Storage of Roots & Tubers

Crop Storage & Preservation (CSP 806)
Storage of Roots & Tubers
By
Ayodele Olatunde Philip
(CRP 98/0188)

Introduction

Roots and tubers such as cassava, yam, and cocoyam ranks next to cereals in provision of the major part of daily energy need of people in the tropics. Together with cereals, they are classed as staple food because they provide the main item of diet for many people. They however differ from grain in possessing high water content, a major drawback in the utilization potential of the crops. This drawback may however be converted to a plus by developing utilization approaches specific to tubers and roots rather than attempting to apply cereals and legume technologies to them. Because roots and tubers are highly perishable, they experience tremendous losses after harvesting resulting from a number of factors.
Mechanical injury: During harvesting and post harvesting handlings, a common occurrence is the sustenance of physical injuries such as bruises to the skin of the tuber, outright breakage of the tuber tips, cuts resulting from sharp harvesting tools and abrasion following the frictional contact with the soil structure. All these injuries create entry points for deteriorating organism and thus aggravate storage losses
Metabolic losses: This is due principally to respiration which produces CO2, H2O and heat. The H2O and heat produced encourage the growth of micro-organism pest, as well as rots. Storage environment particularly temperature and relative humidity are controlling factors.
Spouting: This terminology is used to describe the resumption of active growth in roots and tuber and renders the produce generally less marketable, as well as accelerated respiration. Sprouting occurs at the end of dormancy period which varies from specie to specie.
Exposure to extreme temperature: This is particularly important for yams which are subjected to chilling at temperature below 10˚c resulting to internal discoloration and tissue breakdown followed by rapid decay and loss of quality. Exposure to very high temperature can also lead to physiological decomposition.
Vascular Streaking: This essentially results in internal discoloration and very common in cassava and following physiological changes resulting in blue-black discoloration along the vascular bundles.
Microbial attack: When microorganisms attack tuber & roots, their activities rapidly result in massive tissue breakdown and many times the food reserves is rapidly used up in the metabolic activities of the microorganism. In some instances toxic substances are synthesized by the infecting microorganism such that the tuber has not only loss food reserves but has also become poisonous to human and animal consumers.
Insect and nematode attack: Important insect pests of root and tuber include scale insect, yam beetle, and several nematodes including Scuttellonema bradys, Scuttellonema pratylenchus.
Attack by rodent and other mammals: The magnitude of post harvest losses caused by the factor of rodents and other mammals can be as high as 10% and often the injuries encourages secondary infestation by insect and other microorganisms. About 25% of roots and tubers are lost to the above factors on a global basis during post harvest storage.

Storage Methods
Subsistence farmers by experience have known that most roots and tubers deteriorate rapidly after they are harvested. They have learnt to counteract this by using several cultural techniques to ensure that the culinary qualities are preserved during storage. Basic storage requirements differ crop to crop and from region to region depending on several factors. Even for the same crop, storage method can differ depending on purpose of storage and intend use. The variations are often related to climate but local resources and custom also influence the choice of storage method used. Storage methods can generally be classified into the following:
Traditional Storage Methods
The most common traditional methods for storing roots and tubers after harvesting are: storage in pit, storing in the house, storing on a platform in the open, leaving the crop in the ground where it is grown until it is needed, clamp storage and barn storage for yam in particular.
Improved or Modern Storage Methods
These include the use of chemical sprout inhibitor, ionizing radiations, controlled atmosphere storage and cold storage. These methods are modified in various root and tuber crops.



Yam
A. Yam Barn
The yam barn is the most common method applied for storage in Africa, especially in the humid forest areas of West Africa. It consists of vertical wooden frameworks to which yam tubers are tied individually by the means of strings or local cordage materials such as raffia. In some cases the barns are covered with thatched roofs to protect the tubers from rain and the heat of the sun. Tubers stored are exposed to attack by pest and microorganism and are subject to substantial weight losses due to respiration. Also, placing yam on shelves requires less time and labour than tying. Shelving also has the advantage that insect infested and rotting yams can be removed easily. The disadvantage with shelving lies in the fact that it is easier for rat to hide amongst the yams. Losses due to decay in some cases can amount to 40 – 50 per cent (Olorunda and Adesuyi 1973). Losses can be reduced by protecting the barn from rat by fixing a barrier made of iron sheeting around the barn.

