Root and tuber crops are vital as starchy staples throughout much of the developing world and in some areas are more important than grains. Total production of roots and tuber crops in Sub-Saharan Africa was estimated as 254 million tonnes/annum in 2012 (FAOSTAT, 2013); of this cassava (Manihot esculenta) was the most important with a production of 132 million tonnes/annum followed by yam (Dioscorea spp.) at 56 million tonnes/annum and sweetpotato (Ipomoea batatas) at 17 million tonnes/annum. Their importance is related to their agronomic advantages and limited requirement for inputs. For example, cassava gives a high yield of carbohydrates even on poor soils, has good tolerance to drought, is relatively resistant to pest infestation and disease and can be stored in the ground until required. Sweetpotato produces the highest quantity of energy per hectare per day of any of the major tropical crops (Woolfe, 1992). Yams need more agricultural inputs than the other root crops but are an important source of household income and have great cultural significance, especially in West Africa. Watch the video below for a brief overview of roots and tuber postharvest losses presented by NRI's Professor Keith Tomlins.
Source of losses
Once harvested, roots and tubers are more perishable than grains. This is related to their higher moisture content, greater susceptibility to physical damage and higher metabolic activity. While losses of grains are due primarily to external factors such as insects, rodents and mould damage, roots and tubers are primarily affected by two types of postharvest deterioration: primary physiological deterioration that is the initial cause of loss of market acceptability and secondary deterioration due to microbial spoilage (Booth and Coursey, 1974). Even among roots and tubers there is a wide range in perishability: while some varieties of yam and sweetpotato can be stored for several months, cassava has a particularly short shelf-life (1-3 days). Where cassava is harvested and processed or consumed locally this is not a great problem, but it is a serious constraint for the development of marketing, where the distance between production and consumption/process increases (e.g. Westby et al., 2004). As well as direct physical loss of the crop, postharvest deterioration causes a reduction in quality that results in price discounts and so contributes to economic losses (Naziri et al., 2014; Wenham, 1995; Westby et al., 2002). Furthermore, there can be additional losses due to change in use. For example, if harvested cassava roots cannot be marketed within two or three days of harvest then they may be processed into dried products of low quality, which have lower value (Westby et al., 2002).
There are few reliable estimates of the extent of losses. A systematic assessment of physical losses worldwide by FAO suggests that losses of root and tuber crops are in the range of 30% to 60%. In the case of cassava in Africa losses in 2002 were estimated as 19 million tonnes from a total continent wide production of 101 million tonnes (FAOSTAT, 2004). Yet, the magnitude of losses significantly differs across countries and different value chains within a single country as they depend largely on how cassava is produced, processed and consumed and on the level of coordination among value chain actors (Naziri et al., 2014). It has been argued that in the case of subsistence farmers, PHLs are negligible since, when cassava is for household consumption, the farmers harvest just the quantity necessary for the preparation of individual dishes or immediate processing. Moreover, as the product deteriorates it is used for other purposes such as animal feed (Hall et al., 1998). Nevertheless, this still represents an economic loss.
Using cassava in Sub-Saharan Africa as an example, Westby et al. (2005) listed the following types of loss:
- Physical losses of the dried commodity (cassava chips) range from 3 to 15%.
- There are financial losses when handling older fresh roots, as there is price discounting in anticipation of physical losses. Discounts can be as high as 90% for cassava that is more than 3 days old on wholesale markets.
- Losses due to change in use. For example if harvest fresh roots cannot be marketed within 2 days of harvest they may be processed into dried products of lower value.
- Lost potential because of failure to harvest at the optimum time. Cassava harvest time is flexible but there is a loss in potential earnings if the timing is not optimal.
- The value of dried cassava is related to its quality, so poor qualities represent a loss of income. For example, cassava chips of different qualities sell for different prices in Northern Tanzania, and in Ghana there is a 25% price premium for dried cassava chips that are free of mould.
Naziri et al. (2014) have estimated physical losses of cassava to be 12% in Ghana, 7% in South-West Nigeria, 3% in Vietnam and 2% in Thailand. These losses occur at different stages of the value chain (Figure 1). In Ghana the combined impact of cassava's physical and economic losses in monetary terms has been estimated at about half a US$500 million/annum.
