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Curing allows injured fruits marked by a high water content to heal themselves. The process was initially tested on potatoes and sweet potatoes but a positive effect was also shown for yams (DEMEAUX, 1984). So that the healing process for the wound car occur and the wound is not only dried out it is essential for temperature and humidity to be increased.
Increased temperature and humidity stimulate the yam tubers to from cork cells which can hermetically close the lesions. The cork cells are formed in the cork cambium and then make their way to the wound areas which they close with several layers of wound periderm (BAUTISTA, 1990).
To form the wound periderm certain metabolic processes are necessary. These processes use energy which is gained by expiring starch stored in the tuber. During the respiration processes, water, carbon dioxide and heat are released into the environment (BAUTISTA 1990). Thus the healing of wounds is always connected with a certain loss in tuber weight.
The losses in weight depend on the "curing conditions", i.e. on temperature, humidity, duration of the process and the size of the wound. Experiments in Togo (temperature 35 - 40°C, relative humidity 80 - 95%, duration of treatment 3 days) showed losses in weight due to curing of approx. 1 % of he fresh weight of the tubers (FAO, 1990).
Curing car seal a wound so that neither the water in the fruit can emerge (certain weight loss and shrinking of the tuber) nor can rot enter the tuber. The germination capability of the tuber is not affected by this process so that tubers which have been treated can be used for vegetative propagation.
Healing the wound should be carried out directly after harvesting the tubers (BOOTH, 1978). Clean and smooth cuts heal best of all. Injuries due to squashing do not normally heal but remain as a centre of infection (COURSEY, 1982). All wounds, squashed areas and other injuries should consequently be cleanly cut out.
There are various processes for healing wounds. These differ in their technical methods, their demands on the climate and in the duration of treatment (FAO, 1990; DEMEAUX, 1984; BEEN et al, 1977).
Curing under a jute sheet or under jute sacks is a process developed by the FAO in Togo (FAO, 1990). The tubers, after the appropriate preparation are piled horizontally over each other and covered with a thick layer, about 15 cm, of straw. The whole pile is then covered with a jute sheet or with jute sacks.
This process is very costly since approx. 50 US Dollars have to be estimated for the sacks (FAO, 1990). If the process were carried out with a sheet, the costs would be even higher. The management of this process is also demanding since keeping the temperature (35 - 40°C) and the humidity (80 - 95%) is quite difficult. Cost and management requirements give rise to the question as to whether this process can at all be adopted by the farmers.
Fig. 7 Healing wounds under a sheet made of natural fibres (Source: WILSON, undated)
Another process is the so-called "pit-curing system" which is widespread among yam farmers in Bendel State in Nigeria. For this process, a pit of approx. 2.5 x 1.5 x 1 metre is excavated and the bottom is covered with sawdust. The tubers are put in and covered with a thin layer of soil (NNODU, 1987).
This process shows its best effect at a temperature of 26°C and a relative humidity of 92%. Duration of treatment amounted to 11 and 15 days. In comparison to untreated tubers which were all affected by rot after 4 months of storage in a yam barn, the cured tubers showed only 53% and 40% rot (duration of curing: 11 days and 15 days respectively) (NNODU, 1987).
To define optimum "curing conditions" is very difficult and is influenced amongst others, by the type of yam, the type of wound and the degree of tuber maturity (BOOTH, 1978). It is thus not surprising that the statements on temperature, relative humidity and duration of treatment greatly vary in the relevant literature (DEMEAUX, 1984; FAO, 1990).
Farmers prefer curing processes with low additional costs and a low input of extra work which effect a substantial improvement in the storage behaviour and which are simple to handle. Future activities regarding improvements in the curing process should be oriented to these requirements of the farmers.
The work of BEEN et al. (1977) also goes in this direction. These determined that tubers which had only been placed in direct sunlight for a certain time showed similar storage behaviour to tubers which had been subjected to more extensive treatment. It must be remembered here that the extensive curing processes require a climate which also promotes the reproduction of pathogens and which consequently, under some circumstances, could have a counterproductive effect (ONWUEME, 1978).
