Sweet Potato

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Sweet Potato Links

• GPPIS (FAO)
• AgroIndia

See review in Kotecha and Kadam (1998).

Species: Ipomea batatas

Sweet potato is thought to have originated in southern Central America. It was dispersed through the Pacific region by early Polynesian sea voyages arriving in New Zealand with Maori settlement. Early trading vessels disseminated sweet potato throughout the world. Sweet potato has long been a very important food crop to many peoples and despite advances in plant breeding of cereal crops, remains the seventh most important food crop in the world and fourth in the tropics. Sweet potato was introduced into Australia relatively recently by British colonists while the popular dessert type was first imported as recently as the early 1970's. Only since then has the domestic sweet potato industry developed to its current value of $10-12 million annually. The many attributes and uses of sweet potato have not yet been exploited in Australia (Lester Loader 2000, pers. comm.).

Research and development of the industry in Australia is very recent and limited primarily to plant introduction and evaluation and some aspects of agronomy including plant nutrition. There is a need for a better understanding of specific product requirement and feasibility of industrial uses. Post harvest handling such as curing, semi processing and packaging, including canning, have not been seriously attempted in Australia but would lead to greater consumer convenience, reduced perishability and increased consumption. Efficiencies in production could be achieved with improved mechanisation and handling (Lester Loader 2000, pers. comm.).

Varieties

Hundreds of varieties are grown throughout the world, with a great variety of tuber shape, size, colour of flesh and skin, and texture. Flesh colour varies from white to deep orange and purple (Chew and Morgan 1999).

Vong looking at variety with red skin, white flesh.

Agronomic trials

Lester Loader (Qld) has an ongoing project to assess new varieties, which he does at Gatton, Redlands, Bundaburg, Mareeba and occasionally at other locations. Mainly orange fleshed varieties originating from Louisiana trials. Peter McLaughlin (NSW) is trialling one variety suitable for export to Japan.

Production

Season: The crop requires 4-5 frost free months to grow tubers to full size (Splittstoesser 1990). It is a tropical perennial that is often grown as an annual (Chew and Morgan 1999).

Soil type: Fertile, loose, sandy and crumbly soil is preferable. Heavy or wet soils lead to long and stringy roots (Splittstoesser 1990).

Soil preparation: Soil is often formed into ridges 20-40 cm high and 90-120 cm apart (Splittstoesser 1990). The crop is fairly intolerant to shade (Johnston and Onwueme 1998).

Propagation: Grown from sprouts or stem cuttings (Chew and Morgan 1999).

Plant spacing: Spaced at 30-40 cm between plants (Chew and Morgan 1999).

Glasshouse: Use of charcoal with pyroligneous acid can increase tuber yield by promoting root growth and photosynthesis (Dubey and Ganguli 1998).

Water: Irrigation is essential for maximum yield (Chew and Morgan 1999).

Weeds: It is important to keep the crop weed free for about 30-45 days after planting (Nedunzhiyan et al. 1998).

Harvest: Sweet potato is ready for harvest when the leaves begin to yellow and when the cut surface of the tuber does not yellow (FAO 1989). Irrigate the field before harvest to soften the soil (Chew and Morgan 1999).

Yields of over 30 t/ha were reported in Hong Kong and Israel in 1997, but most developed countries averaged 17-24 t/ha (FAO 1998). Japanese yields have remained at 23-24 t/ha since 1985 (Figure 2).

Figure 1: World production of sweet potato 1990-97 (FAO 1998).

Postharvest

Curing: Tubers should be cured at 27-34°C, 85-95% relative humidity and restricted ventilation for 5-20 days before put into longer term storage. This enables new skin to be formed over harvesting injuries and the whole tuber to mature. Curing is complete when the skin cannot be rubbed off and small buds appear (FAO 1989). Optimal curing conditions vary with cultivar. If decaying still prevails, iprodione (a fungicide) can be applied in conjunction with curing, preferably via super fine mist (Afek et al. 1998).

Temperature: Store at 10-15°C, preferably closer to 10°C. Higher temperatures increase the frequency of sprouting, and lower temperatures cause chilling injury (FAO 1989). The tubers are injured by just one light freezing (Welby and McGregor 1997).