Figure 1. The extent of physical losses at different stages of cassava value chains in Ghana, South-West Nigeria, Vietnam and Thailand
With increasing urbanisation of the African population, transport of fresh products into urban centres has become more significant. Poor marketing systems and poor roads lead to significant losses (Thompson et al., 1997). In the case of sweetpotato in Tanzania, a loss in value of 13% due to handling and transport alone has been recorded (Tomlins et al., 2000).
Loss reduction opportunities
It has been stated that the key to reducing losses is in 'soft' infrastructure investments, improved market integration, and in value addition through processing. This is particularly pertinent to cassava where although in the export markets refrigeration or coating the root with paraffin wax are used to extend shelf-life to over a month, such procedures are too expensive to be adopted by local markets in developing countries. Sweetpotato and yam are different as they have relatively long shelf-lives under ambient conditions. But there is scope for improving shelf-life by a process of curing. If immediately after harvest the product is stored for a few days in a warm, high humidity environment then the healing of harvest damage is promoted which reduces water loss so that quality is retained. Sweetpotato is particularly prone to
preharvest infestation of the sweetpotato weevil (Cylas spp.), which leads to postharvest deterioration. Considerable effort is being expended to overcome this problem both through production of bt-sweetpotato and natural resistance. In the case of yams there would be benefits if cultivars could be developed with tubers that have extended periods of dormancy.
For most root and tuber crops a significant proportion of the crop is processed. Transformation of the crop into a more storable form reduces physical losses as well as providing income and employment. Where processing leads to increased value of the product it can effectively change a situation of postharvest loss to one of net economic gain. Both dried and fermented products are common but here are still subject to physical losses, albeit at a lower rate than the fresh product. Significant volumes of by-products, such as peel, siftings and cassava pulp, are produced during processing. These offer opportunities for loss reduction and value addition, for example the GRATITUDE project is investigating the use of such by-products to generate other foods.
- Booth, R.H. and Coursey, D.G. (1974) Storage of cassava roots and related postharvest problems. In: Araullo, E.V., Nestel, B. and Campbell, M. (eds), Cassava Processing and Storage. An Interdisciplinary Workshop, IDRC, Ottawa, 43-49.
- Hall A.J., Bockett G.N. and Nahdy S. (1998) Sweet potato postharvest systems. In Uganda: Strategies, constraints and potentials. International Potato Centre (CIP) Social Science Department Working Paper Series No. 1998-7 CIP, Lima, Peru. ISSN 0256-87.
- Naziri, D., Quaye, W., Siwoku, B., Wanlapatit, S., Tu, V.P. and Bennett, B. (2014) Not all those who wander are lost: A comparative analysis of postharvest losses in cassava value chains in Ghana, Nigeria, Thailand and Vietnam. Paper submitted for the 14th Congress of the European Association of Agricultural Economists "Agri-food and Rural Innovations for Healthier Societies", Ljubljana, Slovenia.
- Thomson, M., Ndunguru, G.T., Waida, T.D.R., Rwiza, E. and Jeremiah, S. (1997) The relationship between quality and economic value of fresh sweet potato and dried cassava products in Mwanza, Tanzania, NRI Report (A0500). Chatham, UK: Natural Resources Institute.
- Tomlins, K. I., Ndunguru, G., Rwiza, E. and Westby, A. (2000) Postharvest handling and transport of sweet potatoes and their influence on quality in Tanzania, Journal of Horticultural Science and Biotechnology, 75, 586-590.
- Wenham, J.E. (1995). Postharvest deterioration of cassava: A biotechnology perspective. Plant Production and Protection Paper FAO No 130. Rome: FAO.
- Westby, A., Van Oirschot, Q., Tomlins, K. Ndunguru, G., Ngendello, T., Sanni, L., Pessey, D. and Oyewole, O. (2004) Bridging the gap between post-harvest technology development and commercialization of root and tuber crops in Africa. Invited thematic paper at the ISTRC-Africa Branch Triennial Symposium, 31 October – 5 November 2004, Mombasa, Kenya.
- Westby A., Lankford, B., Coulter, J., Orchard, J. and Morton, J. (2005) Rural infrastructure to contribute to African agricultural development: the cases of irrigation and post-harvest. Background paper for the Commission for Africa.
- Westby, A. (2002). Cassava utilization, storage and small-scale processing. Natural Resources Institute, University of Greenwich, UK.
- Woolfe J.A. (1992) Sweet potato. An untapped food resource. Cambridge University Press, ISBN 0 521 4029.