Table 9: The influences of curing processes on the storage losses of D. rotundata
| Climatic
conditions |
Weight
loss after
treatment in % |
After 70 days uncontrolled storage | |
| Weight loss % | Germination % | ||
| Direct sunlight | 11,0 | 22,5 | 77 |
| 26°C/66% RH | 9,1 | 35,5 | 33 |
| 30°C/91% RH | 2,1 | 36,1 | 50 |
| 40°C/98% RH | 4,3 | 20,9 | 73 |
Source: BEEN et al., 1977 (modified)
As already stated earlier, the length of time the tubers can be stored strongly depends on the length of dormancy (cf. 3.5.1). Prolonging dormancy is thus essential in extending the storage of fresh yam tubers.
The duration of dormancy can be influenced to a certain extent by temperature and relative humidity. Low temperatures and low relative humidity rates prolong dormancy (PASSAM, 1977). The possibilities of changing the temperature and humidity to influence dormancy are limited as the tube tissue is destroyed when the temperature falls below 15°C (ibid.). A humidity which is too low also hinders storage quality as early drying of the tubers is induced by this.
Influencing the storage climate by external energy (refrigeration) is restricted economically due to the low product value of yams and the high energy costs.
Another possibility to influence dormancy is by using chemical agents to inhibit germination like are used, for example, in successfully storing potatoes (PERLASCA, 1956). When applied to yams, the substances used for potato storage showed no effect. The reason is that yams, in contrast to potatoes, do not germinate until late and then not in the epidermis but in the cell layers below this. The agents applied in storing potatoes could have a counterproductive effect on yams as they impede the healing process and can promote the formation of rot (DEMEAUX, 1984).
Experiments to influence germination with natural and synthetic growth hormones showed positive beginnings. Amongst others, gibberellic acid, a synthetic hormone available in several compositions, and batatasins were tested. The latter are natural growth hormones which occur, amongst others, in different Dioscorea varieties. Batatasins applied endogenously showed no or a very limited effect on dormancy (PASSAM, 1984) so that further progress in this direction is doubtful.
Experiments with gibberellic acid were positive in some cases, i.e. dormancy was clearly prolonged by the effect of this hormone. If gibberellic acid is applied to the foliage prior to harvesting, dormancy is only extended for Dioscorea esculenta. Applied to Dioscorea alata, gibberellic acid showed no effect at all (WICKHAM, 1984,a). If gibberellic acid is applied after harvesting the dormancy of Dioscorea esculenta as well as Dioscorea alata can be extended. Use on Dioscorea bulbifera remained without effect (WICKHAM,1984,b).
Table 10: The effect of various chemical growth regulators (germination inhibiting agents) on the storage quality of yam tubers
| Yam variety | Chemical agent | Effect on the storage quality |
| D.alata | methyl-a-NAA | + 1,5 - 2 months |
| chlorethanol | promotes germination | |
| gibberellic acid | + 4 weeks | |
| D. rotundata | methyl-a-NAA | no effect |
| gibberellic acid | no effect | |
| IAA | no effect | |
| kinetin | no effect | |
| D. esculenta | gibberellic acid | + 6 weeks |
Source: PASSAM, 1982 (modified)
According to WICKHAM (1984,b) the best effect occurs when the tubers have been treated for 22 hours in a solution of 150 mg/litre gibberellic acid. Other authors recommend other concentrations in some cases for the same agent (MARTIN, 1977; DEMEAUX and VIVIER, 1984). According to OSIURO (1992), dormancy can be extended for longer, the higher the concentration of the agent is.
Apart from the concentration of the agent, the point of time when it is applied is a critical factor in influencing the hormones for dormancy. MARTIN (1977) defines application towards the end of natural dormancy to lengthen this, a fact which is disputed by WICKHAM (1984,a). For PASSAM (1985), the condition of the tuber is a decisive factor in the effect of gibberellic acid. If gibberellic acid is applied to freshly germinated tubers this will promote the formation of germs. If the germs are removed prior to application it will delay re-formation of germs. The most favourable time for the application of gibberellic acid according to DEMEAUX and VIVIER (1985) is just after harvesting.
