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Top of storage pile in a curved building. Overhead fans provide extra airflow if needed to ensure free moisture does not accumulate. (Photo: Patrick Morris, Techmark, Inc.)

Potato Storage: Keeping an Eye on the Prize

The case could be made that caring for potatoes and for children have a lot in common: Let your guard down for a minute and you’ll pay for it in the end. Like parenting, potato storage is an area in which expert advice can save a lot of headaches and wrong turns.

In this edition of Roundtable, Spud Smart turned to three experts on the ins and outs of potato storage:

  • Robert Coffin, potato researcher at Potato Consulting Services Inc., Prince Edward Island.
  • Todd Forbush, engineering/storage specialist, fruit and vegetable systems, Techmark, Lansing, Michigan.
  • Mary LeMere, manager, agronomy mid-west, McCain Foods, Wisconsin Rapids, Wisconsin.

Millions of pounds of potatoes rot needlessly in storage every year.

“This results in financial losses for growers, consumer complaints of poor quality table potatoes, and problems for scheduling and delivery to processing plants,” says Coffin. “With sound storage practices, much of that rot can be avoided.”

According to Forbush, potatoes are the perfect target for pathogens, “so you must keep diseases in check, and you do that through proper storage. Our main jobs are to protect potatoes from pathogens, maintain the internal water content, and preserve potato quality in terms of skin, blemishes and sugar concentration.

Simply put, “proper storage leads to high quality in the final product. You can’t have a good french fry if you don’t have good storage,” says LeMere.

Dampness means Danger

A wet year poses unique challenges for potato storage.

It starts with logistical challenges during harvest,” says LeMere. “Wet potatoes tend to have soil stuck to their surface and when you put them in a storage building, the soil will clog the air spaces between potatoes. This can prevent proper airflow, so what happens during storage can be detrimental for the entire storage season.”

The other great concern on LeMere’s mind is the potential for disease.

“Several storage diseases – such as pink rot and Pythium leak – are worsened in wet weather. Moreover, the diseases can compound to cause a bigger problem. For example, even though leaf blight is a slow-moving disease, it produces tiny wounds on the face of the potato and leaves the door open for faster pathogens to penetrate. The key here is good handling practices during harvest. Producers must do anything they can to minimize the damage to tubers. Make sure that the drop between the truck and conveyor belt is no more than six to seven inches.”

In the face of pathogen loading, Forbush has some advice. Pathogen loading is a common problem in a wet year. Try to harvest potatoes from 10-18 C, handle them carefully and ventilate to rid your storage of any surface moisture.”

Keep Cold at Bay

According to LeMere, tubers like warm days and cool nights. “In cold conditions, potatoes are more susceptible to seed piece decay and blackleg, and you will have reduced bulking rates and lower yields.”

In these conditions, “first try and grade out any green potatoes as they are closer to the surface and thus will have more exposure to the cold,” says Forbush. “If possible, harvest in optimal conditions by allowing potatoes to warm to above 10 C prior to digging.”

The Heat is On

Too much warmth, however, can also be a problem.

“Just as in cold weather, potatoes won’t grow as fast or get as big as usual,” says LeMere.

When the heat is on, it’s critical to maintain field moisture and irrigate accordingly. “Note that as the temperature rises, potatoes peak and then moisture demands actually decrease,” she adds. “For this reason, you must pay close attention to moisture levels and what your plants are demanding, as it’s very easy to either over or under water in extreme heat.”

Forbush recommends digging at a cooler time of day and avoiding the temptation to cool potatoes rapidly as that will limit the amount of cooling air available during warm nights. Aim for at least six hours of ventilation air at night and get fresh air daily, minimizing free surface moisture in storage.

Something in the Air

“You could write an entire book just on ventilation and storage,” says LeMere. “Adequate ventilation is crucial through the entire storage season. Ensure you have ventilation systems that are properly sized for the height of the pile and the number of potatoes. Any new storage building needs a minimum of 1.5 cfm [cubic feet of air per minute] per hundredweight.”

That advice is echoed by Forbush. “Good vent rates range from 1.25-2.0 cfm per hundredweight. Lower airflow rates can subject your crop to potential harm as there isn’t enough air to rid potatoes of moisture, heat and CO2 produced by the crop.”

When it comes to airflow, growers should keep in mind that theory and reality don’t always match.

