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Want to make your sweet potatoes even sweeter? Try curing them

The other day I was at the Horticulture Research Farm talking with Corey Dillon and Merle Barto about harvesting Dr. Luis Duque’s sweet potato research plots. Duque is investigating different colored sweet potato clones to see if he can produce small, colored sweet potatoes for the specialty market. That led to a discussion about curing sweet potatoes.

Most of the commercial acreage of sweet potatoes is located in the southeastern part of the United States. While at North Carolina State University, I had the opportunity to visit several large sweet potato growers whose sweet potatoes can probably be found on the shelves of our stores in State College.

The sweet potato, Ipomoea batatas, is a tender, warm-weather vegetable that requires a long frost-free growing season to mature large, useful roots. That is why in the Northeast we recommend using black plastic mulch and drip irrigation to grow sweet potatoes in the gardens and is the system that Duque used at the Horticulture Research Farm. Sweet potato is native to Central and South America and is also a rich source of vitamin A.

Though orange-fleshed varieties are most common today, white or very light yellow-fleshed types were once considered the finest types for sophisticated people. Some white-fleshed types are still available, though they may be hard to find outside the Deep South.

The sweet potato is not related to the yam, though in the marketplace the two names are often used interchangeably. The true yam, Dioscorea sp., is an entirely separate species that grows only in the tropics. This situation is similar to the use of the terms muskmelon and cantaloupe. We only grow muskmelons in our gardens.

Sweet potatoes are warm-season plants that are very sensitive to cold temperatures. The tuberous roots should be harvested by the time frost kills the vines or soon thereafter. Sweet potato roots continue to grow until frost kills the vines. Roots can be left in the ground for a short while; however, an extremely hard frost can cause damage to roots near the surface. Chilling injury also results to roots when soil temperatures drop to 50 degrees or lower, and this can result in internal decay in storage. The greatest danger from delayed digging is the risk of cold, wet soil encouraging decay of the roots.

Depending on how early you were able to plant, you may find an assortment of “baby baker” or smaller roots, as well as full-size potatoes. Although you can cook newly dug sweet potatoes right away, their flavor and storage quality is greatly improved by curing at warm temperatures first. It is during the curing process that starch is converted to sugar.

Care should be taken during digging and handling to avoid skinning and bruising the roots. Even a small wound can easily become infected with decay organisms. Line containers with rags or other soft material, if possible, to avoid scratching the roots. Do not store badly injured or diseased roots. You do not want large amounts of soil clinging to roots during storage but be careful in removing the soil as sweet potatoes are easily damaged during the washing process when freshly dug. It is recommended to allow roots to dry and cure before removing excess soil.

Sweet potatoes can be cured by holding them for about 10 days at 80-85 degrees and high relative humidity (85-90 percent). In the absence of better facilities, they can be cured near a furnace to provide warmth. If the temperature near your furnace is between 65-75 degrees, the curing period should last two to three weeks. To maintain the required high humidity (85-90 percent relative humidity), stack storage crates or boxes and cover them with paper or heavy cloth. Packing in perforated plastic bags will also keep humidity high, yet the perforations will allow excess moisture to escape.

Once the sweet potatoes are cured, move them to a dark location where a temperature of about 55-60 degrees can be maintained during storage. Sweet potatoes are subject to chilling injury, so keep them out of the refrigerator. Wrapping cured sweet potatoes in newspaper and storing them in a cool closet can obtain good results. Then sit back and enjoy the great nutritional value of sweet potatoes and also some excellent sweet potato pies. I am looking forward to the results of Duque’s research.

Bill Lamont is a professor emeritus in the department of plant science at Penn State and can be reached by email at wlamont@psu.edu.