The roof should be waterproof to protect the produce from rain during storage, the storage area should be cleaned before the produce is put into the store, and only sound, healthy roots and tubers should be considered for storage (Wilson, 1987). In addition, it is important to occasionally inspect the produce with the aim of ensuring continued sanitation of the store.
6.4.1 Cassava
In most areas where cassava is grown under subsistence farming conditions, the problem of storage is overcome by leaving the mature roots in the ground until needed (IITA, 1990; Knoth, 1993), where they may remain for up to 2 years (UNIFEM, 1993). The main disadvantages of this method is that large areas of land cannot be utilized for further cropping, and the roots become more fibrous, the starch content declines and they are also vulnerable to attack by biotic factors such as rodents, insects and nematodes (IITA, 1990; Knoth, 1993).
Other traditional storage methods include burying harvested roots in the soil (Knoth, 1993) or in pits lined with straw or other vegetative material (IITA, 1990). The roots are carefully harvested, ensuring that part of the aerial stem is still attached (Etejere and Bhat, 1986), a practice that ensures they are preserved in good condition since pathogen infection starts at the cut end. Roots may also be piled into heaps and kept fresh through daily watering or coating them with a paste of mud (Etejere and Bhat, 1986; IITA, 1990). To ensure that the temperature remains as low as possible and daily fluctuations are balanced out, the heap of roots is usually positioned at a corner of the farmhouse or under a shady tree (FAO, 1990). Storage of harvested cassava roots in pits and heaps is restricted to a maximum period of one week, because ventilation is poor and roots cannot be checked regularly (Etejere and Bhat, 1986).
Cassava roots are also stored under water, but for a very limited period since they normally begin to ferment or spoil in about 3 days (Knoth, 1993).
Several improved methods have been tested and used for storing fresh cassava roots for up to 3 months, including burying the roots in specially prepared trenches and in moist sawdust (Etejere and Bhat, 1986; IITA, 1990). In the trench silos, the harvested roots are arranged between oil palm or raffia leaves, and the trench is covered with a top layer of soil which is watered at intervals to keep it moist. Cassava roots stored in sawdust must be freshly harvested with 15–20 cm of the stem attached (Knoth, 1993). Different types of containers such as woven baskets, paper cartons, wooden boxes with covers (IITA, 1990) and wooden crates (Knoth, 1993) can be used for storage. The containers can be stored or transported in this way, thereby reducing handling costs and injuries to the roots (Knoth, 1993). Fresh cassava roots can also be stored in sealed polyethylene bags for 2 weeks or more. The storage method allows “curing” to take place, since the roots create the necessary temperature (30–40oC) and humidity (>80 %) environment inside the sealed bag (IITA, 1990).
Among the improved storage methods for fresh cassava, techniques involving refrigeration and freezing, waxing, gamma irradiation, and control of storage environment (relative humidity and temperature) have been reported (IITA, 1990). Reduced temperature can extend storage of cassava roots. The most favourable conditions for long-term storage of fresh cassava roots is 0–2 °C at 85–90 % relative humidity (Snowdon, 1990). Coating fresh cassava roots in wax helps to extend storage duration by reducing respiration, and addition of a fungicide to the wax has been found to extend the shelf life of the roots by a further 10 days (Knoth, 1993).
6.4.2 Sweet Potato
Harvested sweet potato roots are difficult to store and their shelf life may range from a few days to a few months, depending on the cultivar and storage conditions (Ray et al., 2010). In dealing with this difficulty, roots are left in the ground after maturity and only harvested when needed (Woolfe, 1992). However, this practice is generally not recommended because of pest infestation which intensifies when the crop remains in the field and also the root quality deteriorates due to starch depletion, fibrous development and crack formation on the surface (Pillai, 1991). Freshly harvested roots are stored in heaps on the house floor, covered with straw which may also be plastered with a mixture of clay soil and cow dung, laid on shelves or placed in pits or shallow trenches with alternative layers of grass or wood ash (Etejere and Bhat, 1986; Ray and Balagopalan, 1997). The wood ash is believed to have antifungal properties (Ray et al., 2010). Packing sweet potato roots in bamboo baskets and storing them in the roof spaces, storage in barns and tying roots on racks covered with either dried grass or oil palm leaves have also been reported (Etejere and Bhat, 1986).
Sweet potato storage in well-ventilated storehouses on raised platforms, in earthenware pots, wood ash and fine sand has been successful in the short term (Pillai, 1991). Other improved methods are storage in closed cardboard boxes, constructed pits or clamps and evaporative double-walled cool brick chambers (CIP, 1997; Ray et al, 2010). In the long term, storage for up to 12 months is possible without loss of quality at low, non-freezing temperatures in the range of 12–16 °C and 85–95 % relative humidity (Eckert and Ogawa, 1988; Snowdon, 1990; Woolfe, 1992).
6.4.3 Yam
There are well-developed traditional yam storage systems in the major growing regions such as West Africa, the Caribbean, parts of Southeast Asia and the Pacific (UNIFEM, 1993). Yam tubers are mature when the forage has died, but harvesting need not take place immediately since the tubers can stay underground for a further 1–4 months without much loss (Knoth, 1993). This method of storage is feasible since no costs are incurred but, as in cassava underground storage, there is opportunity cost and produce is without protection from pests. Special structures are built for yam storage consisting of a sheltered vertical framework of timber with a table made of cross pieces of bamboo or palm leaves where the tubers are placed or hanged one by one (Wilson, 1987). As in sweet potato and cassava, yams are also stored in trench silos or pits that are excavated in the ground and lined with straw. The top layer of yams is also covered with straw or similar materials and protected with soil or a conical roof arrangement made of maize or millet stalks (Knoth, 1993). Other methods include piling the tubers on the ground, underneath rocky outcrops, household floors, inside huts, underneath houses built on stilts or underneath soil and humus (UNIFEM, 1993), and then covering the heap with maize stalks, millet stalks or similar materials (FAO, 1990).
Yam tubers are stored in an improved yam barn that is designed with a roof made of organic locally available materials such as straw or palm leaves, and the tubers stored on multi-level shelves (Knoth, 1993). In high rainfall areas during the storage seasons, yams are kept dry with water-proof roofs such as iron sheets (Wilson, 1987). Highly technical yam storage methods include cold storage at 15oC and irradiation with gamma rays at low dosages before storage at ambient conditions (Etejere and Bhat, 1986).
6.4.4 Taro
Dormancy in taro is poorly understood, which creates problems in its storage. Moreover, this is likely to differ in the cultivars of tropical and temperate regions. Environmental conditions, crop rotation within the agricultural systems and many other factors make storage systems desirable for longer periods. To preserve taro tubers, the same precautions should be taken as usually adopted for other roots and tubers (Janssens, 2001). Overall storability of taro tubers has been improved and shelf life extended, when placed in perforated plastic bags (Bikomo, 1994). To maximize the length of storage in fresh taro, various combinations of relative humidity and temperature are recommended by various researchers. Taro stored at 60 % RH and 20 °C can be stored for 2 to 3 weeks. However, the storage period can be further extended by changing the temperature and relative humidity. At 7-10 °C, 85–90%RH, taro can be stored for 120–150 days (Thompson, 1996) (Table 6.7). Pit and fermentation are the traditional storage methods for taro. Others include drying, freezing and storage in raised shelters. Above-ground storage in a shelter probably helps to keep the taro aerated, dry and slightly cool. The individual or combination of modern techniques may be explored for taro for its long-term storage.