Sweet potato storage

The sweet potato (Ipomoea batatas) is a very important crop for subsistence farmers in Africa and other developing countries.[1][2] 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.[1][2] However, it is a highly perishable food source that is susceptible to destruction by microorganisms, metabolic spoilage, physical destruction and pests.[3] As such they are not generally stored for extended periods of time following harvest.[4] This is a major barrier for the optimal use of the crop and causes a lot of waste.[3]

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.[5][6] 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.[3][7] Maintaining proper storage temperature is probably the most significant barrier to good storage in subsistence farmers.[3]

Pretreatment of sweet potato can help to minimize risk of losses.[3] Curing can toughen the skin and heal minor physical damages while drying can reduce spoilage and inactivate metabolic degradation.[3]

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.[3] At these conditions sweet potatoes have been shown to last 5 months to a maximum of a year compared to 2–3 months normally.[3]

Susceptibility to spoilage

Sweet potatoes are a highly perishable crop and difficult to store for extended periods of time.[4] This is mainly due to their high moisture content, metabolic activity following harvesting and thin, permeable skin.[3] Following harvest, sweet potatoes are susceptible to spoilage by physical, physiological, pathological, pests and environmental means.

Physical

Physical losses refer to sweet potatoes damaged by mechanical force during harvest, handling, and transportation of the tubers.[3][8] 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.[3] Damage to tubers by physical means can be minimized careful handling and storing in boxes and cartons instead of cloth sacks.[3]

Physiological

Physiological processes within the plant can contribute highly to spoilage. Natural transpiration causes water loss and results in an unappealing pithy texture.[3] 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.[3]

Pathological and pests

Further information: List of sweet potato diseases

Sweet potatoes are susceptible to a number of fungi, bacteria and viruses.[3] Infection can cause the production of toxins, pathogenic bacteria, bitter flavours, tissue breakdown, and surface blemishes.[3] 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.[3][9] 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.[3] Dipping of tubers in hot water also been shown as an effective intervention to control infestations .[3] Damage to tuber by physical and physiological means can increase chance of pathological infection and pests.[3]

Environmental

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.[3] This is characterized by discoloration of the insides of the tuber, which can range from brown to black instead of the usual orange.[3] Formation of hard areas also occurs that remain even after cooking.[3] 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.[3] 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.[3] Relative humidity under 80% will cause excessive water loss while RH above 90% will cause condensation to form, which accelerates rotting.[3]

Ideal storage conditions

To minimize potential spoilage, precise storage conditions should be met. Sweet potatoes should be stored at between 12 and 16 °C (54 and 61 °F) with a RH of 85-90%.[3][8] Proper ventilation of the storage area is a key factor in maximizing potential storage time. Respiration causes the production of CO2, which will act to increase the rate of spoilage.[3] Sweet potatoes can convert approximately 57dm3 of oxygen/ton/day to carbon dioxide and need ventilation to compensate for this.[3] The O2 in the storage environment should not fall below 7% and CO2 must not exceed 10%. Due to lack of proper equipment, maintaining adequate temperature conditions are the largest hurdle that farmers in a developing nation will have to face.[3]

Not all sweet potatoes are suitable for storage. Tubers that have been mechanically damaged, infected with pests or pathogens, or are rotten or sprouting should not be kept.[10] These factors will increase rate of spoilage and can increase losses in the rest of the stored crop.[3] Under these optimum conditions sweet potatoes have been shown to keep for 5 months up to a max of about a year.[8]

Storage methods

In developed countries proper refrigeration technology is used to keep sweet potatoes for long periods of time. Many poor farmers do not have access to refrigeration or even electricity so must rely on other storage methods. There are several methods of storage that only require simple and cheap building materials. The most commonly used are pit storage, clamp storage and indoor storage. These techniques are used only sporadically however as most farmers practice sequential harvesting in which tubers are left in-ground until needed.[4]

In-ground

In subsistence farming, sweet potatoes are commonly left in the ground and eaten or sold directly following harvest; this is called piece-meal or sequential harvesting.[11] Sweet potatoes are delicate and easily damaged.[3][4][8] In-ground storage is used to protect the tubers while reducing the work required to set up storage facilities. In areas with reliable rainfall patterns farmers are able to maintain a supply of fresh sweet potatoes for most of the year by continuous cultivation and harvesting.[3] In drier places, however, such as northern Uganda, this is not a practical solution.[8] This is not a long-term option and the plants will degrade after a short period of time.[3] There are numerous disadvantages to this method as it takes up field space that could otherwise be used for growing more crops.[3][8]

Pit storage

This is a simple, cheap method that involves digging a hole in the ground for storing the potatoes. Pit storage differs from in-ground as tubers are collected and kept together and considerations are made to control the storage environment.