Yam tubers tied to a vertical wooden framework in the shade for storage
B. Burying (Clamp) or Pit Method
This involves the digging of holes or pits about 1m deep and 1m diameter in the ground and lining them with dry leaves and grasses. After selecting good quality yam tubers, they are placed in the pit, alternating each layer of good quality yam tubers with dried grasses. They are finally covered with a layer of dried grasses and a top layer of soil. This method is not very effective due to lack of sufficient ventilation, and difficulty of inspecting the tubers.
C. Trobriand Yam House and Crib for Yam Storage
Outside the forest zone, for example in savanna region of West Africa, yams can be stored either on the field or in the farm compound. Yams stored in the farm compound are stacked in crib like those of used for maize. The cribs are well raised well off the ground with rat guards fitted to the legs. In Papua New Guinea, the so-called Trobriand yam house is widely used. The typical Trobriand yam house consist of platforms or ‘shelves’ of wicker (bamboo) or of light poles laid together, supported on vertical wooden poles a meter or two above the ground level. The vertical poles, which support the shelves, also carry a pitched roof of thatch, which protects the yam tuber from the sun and, to a certain extent, from the rain. The tubers are placed on top of each other on the open shelves. Since individual shelves would normally contain only a few dozen tubers, adequate ventilation is ensured. However, if the structure is not raised adequately above the ground level, the tubers may not be sufficiently protected from rodent attack or from danger of flooding. The Trobriand yam house is especially suitable for storage of firmer or harder-fleshed yam varieties, which do not bruise easily.

Crib for yam storage (left) and Typical Trobriand yam house (Right)




D. Field Storage
Leaving the yam in the field is the simplest method of storage. Tubers are stacked in pile and covered with grass, sorghum or millet stalks. A tent made of sorghum stalk is also used as a yam store.
E. Cold Storage of Yam Tubers
The application of low temperature storage to yam is limiting by the fact that yam are susceptible to low temperature injuries at temperature of 10-12˚C or less. Temperature of 16-17˚C have been successfully employed to prolong the storage time of yams. At 16˚C the dormancy and hence the storage life of D. alata tuber may be extended by as much as four months, particularly if the tuber is cured prior to storage so as to reduce infection by wound pathogens. The storage of yam tuber at lowered temperature has the advantage of reducing the major sources of storage loss (respiration, sprouting and rotting). However, the widespread application of cold storage for yam is not yet economically feasible due to the relative high cost of the technology involved.

In the recent past however, several practices aimed at improving the storage of yam have been attempted and they include techniques that can be applied by farmers for small scale storage of yams and those that can be applied by farmers for large scale storage.
Small Scale Storage Method of Yam
Manual removal of sprout at the time of first development: This reduces weight loss and conserves moisture and starch food content, improves palatability especially when stored for more than 6 months where good tubers are stored initially. The method is simple, requires no special training, and has no input cost except that of time. Another advantage is that it requires no electricity which may b e limiting for other storage methods in rural areas. The only disadvantage is that it can not be applied to large quantities of tubers and that the first sprout itself is associated with loss.
Application chemical Sprout inhibitors and fungicides: Maleic hydrazide can be used as a pre-harvest foliar spray and as a dip for harvested tuber before storage to suppress sprouting, Tetracholoronitrobenezene, methyl ester of 1- naphthylacetic acid, naphthalene and acetic acid have been found effective at high concentration. Benlate and thiabendazol are effective fungicides for yam storage. Curing at 25˚c and 30-60% relative humidity for 5 days prevents storage rot.
Large Scale Storage Method of Yam
Controlled atmosphere: Storage at low temperature particularly 15˚C suppresses sprouting in yams. This same temperature reduces weight loss, moisture loss, respiration rate, and keeps the tuber palatable
2. Irradiation treatment: The use of gamma ray at 7.5- 15.0 k rad applied 4 weeks after harvesting inhibits sprouting for up to 8 months without affecting acceptability.

Cassava (Manioc)
Most farmers leave the roots of their cassava in the ground until it is needed but this leads to reduced starch quality beyond optimum time. This method is cheap and easy to apply but exposes the tuber to pre-harvest pests. Harvesting may also become difficult if a hard pan forms due to dry spell. Flooding may also lead to decay during subsequent storage. Deterioration in cassava can be primary which include vascular streaking or secondary resulting from pathogenic rot, fermentation, or soften of roots. Curing of the roots prevent the onset of primary deterioration by healing the wounds on cassava roots. It involves keeping the roots at 80-85% relative humidity 25-40˚c for 4-9 days.
The use of field clamp boxes containing saw dust have also been proved effective for up to 2 months. The basic design of these clamps consist of circular bed of straw on which freshly harvested tubers are heaped in a conical style then covered with another layer of straw overlaid with soil which has been dug to form a drainage ditch round the clamp. The temperature is kept below 40˚c by varying the thickness of the straw and soil layers and by inserting ventilators made from straw or hollow bamboo. Storage in moist saw dust requires that the saw dust must be moist enough to maintain high and keep the root fresh but not damp as to make them wet. This can work up to 1 month and it is useful in markets and transportation.
Conclusion
Generally, development of better harvesting, handling and storage technique appear to be critical to reducing losses in roots and tubers. This is achievable by breeding varieties that are less branched, thicker- skinned and uniform size and shape