In an effort to illustrate the importance of storage, Booth and Shaw (1981) compared storage to a reservoir that controls stream water. The reservoir is designed to hold excess flood water during the rainy season, and to release it gradually during the dry season when downstream water needs are greatest. The goal is to maintain a minimum flow at all times, therefore knowing when and how much water should be released is critical in ensuring that the flow is regulated and there are no excesses in times of floods or droughts.
Storage is also necessary for the purpose of preserving plants for vegetative propagation in the following season, especially in yam (Knoth, 1993). Short-term storage is important when harvested crops take time to reach the market or the intended consumer (Etejere and Bhat, 1986). The adequate planning and controlling of storage can help to avoid gluts and scarcity of produce in the market, even where seasonality of production is a reality (Wills et al., 1998).
6.2 Problems faced during Storage and their Preventive Measures
Post-harvest losses of fresh produce result largely from physical, physiological and pathological damage (Booth and Burden, 1983) and recognizing their significance and reducing them will make a greater contribution to increasing food supply than what might be achieved by increasing primary production (Wills et al., 1998). Malformed shape of tubers is one of the major physiological defects.
The losses are often substantial, but they are difficult to quantify since they are cumulative and occur at all stages between harvest and final consumption (Snowdon, 1990). The nature of the losses may be quantitative, qualitative or both, and arise from methods of harvesting, handling, transportation and storage (Booth and Burden, 1983; Kader et al, 1985). However, the behaviour of the particular produce is influenced by genetic factors, its physical condition, agronomic practices during growth, levels of pest and disease attack and the storage environment (Booth and Shaw, 1981). The losses may occur throughout all stages of the food system, from crop maturity through harvesting, transport and storage (Table 6.1).
Table 6.1 Major causes of loss in roots and tubers
Factor | Mechanism | Stage affected | Resulting losses
Mechanical | Rupture | Harvesting | Loss of moisture
« ― | Bruising | Harvesting, transportation and storage | Access to pests and diseases may cause partial to complete losses
« ― | Crushing | Storage and transportation | Totality loss
Physiological | Transpiration | All stages before processing | Water loss
« ― | Chilling | Cold storage | Loss of palatability
« ― | Inversion of starch | End of dormancy | Increased transpiration and respiration may cause water loss
Pathogenic bacteria and fungi | Tissue degradation | Pre-harvest | Partial to complete loss
Insect/pest infestation | Chewing and boring | Pre-harvest | Partial loss
Rodent and bird damage | Chewing | Pre-harvest | Partial loss
« ― | Packing | Storage | Access for decayed organisms which may cause partial to complete loss
Source: FAO (1981)
Pre-harvest factors are maximally responsible for post-harvest losses observed in roots and tubers. The factors include field pests, infection by disease organisms, infestation by insects and pests, environmental and cultural practices and also genetic factors. A further complication is the interrelationship and interaction between the different components of production and harvesting. Their effects are greatly influenced by the condition of the product itself and, during storage, the temperature and ambient relative humidity. For these reasons, the total production and marketing system (local as well as urban) needs to be addressed as a whole. The various conditions for storage of roots and tubers are presented in Table 6.2.
Table 6.2 Conditions for storage of roots and tubers
S. no. | Required conditions for storage
1 | Desiccation should be avoided
2 | Proper control of humidity
3 | Avoid chemical changes that affects firmness and taste
4 | Special care of dormancy period
5 | Proper protection from pests during storage
6 | Proper control of environmental conditions to maintain keeping quality
6.2.1 Physical Factors
Harvested roots and tubers spoil rapidly because their main component is water at 60–90 % and the skin is easily damaged during harvest and handling operations (UNIDO, 2004). Physical injury removes the skin, an effective barrier against spoilage microorganisms, thereby accelerating water loss leading to shrivelling and drying out of the root or tuber (UNIFEM, 1993; Wills et al., 1998). Most physical injuries occur as a result of careless handling of roots and tubers during harvesting, transportation, exposure in the market and in the hands of the consumer (Snowdon, 1990; UNIDO, 2004). Bruising and abrasion damages the protective skin, which may directly expose the underlying tissues to the atmosphere leading to accelerated moisture loss through the damaged area (Wills et al., 1998). It is important to avoid injury caused by the harvesting tools (Woolfe, 1992), such as wooden sticks, machetes, hoes or forks. Roots or tubers once lifted from the soil should be safely dropped into a harvesting container. The soil condition at harvest may greatly influence the damage levels of produce, with damage increasing with both extremes of wet and dry soil conditions (Booth and Shaw, 1981). Damage may be minimized by harvesting time, such as when the soil is relatively soft.
6.2.2 Physiological Factors
Roots and tubers, like all plant organs, are living biological systems composed of cells, which continue to respire and function after harvest (Wills et al., 1998). Consequently, physiological losses may occur due to the processes of respiration, transpiration and sprouting (Osunde, 2008). The permeability of the skin of the tubers is a function of its maturity and is a very significant factor in the rate of respiration. The periderm of freshly harvested immature tubers is most permeable and thus permits greater levels of respiration than similarly harvested mature tubers. Immature sweet potatoes are reported to respire at a rate of about 17 mL O2/kg/h immediately after harvest, compared to a rate of 5 ml O2/kg/h when physiological mature (Rastovski et al., 1981).
The magnitude of the resultant losses is largely influenced by the storage environment and any effort towards lowering temperature results in reduction of losses and extension of the shelf life (Booth and Shaw, 1981). Once roots and tubers are dug out of the soil, they become exposed to the drying effect of the air and substantial weight loss may occur. This has a direct effect on marketability, because weight loss before the produce is sold translates to a loss of income where sales are made by weight. In addition, losses in sales may occur due to the resultant unattractive appearance of visible symptoms on produce that has lost excessive moisture (Van Oirschot, 2000). Sprouting during storage is also undesirable, because it reduces acceptability and marketability, besides causing an increase in respiration and weight loss (Knoth, 1993; Snowdon, 1990). With the exception of cassava, tropical roots and tubers may develop “chilling injury” when held under low temperature conditions (Cooke et al., 1988). The complex sequence of biochemical reactions taking place in the plant tissues are disrupted, causing undesirable irreversible damage (Snowdon, 1990). The simplest way of avoiding chilling injury disorder is to ascertain the critical temperature for the commodity in question, and to ensure that an appropriate temperature is maintained during storage (Picha, 1987).