Many farms near Kentucky State University didn't get electricity until the mid-1940s. A few farmers in the region still remember the strategies their families used to store food without freezers or refrigerators.
Times have changed. Kentucky now has some of the cheapest electricity in the country, and we use more of it than most other Americans. Almost all of it comes from coal-fired power plants. As a result, Kentucky releases 4% of the country's greenhouse gasses, but accounts for just 1.3% of the population. Lexington, the nearest urban center to Frankfort, has the largest per capita carbon footprint of the nation's 100 biggest cities.
Kentucky State University hosts an annual conference in November for limited resource and minority farmers. During a lunch break a few of the conference participants ventured out into the cold to guide construction of a traditional sweet potato storage pit. Cured sweet potatoes were arranged in a well-drained pit (~3 feet deep) on a thick bed of straw. Layers of sweet potatoes were separated by layers of straw, like a giant lasagna underground. The final layer of straw was covered by a thick (~1.5 foot) layer of soil and a tarp to keep the potatoes from freezing (too cold) or sprouting (too wet). As we worked, some of the farmers reminisced about other strategies their families used to store food without refrigeration.
We used thermocouples attached to a datalogger to continuously monitor temperatures at the soil surface and at all layers of the pit from December through March. Over the course of the winter we removed sweet potatoes periodically to serve at university meals and to assess their condition.
Daily average temperature in a sweet potato storage pit (orange) and surrounding air (blue), December 2008 through March 2009. Blue bars show daily air temperature range. Yellow area shows ideal range for sweet potato storage.
The pit temperatures stayed in the ideal range — between 13 and 16 °C (55-60 °F) — through December. They fell a little below optimum in January, returned to the ideal range in February, and got a little warmer than ideal in March. Air temperature routinely dipped below freezing during the storage period, but pit temperature stayed well above freezing.
We collected the rest of the sweet potatoes in late March, to start slips for the 2009 crop. By the final collection most of the sweet potatoes were rotting, leaving about 25% for slips. Rot appeared to be associated with excessive moisture in the pit. The tarp that covered the pit was not enough to keep it dry during the spring rains.
The thermal mass of the soil kept storage temperatures near optimal for sweet potatoes, but digging the sweet potatoes up whenever we wanted to serve them was a lot of work. Our experience convinced us that we need a proper root cellar on the land, to keep moisture out and make collection easier.
Sweet potato storage pits have been used for centuries in South and Central America, Polynesia, and Africa. The size, shape, and structure of sweet potato storage pits typically used in the American South is very similar to that of pits used in West Africa, suggesting an African origin for the method we used.
Updated April 17, 2009