Construction of the pit can vary according to what materials are available but commonly used are; grass, soil, wood, lime, sawdust and ash.[3][4][12] Grass is typically used to line the bottom and sides of the pit in order to insulate against temperature change and absorb moisture.[12] Before usage, grass should be flamed in order to destroy any pests that may be hiding within.[3] Soil is used to seal the roof of the pit and as filler.[3] Wood and plant material can be used to strengthen walls as well as create a roof covering the pit.[3] Roofs built over the pit structure are beneficial in that they can help keep rain out and provide shade to lower temperature.[3] Lime may have some effectiveness in absorbing CO2 and removing it form the environment.[3] Sawdust is used as a cushioning material and to help control condensation on the tubers.[12] Wood ash can applied to potatoes prior to storage and has shown some effectiveness in protecting against insect attacks and mould.[3][12]

In order to prevent rotting the pit should ideally be dug into a slope to promote drainage. It should also be situated at least 0.35 metres (1.1 ft) above the water table during dry season and must remain above during wet/rainy season and be approximately 0.5 metres (1.6 ft) below the surface.[3]

There can be ventilation problems with this storage method.[3] Commonly the pit will be sealed shut in order to maintain a good RH. The draw back to this is that CO2 will collect and cause spoilage.[3] To help this, a sizable headspace should be maintained in the pit to help promote airflow.[3]

Pits can be reused but they should be cleaned, the soil turned over and disinfected with fire or sulphur to rid the area of any microorganisms.[3]

Losses are still fairly high for this technique, though it has been shown in some instances to keep roots for 2–4 months.[3][12]

Clamp and mound storage

Another simple and low-cost method, clamp storage consists of covered piles of sweet potatoes.[3] After selecting the tubers in the best condition, they are stacked in a heap on a layer of grass and covered in layers of grass and soil.[3] As with pit storage, ash, lime and sawdust can be used for added effect. The piles may be made at ground level or in shallow or deep trenches.[3] Drainage should be considered and ruts may be made in the ground to lead off water.[3] The clamps may be covered by a roof or kept in a building for added protection.[3] To minimize losses due to respiration a ventilation shaft can be added.[3] Results are fairly poor with this technique and estimated storage time is 2–3 months.[3]

Indoor storage

Sweet potatoes may be harvested and stored in building. This could include in the living area or in a granary built specifically to store produce.[3] In home storage is typically done in straw woven baskets, cloth bags or wooden boxes.[3] Baskets and boxes have been shown to be more effective at minimizing mechanical damage.[3] If possible tubers should be kept off of the ground to keep them away from rodents and other pests.[3] This is an effective technique for maintaining proper ventilation though depending on the type of building maintaining proper storage temperature and RH may be difficult.

Granaries or other storage buildings typically consist of a round hut with walls made of straw, mud, clay and wood and a conical straw roof.[3][7] These are commonly supported above ground by a system of legs to keep the crop dry and away from animals, rodents and pests.[3][7]

Pretreatment of sweet potatoes

Typically sweet potatoes are stored and eaten fresh.[3][12] However, there are some simple methods used to increase their storability that can be used in conjunction with other storage methods. Drying and curing of sweet potatoes are two common methods to prepare them prior to storage.[3][11]

Drying of tubers is done on those that are too damaged to be stored fresh but still have edible material on them.[3] This involves slicing tubers to a thickness of approximately 2–4 mm and then laying them out in the sun for four days or until they are rid of most of their moisture.[3] During drying the potatoes can be covered in prickly bushes or thorns to ward off animals.[3] Dried slices can be kept in-doors or in raised silos until eaten.[3] Drying removes moisture, reduces bacterial growth, and inactivates metabolic processes and enzymatic decomposition.[3]

