The sweet potato (Ipomoea batatas) is a very important crop for subsistence farmers in Africa and developing countries in other regions. Its relatively short growing period, tolerance to drought and high yield from poor soils lead to its use as a famine reserve for many of these households. However, it is a highly perishable food source that is susceptible to destruction by microorganisms, metabolic spoilage, physical destruction and pests. Therefore, it is not generally stored for long after harvest. This is a major barrier for the optimal use of the crop and causes much waste.
Most subsistence farmers who eat sweet potato do not use a storage technique of any kind, instead using a method of continuous cultivation and leaving the tubers in the ground until they are ready to be used. While less common, simple methods of storage do exist. The common ones include pit, clamp, and indoor storage and all extend the storage time of sweet potatoes by some degree. Maintaining proper storage temperature is probably the most significant barrier to good storage for subsistence farmers.
Pretreatment of sweet potato can help to minimize risk of losses. Curing can toughen the skin and heal minor physical damage, while drying can reduce spoilage and inactivate metabolic degradation.
Optimum storage of sweet potatoes occurs at and 85–90% relative humidity (RH) and requires proper ventilation to remove excess carbon dioxide () and bring in oxygen () for respiration. At these conditions sweet potatoes have been shown to last 5 months to a maximum of a year compared to 2–3 months normally.
Susceptibility to spoilage
Sweet potatoes are a highly perishable crop and difficult to store for extended periods of time. This is mainly due to their high moisture content, metabolic activity following harvesting and thin, permeable skin. Following harvest, sweet potatoes are susceptible to spoilage by physical, physiological, pathological and environmental means, including by pests.
Physical
Physical losses occur when sweet potatoes are damaged by mechanical force during harvest, handling, and transportation of the tubers, including injury by cutting, bruising, and skinning. Delicate skin and poor harvesting and handling techniques are the main cause of these problems. Damage to tubers by physical means can be minimized by handling the tubers carefully and storing them in rigid boxes or cartons rather than cloth sacks.
Physiological
Physiological processes within the plant can contribute highly to spoilage. Natural transpiration causes water loss and results in an unappealing pithy texture. Respiration also continues after harvesting, and a buildup of around the crop can cause early spoilage. During long-term storage, tubers sprout, which is useful if they are to be planted but not if they are to be eaten, as sprouting consumes nutrients in the tubers. Sprouting can be inhibited by preharvest spraying with maleic hydrazide, treatment of tubers with methyl ester of naphthalene acetic acid (MENA) in acetone on paper spread between tubers and 0.5–4.0% thiourea solution.
Pathological and pests
Sweet potatoes are susceptible to a number of species of fungi, bacteria and virus. Infection can result in the production of toxins, pathogens, bitter flavours, surface blemishes, and decayed tissue. Two common sources of infection are the fungus Rhizopus oryzae (soft rot) and Botryodiplodia (Java black rot). These are responsible for 78% of sweet potato infections in Bangladesh. The pest of greatest concern for farmers is the sweet potato weevil (Cylas spp.), which can enter the tuber prior to harvesting, proliferate in storage, and cause up to 50% loss in yields. Simple techniques can be used to minimize the threat of the sweet potato weevil. Storage of the potatoes in temperatures under can kill up to 89.5% of weevils in an infected crop. Dipping the tubers in hot water also has been shown as an effective intervention to control infestations. Damage to tubers by physical and physiological means can increase the chance of pathological infection and pest infestation.