There can sometimes be a considerable difference between the theoretical airflow in a storage building and the actual airflow,” says Coffin. “For example, the grower may have been told by the contractor/builder that the building and ventilation system should provide an airflow volume of 20 cubic feet per minute per ton of potatoes. However, when the total airflow through the laterals was measured and divided by the tons of potatoes in the building, the actual airflow may have been only 15 cubic feet per ton. In some instances, adding an additional fan or increasing the horsepower of fans results in increased airflow volume through the potato pile.”

Humidity is Key

Potatoes are over 80 per cent water, so high humidity in storage is critical, as high as possible without causing surface condensation,” says Forbush.

But as Coffin points out, one area that is poorly understood by many growers is relative humidity and the serious implications if this parameter is not correctly included in decision making during storage management.

“For example, some growers have the control settings in a mode where warm humid air is imported into the storage in November or March,” says Coffin. “When the warm, moist air encounters the cooler potatoes, condensation occurs, and this often results in extensive rot.”

Checking all temperature sensors and replacing units that have inaccurate readings is central to success in potato storage.

Another critical area is cooling and holding periods, and the numbers depend on your objective.

Final holding temperature and cool down rate is a function of the crop’s end use, outside conditions and whether you have refrigeration,” notes Forbush.

Without refrigeration, you must watch how quickly you cool the crop because if your rate is too fast and it’s warm outside, you won’t get a sufficient supply of fresh air.

According to Forbush, with seed crops, you want to cool them down to 3 C or 4 C at a rate of no more than 0.4 C per day. You would use the same cooling rate for table stock potatoes, but only cool them to 4 C or 4.5 C depending on the variety.

Last but certainly not least, french fries can be cooled to 6 C using a rate of 0.23 C per day, while chip stock is cooled to 7 C or 8 C at a maximum cooling rate of 0.2 C per day.

Thus, it’s very important to look at your end use in determining cooling rates.

As you work with these numbers, LeMere stresses growers must also work with their customers.

If you have a storage building with some diseases developing or rot problems, it’s best to cool your crop as quickly as possible. I strongly recommend that before growers do that, however, they consult their end user and make sure they are on board. The worst thing you can do is to solve one problem through faster cooling and end up with a quality problem instead.”

Monitor, Monitor, Monitor

LeMere doesn’t mince words here with her advice: don’t get lazy. As she points out, you put so much effort into planting and fertilizing and irrigating your crop. Yet there is sometimes a tendency once it’s in storage to think you just have to check your laptop screen each day and you’re good.

She says there is no substitute for entering the building each day and checking your potatoes first-hand. “The longer you can store them, the more money you can make, so keep your eyes on the prize.”

The Future of Storage

These days, growers must stay on the cutting edge to be competitive. So what storage innovations make the cut as worthy of mention?

“Prescriptive air is quite new on the market. If you have a section of potatoes going bad in storage, a prescriptive air system can provide dry air through three to five pipes to the diseased area while maintaining regular humidified area to the rest of the crop,” says LeMere.

“I’ve seen rotten sections completely dried out and buildings saved with this system. It’s a simple yet very impressive concept and it’s all about delivering the right kind of air for what your potatoes need.”

If delivering the right air is half the battle, delivering more of it may be the other half.

While most conventional storage buildings and ventilation systems deliver from 15 to 25 cubic feet per minute per ton of potatoes, some of the Dutch systems now are built with the capacity to deliver four times or more airflow volume,” notes Coffin.

The rationale for the increased capacity is to have the additional airflow available when rapid drying of the potatoes is required to minimize the spread of rot. As well, the high airflow capacity allows the system to import cool outside air when it is available only for short periods of time.

In Coffin’s view, this is an example of effective risk management. “When the airflow is greater than 15 cubic feet per minute per ton of potatoes, it does not increase the moisture loss per unit time from healthy potatoes. The high airflow capacity is not always required but is valuable for early control of storage rot (drying up ‘leakers’). It could be compared to a four-wheel drive truck. There are times when the four-wheel drive is very helpful to assure travel and other times it is not needed.”

While there’s much more to proper potato storage than you may think, Mary LeMere sums up the key principles for stored potatoes as follows:

  • Handle them like they are your children.
  • Keep an eye on them like they are going to get in trouble.
  • Hope for your just rewards when you send them out the door.

The only advantage with children is that they are more likely to thank you for your efforts when they grow up; well, if you’re lucky.

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