Sustainable Practices for Vegetable Production in the South
Dr. Mary Peet, NCSU
Home

Sweetpotato

HARVEST AND POST-HARVEST

Storage roots continue to grow until the leaves are killed by frost. Depending on the cultivar, roots develop to marketable size in 90 to 150 days after transplanting. Time of harvest is often determined by digging up a few representative plants and determining the percentage of roots in the size classes listed below. Normally, harvest begins when most of the roots are in the No.1 size class, as No.1 sweetpotatoes command the highest price. In harvest systems where the vines would tangle in the harvester, vines are cut immediately before harvest. Vine killing in hot, wet weather and/or in poorly drained soils may result in anaerobic conditions and subsequent souring of roots either in the ground or in storage, so roots should not be left in the ground for long periods after the vines are killed.

Sweetpotatoes are very susceptible to damage at harvest. Harvested roots left in the sun at temperatures above 90 degrees F sunscald in 30 minutes. Scalded areas turn purplish-brown and are more susceptible to storage rots. In very dry soil, the root periderm or outer layer of skin becomes more fragile and easily abraded or 'skinned' on the hard soil clods during harvest. Sweetpotatoes do not have a thick protective outer layer of cells such as that on white potato tubers. Any abrasion can lead to rots in storage. Skinning injury in dry soil can be avoided either by waiting for rain or by irrigating the field before harvest. Rough handling by workers also tears the delicate skin. Workers should wear gloves and avoid dumping the roots roughly into containers.

Skinned areas can become dark and sunken and surrounded by a narrow brown border. These scars offer opportunities for storage rot pathogens such as Fusarium to enter the root. Curing roots (see below) after harvest allows the periderm to reform, reducing subsequent storage damage. Skinning also takes place in packing and shipping to markets so packing lines should be designed to reduce injury.

Harvest must take place before a killing frost because if vegetation is killed by the frost, roots left in wet soil may sour. Light frosts are not serious unless soil temperatures fall below 50 to 55 degrees F for several hours. If storage roots are exposed to temperatures below 40 degrees F overnight, internal breakdown may also occur in storage.

To harvest, the field is usually plowed with a modified disk or moldboard plow with a spiral attachment. Roots are then hand harvested and graded in the field.

Sweetpotatoes can also be dug by a chain digger or a riding harvester which conveys the roots to a sorting crew using a harvest aide. Potato harvesters are sometimes used to harvest sweetpotatoes but damage is usually unacceptably high.

After harvest, the sweetpotatoes are transferred to 20-or-40-bushel bulk containers which hold about 1000 pounds. Approximately 15 to 20 percent of the 1993 North Carolina crop was mechanically dug and transferred directly to bulk bins.

Curing and Storage

A small portion (15 to 20 percent) of the marketable roots are washed, graded and packed within a few days of harvest and immediately shipped to the buyers. Such roots are referred to as 'green' and are usually not as sweet as cured sweetpotatoes. Most jumbo and No.1 roots are cured immediately after harvest to improve flavor and storage life. Curing heals cuts and reduces decay and shrinkage in storage because it allows the periderm to thicken and to reform. Curing also converts some starches to sugars, enhancing flavor.

Curing should be started within 1 to 2 hours of harvest and continued for 4 to 7 days at 85 degrees F and 90 to 95 percent relative humidity with periodic venting. Earlier in the season, soil and air temperatures are warm, and the roots will cure in a shorter time than later in the season when the roots start out cooler. Sweetpotatoes are normally stored in bulk containers, holding from 20 to 60 bushels.

Storage temperatures are very important. The storage area should be maintained at 55 to 60 degrees F and 85 to 90 percent relative humidity with sufficient venting to produce a total volume change of air at least once a day. Above 60 degrees F, internal breakdown, shrinking and sprouting can occur. Temperatures below 55 degrees F may cause hardcore, a disorder where a whitish, hard area appears in the cooked sweetpotato. Properly cured and stored sweetpotatoes can be held up to 10 months with little reduction in quality. Shrinkage occurs at 1 to 2 percent per month if cured, 2 to 5 percent if uncured. In some cultivars, pithiness also increases with length of storage.

When brought out of storage for marketing, the bruising the roots suffer as they are moved out of the bins, dumped in washing vats, graded, and packaged re-exposes them to rot organisms which penetrate through surface injuries. Wash water is usually chlorinated to kill spores of rot fungi, but this wash does not protect against subsequently introduced pathogens. When registered products are available, a fungicide such as Botran is also used commercially to reduce postharvest rots.