According to research findings so far, it can be assumed that gibberellic acid delays the formation of genes, i.e. prolongs dormancy However, there is a necessity for application methods, times and agent concentrations to be clarified. Only when these points have provided precise results and the economic efficiency of the process has been proven can recommendations on practical application be expressed.
Until such information is available, the germs should be removed manually. Since too frequent removal of the germs stimulates re-growth, the germs should not be removed until these have attained a length of approx. 50 cm.
4.7.4 Influencing the storage climate
During storage, certain metabolic processes have to take place so that the tuber retains its viability and reproductive quality. The intensity of respiration and transpiration is partly dependent on the "storage phase" at which the tuber is (cf. Chapters 3.5.2 and 3.5.3). In addition, the storage climate, i.e. temperature and humidity, have an effect on this. These two determinants in storage behaviour are not given quantities but can be manipulated by means of certain methods.
4.7.4.1 Influencing the storage temperature
In general, it can be determined that a longer storage period is possible at lower temperatures. At lower temperatures, respiration is lower and simultaneously, the formation of germs is delayed (DEMEAUX and VIVIER, 1984).
For many tropical fruits there is a "critical" temperature. Below this, an irreversible change in tissue occurs resulting in rapid deterioration of the fruit. The critical temperature for tropical fruits, also referred to in literature as causing me irreversible "chilling injury" alteration to the tissue, is well above freezing point. For yams, depending on me variety, it is between 13 and 15°C (DEMEAUX and VIVIER, 1984). Other authors state me critical range as being between 10 and 12°C (DEMEAUX and VIVIER, 1984).
Consequently, a reduction in temperature to improve me storage quality of yams is very limited and this should not fall below 15°C. Without using external energy for cooling even this value can hardly be retained under tropical conditions. The use of external energy also making the construction of closed and insulted storage structures essential, cannot be considered a possibility to improve the storage of yams on a small-farm level for reasons of cost.
Even low reductions in temperature lengthen the period of storage for yams. For this reason, all possibilities available for this which are economically feasible should be made use of Initially and primarily to be thought of here are simple changes in the construction of traditional storage structures to take advantage of me natural temperature fluctuations between day and night. Planting shady trees and me use of air currents can also lead to a noticeable reduction in storage temperatures and thus provide a contribution to improving me storage climate.
4.7.4.2 Influencing humidity of the air
There is an exchange of water vapour between the stored produce and their environment for the purpose of balancing out the moisture content of the produce and its surroundings. Dried crops e.g. cereals, tend more to re-absorb moisture from the surrounding air. Crops like yams which have a high moisture content tend to emit moisture to their environment during storage.
Loss of moisture from stored yams is not desired since this leads to economic loss (loss in weight and shrinking of the tubers) without improving storage quality. Consequently, me air in storage should have a humidity rate at which the exchange of water vapour is minimal. At a storage temperature of 26 - 28 °C which can be assumed typical for West Africa, a relative humidity of 70 - 80% leads to an equilibrium, in which the exchange of air between the tuber and its surroundings is very low.
With these storage conditions, the tuber retains the properties which define its quality like colour, aroma, flavour and chemical composition. At a higher relative humidity, there is the risk of water vapour condensing which promotes the formation of mould on the tubers.
When considering measures to influence humidity, only those should be taken into account which are technically and financially feasible for me target group of farmers. In the foreground here are alterations in construction which promote me exchange of air and thus remove superfluous air moisture from storage. The changes in construction can be supported by selecting a location which encourages air exchange.
4.7.4.3 Promoting ventilation
Normal atmospheric air consists of 78% hydrogen, 21% oxygen, 0.03% carbon dioxide and a varying content of water vapour.
The supply of oxygen from me air is essential for the metabolic functions to preserve the life of the tuber. At me same time the tuber releases water vapour and carbon dioxide. If the composition of me atmosphere in storage deviates from me normal state of the air as a result of the metabolic functions this can have an unfavourable effect on the condition of me stored produce.
Excessive air moisture which can condense if the temperature falls, promotes the formation of rot. Very low concentrations of oxygen prevent respiration and promote an undesired fermentation of me tubers in storage. Increased carbon dioxide and ethylene concentrations where yams are stored are not desired either. Increased carbon dioxide concentrations cause destruction of the tuber cell structure. Ethylene is a growth hormone which promotes germination (BATISTA, 1990).
