Sweet potato storage

The sweet potato (Ipomoea batatas) is a very important crop for subsistence farmers in Africa and other developing countries. Due to its relatively short growing period, tolerance to drought and high yield from poor soils, it is used 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. As such they are not generally stored for extended periods of time following harvest. This is a major barrier for the optimal use of the crop and causes a lot of waste. The sweet potato (Ipomoea batatas) is a very important crop for subsistence farmers in Africa and other developing countries. Due to its relatively short growing period, tolerance to drought and high yield from poor soils, it is used 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. As such they are not generally stored for extended periods of time following harvest. This is a major barrier for the optimal use of the crop and causes a lot of waste. The majority of subsistence farmers who eat sweet potato do not utilize 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 in subsistence farmers. Pretreatment of sweet potato can help to minimize risk of losses. Curing can toughen the skin and heal minor physical damages while drying can reduce spoilage and inactivate metabolic degradation. Optimum storage of sweet potatoes occurs at 12–16 °C (54–61 °F), 85–90% relative humidity (RH) and requires proper ventilation to remove excess carbon dioxide (CO2) and bring in oxygen (O2) 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. 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, pests and environmental means. Physical losses refer to sweet potatoes damaged by mechanical force during harvest, handling, and transportation of the tubers. This includes injury by way of cuts, bruising, and skinning. Delicate skin and poor harvesting and handling techniques are the main cause of these issues. Damage to tubers by physical means can be minimized careful handling and storing in boxes and cartons instead of cloth sacks. 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 to occur after harvesting and a buildup of CO2 around the crop can cause early spoilage. Sprouting will also occur during long term storage, which is useful for planting but will utilize nutrients in the tuber. Sprouting can be inhibited by preharvest spraying with maleic hydrazide, treatment of tubers with methyl ester of napthalene acetic acid (MENA) in acetone on paper spread between tubers and 0.5–4.0% thiourea solution. Sweet potatoes are susceptible to a number of fungi, bacteria and viruses. Infection can cause the production of toxins, pathogenic bacteria, bitter flavours, tissue breakdown, and surface blemishes. Two common sources of infection are from 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.). This can enter the tuber prior to harvesting and proliferate in storage and has been known to cause up to 50% loss in yields. Simple techniques can be used to minimize the threat of the sweet potato weevil. It has been shown that storage of the potatoes in temperatures under 20 °C (68 °F) can kill up to 89.5% of weevils in an infected crop. Dipping of tubers in hot water also been shown as an effective intervention to control infestations . Damage to tuber by physical and physiological means can increase chance of pathological infection and pests. The length of time that sweet potatoes may be stored is highly dependent on temperature and relative humidity. Storage in temperatures less than 10 °C (50 °F) will cause chill injury in sweet potatoes. This is characterized by discoloration of the insides of the tuber, which can range from brown to black instead of the usual orange. Formation of hard areas also occurs that remain even after cooking. Other signs of chill injury include production of off flavours, internal cell structure breakdown and increased rotting due to reduced skin formation, which allows fungal infection. On the other end of the scale, temperatures above 16 °C (61 °F) will cause a rise in the rates of respiration and sprouting in tubers as well as increased activity by microorganisms. Relative humidity under 80% will cause excessive water loss while RH above 90% will cause condensation to form, which accelerates rotting.