Curing is a technique done to toughen up the outer layer of skin (periderm).[3] It is a measure that prevents against excessive moisture loss, entry of microorganisms into the plant and even facilitates healing of mechanically damaged tubers.[1][3][8][13] Curing is done by exposing the whole potatoes to a moderately high temperature for several days immediately following harvest.;[1][3][8][13] 30–32 °C (86–90 °F) and 80–95% RH for 4–10 days has been found to be adequate curing conditions [3] A common method to cure without use of incubation equipment is to wrap tubers in black polythene sheets and leave out in the sun for 5 days.[3] The black sheet keeps moisture in and collects heat to reach necessary conditions for curing.[3] Immediately following curing the temperature must be dropped quickly in order to avoid sprouting damage.[3]

References

1. Ewell, P. 1993. Sweetpotato in Africa : Research priorities to stimulate increased marketing. Paper presented at the International Workshop on Methods for Agricultural Marketing Research, 16–20 March 1993, IARI Campus, New Delhi, India
2. Bashaasha, B., Mwanga, R.O.M., and Ocitti p’Obwoya, C.N. 1993. Sweet potato in the farming and food systems of Uganda. A farm survey report. Kampala, Uganda
3. Devereau AD. 1994. Tropical sweet potato storage: A literature review. Report. Natural Resources Institute, Chatham
4. Karuri, E.G. and Ojijo, N.K.O. 1994. Storage studies on sweet potato roots : Experiences with KSP20 cultivar. Acta Horticulturae 368: 441-452
5. Onwueme, I.C. 1982. The tropical tuber crops: Yams, cassava, sweetpotato, and cocoyams. English Language Book Society and John Wiley and Sons. Chichester, Britain
6. Smit, N.E. and Ocitti p'Obwoya, C.N. 1994. Piecemeal harvesting of sweetpotato: Its effect on yield and yield loss due to sweetpotato weevils. Paper presented at the third triennial conference of the African Potato Association, 9–13 May 1994, Sousse, Tunisia
7. Dandago MA, and Gungula DT. (2011). Effects of various storage methods on the quality and nutritional composition of sweet potato (Ipomea batatas L.) in Yola Nigeria. International Food Research Journal 18:271-278
8. Hall AJ and Devereau D. (2000). Low-cost storage of fresh sweet potatoes in Uganda: Lessons from participatory and on-station approaches to technology choice and adaptive testing. Outlook on Agriculture. 29(4):275-282
9. Ndamage, G. 1988. Developpement et amelioration de la production de la patate douce au Rwanda. In: Seminaire-atelier sur la production, la conservation, la transformation et l'utilisation des racines et tubercules; UNICEF-IWACU, 8–12 February, Kigali, Rwanda
10. Ramirez P. (1992). Cultivation harvesting and storage of sweet potato products. In: Roots, tubers, plantains and bananas in animal feeding. (Eds Machin, D.; Nyvold, S.), Proceedings of the FAO Expert Consultation held in CIAT, Cali, Colombia 21–25 January 1991; FAO Animal Production and Health Paper - 95
11. Karuri, E.G. and Hagenimana, V. (1995). Use of ambient conditions and sawdust in storage of sweet potato (Ipomoea batatas L.) roots in Kenya. Zimbabwe Journal of Agricultural Research. 33(1):83-91
12. Mutandwa E, and Gadzirayi CT. 2007. Comparative assessment of indigenous methods of sweet potato preservation among smallholder farmers: Case of grass, ash and soil based approaches in Zimbabwe. African Studies Quarterly 9, no. 3: [online] URL: http://web.africa.ufl.edu/asq/v9/v9i3a4.htm
13. Kapinga RE, Rees D, Westby A, Ndunguru GT, Rwiza E, Tomlins KI, Stathers T, Jeremiah S, Mbilinyi L. 2003. Increasing the contribution of sweetpotato to sustainable rural livelihoods in Tanzania. Proceedings of the Twelfth Symposium of The International Society for Tropical Root Crops: Potential of Root Crops for Food and Industrial Resources, Tsuk, pp. 285-291