Pictured above: Pallet bins of sweetpotatoes stacked in a warehouse.
October 2008
North Carolina sweetpotatoes are typically harvested in October, but the sweetpotatoes pulled from the ground in the fall of one year may not turn up in grocery stores until as late as August of the following year.
That’s because of sweetpotato storage technology developed in the College of Agriculture and Life Sciences at North Carolina State University. Dr. Mike Boyette, Philip Morris Professor of Biological and Agricultural Engineering, has been working on sweetpotato curing and storage technology since the late 1980s and is largely responsible for developing the technology that is used today not only in North Carolina but around the world.
A sectional drawing of a sweetpotato warehouse. The cavity on the right of the building is the plenum, Arrows indicate air circulation.
Long-term storage is an advantage to sweetpotato growers because it allows them to sell their sweetpotatoes when they want. They can take advantage of market windows when prices are higher.
Prior to 1990, North Carolina sweetpotato growers were storing their sweetpotatoes in buildings designated for that purpose, but the system growers used to try to control temperature and humidity, which determine how long sweetpotatoes can be stored, didn’t work well, and sweetpotatoes could be stored no more than a few months, until the next January or February.
Boyette recalls visiting a storage building in the late 1980s where the sweetpotatoes stored at the top of the building had sprouted while those at the bottom had not.
That Boyette says, told him the temperature was not consistent throughout the building.
“It’s hot up there,” he explains. “The sweetpotato thinks its spring time, time to grow.”
Dr. Mike Boyette
At about that time, Boyette took a course at the University of California – Davis on postharvest handling of fruits and vegetables. While in California, he met Dr. Jim Thompson, a Cal-Davis scientist who had been working on forced-air cooling of fruits such as peaches and plums stored in pallet bins. Pallet bins are wooden boxes attached to a pallet so that the box can be moved easily with a forklift. Sweetpotatoes are typically stored in pallet bins.
Thompson suggested Boyette confer with Dr. Robert Pringle, a Scottish scientist working on storage methods for Irish potatoes. Boyette corresponded with Pringle and, using what he had learned from Thompson and Pringle, came up with a plan to more evenly control temperature and humidity in sweetpotato storage buildings, thus extending storage life.
Boyette put that plan into action in 1990, thanks to Tony Johnson, a sweetpotato grower in Benson. Johnson had an older building he wanted to renovate for sweetpotato storage and agreed to act as a guinea pig.
Boyette’s design called for an open space at one end of the building called a plenum. The building was equipped with fans to draw air through the plenum. The pallet bins in which sweetpotatoes were stored were stacked so that the spaces where the pallets would be picked up by a forklift served as channels through which air flowed. The design allowed air to circulate more evenly and thoroughly through the building, and the sweetpotatoes could be stored longer.
The system has been improved considerably over the years. In Boyette’s original design, the plenum was defined by an exterior wall and a parallel interior wall. Boyette soon realized that most of the interior wall wasn’t necessary. It could be formed by the stacked pallet bins of sweetpoatotes. A short section of wall is left hanging from the ceiling. This piece of wall holds the fans that circulate air through the building. Boyette improved the efficiency of early storage building designs by positioning fans so that they pull air through the building rather than push it.
Sweetpotato storage buildings today are usually equipped with sophisticated sensors that measure and control temperature and humidity.
After harvest, sweetpotatoes are typically cured for a about a week at 85 percent relative humidity and 85 degrees F. Curing coverts some of the starch in a sweetpotatoe to sugars, changing the taste slightly. Curing also allows any cuts or bruises to heal, which lessens the chance of infection and prolongs storage. Once cured, the temperature is dropped to 59 degrees F, while the relative humidity remains at 85 percent.
How effective is the system that most growers now use. Where the pack-out rate (the percentage of sweetpotatoes that come out of storage buildings) used to be around 65 percent, it is now 90 to 95 percent.
If you’re a sweetpotato grower, that’s a sweet story.
- Dave Caldwell