Relative humidity: Optimum is 85-90% as the tuber has a medium susceptibility to moisture loss (Welby and McGregor 1997).

Storage life: Up to 4 months (FAO 1989).

Quality assessment

Level of carotenoids, and hence vitamin A, is strongly linked to colour. Drying and/or storing sweet potatoes reduces carotenoid content, but this effect is small compared to the initial carotenoid content (Hagenimana et al. 1999).

Pests and diseases

Potential viruses are listed at Plant viruses online (Brunt et al. 1996). See Hawaiian pests & diseases.

Domestic market

Australian annual per capita consumption of sweet potato is only 0.5 kg while in the USA it is approximately 3 kg. In China it is estimated to be as high as 100 kg (60 kg fresh plus 40 kg via stock) and up to 200 kg in parts of Papua New Guinea. Consumption in Australia is increasing and is supported by strong demand from our increasing ethnic Asian and Islander population to whom it is a traditional food (Lester Loader 2000, pers. comm.). Domestic market has declined because supermarkets aren't paying enough - price fell due to increase in production in Bundaberg and Atherton (Kevin Blackburn 1999, pers. comm.).

Export markets

(click here for exchange rates).

111 countries were producing sweet potato commercially in 1997, but the largest producer by far was China (Figure 1), which had 6.5 million ha planted in 1994 (FAO 1998).

Japanese production is declining, falling to 1.1 million t in 1996 (Figure 2). It appears that a range of varieties are acceptable to the market (Lester Loader 1999, pers. comm.).

South Korea planted 15 000 ha in 1994 (FAO 1998).

Japan, Congo and USA each produced about 40 000 tons of seed in 1997, with smaller amounts being produced in seven other countries (FAO 1998).

Figure 2: Japanese production of sweet potato (MAFF 1999).

References

Afek, U., Orenstein, J. and Nuriel, E. (1998). Increased quality and prolonged storage of sweet potatoes in Israel. Phytoparasitica 26(4): 307-312.

Brunt, A.A., Crabtree, K., Dallwitz, M.J., Gibbs, A.J., Watson, L. and Zurcher, E.J. (eds.) (1996 onwards). `Plant Viruses Online: Descriptions and Lists from the VIDE Database. Version: 16th January 1997.'

Chew, M and Morgan, W. (1999). Sweet potato (Ipomea batatas). Access to Asian Vegetables. (20): 2.

Dubey, V. K. and Ganguli, R. N. (1998). Effect of the mixture of charcoal with pyroligneous acid on shoot and root growth of sweet potato. [Japanese]. Japanese Journal of Crop Science 67(2): 149-152.

FAO (1989). Prevention of post-harvest food losses: fruits, vegetables and root crops. FAO Training Series: no. 17/2. Rome FAO

FAO (1998). FAOSTAT statistics database.

Hagenimana, V., Carey, E. E., Gichuki, S. T., Oyunga, M. A. and Imungi, J. K. (1999). Carotenoid contents in fresh, dried and processed sweetpotato products. Ecology of Food & Nutrition 37(5): 455-473.

Johnston, M. and I. C. Onwueme (1998). Effect of shade on photosynthetic pigments in the tropical root crops: yam, taro, tannia, cassava and sweet potato. Experimental Agriculture 34(3): 301-312.

Kotecha, P. M. and Kadam, S. S. (1998). Sweet potato. In: Handbook of vegetable science and technology: production, composition, storage and processing. (Ed; Salunkhe, D. K. and Kadam, S. S.). New York, Marcel Dekker. 71-98.

MAFF (1999). Abstract of statistics on agriculture, forestry and fisheries. Statistics and Information Department, The Ministry of Agriculture, Forestry and Fisheries of Japan.

Nedunzhiyan, M., Varma, S. P. and Ray, R. C. (1998). Estimation of critical period of crop-weed competition in sweet potato (Ipomoea batatas L.). Advances in Horticultural Science 12(2): 101-104.

Splittstoesser, W. E. (1990). Vegetable growing handbook, organic and traditional methods. 3rd Ed. Melbourne, Thomas Nelson Australia 362 pp.

Welby, E. M. and McGregor, B. (1997). Agricultural Export Transportation Handbook. USDA Agricultural Handbook 700, United States Department of Agriculture.