A small portion (15 to 20 %) 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 sweet potatoes. Most 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 80 to 85 degrees and 90 to 95 % relative humidity with ample ventilation for about 5 days. Rooms with 100 % relative humidity should be avoided so that the surface of the sweet potatoes will not be completely wet, resulting in more disease. Earlier in the season, when the soil and air temperatures are higher, the roots will cure in a shorter time than later in the season when the roots start out cooler. Sweet potatoes are normally stored in bulk containers that hold from 20 to 60 bushels.
Storage temperatures are very important. Long-term storage areas should be maintained at 55 to 60 degrees with 85 % relative humidity and with sufficient venting to produce a total volume change of air at least once a day. Above 60 degrees, internal breakdown, shrinking and sprouting can occur. Temperatures below 55 degrees may cause hardcore, a disorder where a whitish, hard area appears in the cooked sweet potato. Properly cured and stored sweet potatoes can be held up to 12 months with little reduction in quality. Shrinkage occurs at 1 to 2 % per month if cured, 2 to 5 % if uncured. In some cultivars, pithiness also increases with length of storage.