burbank potatoes have been kept  at high integrity in 2013 through August with no reduction in weight or integrity.  10,000 cwt of a yellow variety have been kept at high integrity without refrigeration until they were finally removed on September 15, 2014. The temperature in the cellar was as high as 67º F in August and the RH was at 95%.

More data will be collected as we monitor our growers throughout the coming season but so far, indications are that potatoes can be kept healthy and well hydrated at higher temperatures with Humigation® than without.

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