For the above reasons it becomes clear that changes in the composition of me atmosphere in storage are not desired as these can have a negative effect on storage. To avoid undesired changes in the atmosphere me store must be sufficiently ventilated. Ventilation is not only for the purpose of gas exchanges between me store and the environment but also affects the temperature in storage.
Controlling ventilation is not simple and easily leads to counterproductive effects. If, e.g. the store is ventilated during me day, this can, at raised temperatures, lead to undesired heating of me stored produce. Inadequate ventilation at very low humidities promotes drying out of me tubers in storage. The store should consequently be ventilated at night as far as possible since temperatures are lower during this time and me relative humidity is normally higher (SADIK, 1987).
As with other improvement measures, the improvements in ventilation should be as simple as possible to carry out and not incur any additional cost. Where storage facilities are to be newly erected, locations allowing natural ventilation by means of air currents should be selected. Apart from this, the tubers should be stored so that ventilation is not hindered. Storage in huge heaps and in trench silos are consequently not suitable to meet the demands of sufficient ventilation.
4.7.4.4 Providing shade for storage facilities
On the one hand, the direct effect of sunlight on stored produce increases storage temperatures. On the other hand, me formation of germs is promoted by this. For this reason, the store should be sufficiently m the shade.
Sufficient shade can be attained by constructions where storage structures are covered by a roof. Roofs should be made of plant materials available locally for cost reasons but also due to the high heat insulation provided by these. A roof not only keeps the rays of the sun out but also protects me stored produce from rain showers which promote the formation of rot.
In addition to building roofs, natural shade should also be made use of, as protection for the produce e.g. evergreen trees. When mounting roofs for shade and taking advantage of natural sources of shade, it must be observed that the ventilation of the store is not affected negatively.
As already stated, rot is caused particularly by fungus and bacteria pathogens. These can however only penetrate me skin of the tuber through damaged spots, like injuries, lesions and holes made by nematodes.
An important precaution is consequently to minimise me risk of injury to the tuber during harvest, transport and storage by treating it carefully. Tubers already showing rot at the time of being stored should be put to some other purpose.
The danger of rot can be reduced by curing processes (cf. 4.7.2). In this way wounds are closed so that agents causing rot can no longer enter the tuber. In addition to curing, the wounds can be treated with traditional means like ash and limedust (ONWUEME, 1978).
Since rot can be passed from tuber to tuber the stored produce must be checked on a regular basis so that infested tubers can be removed from the store in good time.
Treating the tubers with fungicides is also a measure which can be used to control rot. Satisfactory results have only been achieved with thiabendazol and benomyl (DEMEAUX and VIVIER, 1984). These substances only have a low degree of toxicity and remain locally in the tuber skin, i.e. they do not move into the flesh of the tuber (DEMEAUX and VIVIER, 1984).
Treatment with fungicides is recommended as a bath. The concentration of me agent is stated as 250 - 2500 ppm at a treatment duration of 2 - 30 minutes (ibid.). It is considered necessary that further experiments to define the treatment with fungicide be carried out in view of the wide range of agent concentrations.
To avoid subsequent damage but also to achieve me appropriate effect, the treatment with fungicide requires a very precise procedure. This necessitates a high degree of extension and backstopping for small African farmers in me use of fungicides.
The control of nematodes is simultaneously also a precautionary measure against rot agents who follow the nematodes and often cause greater damage than me nematodes themselves.
Nematodes as parasites on roots and tubers are spread by plants which are infested. For this reason, only plants which are free of nematodes should be used for vegetative propagation.
As nematodes are also freely existent in the soil the relevant crop rotation (long periods between the planting of two yam crops) can reduce the pressure of the pests. To qualify this, it must be said mat most nematodes which are parasites on yams also have other host plants. Control of the nematodes by appropriate rotation is thus made more difficult.
Measures like chemical control or me treatment of tubers with hot water (CENTRE FOR OVERSEAS PEST RESEARCH, 1978), seem less suitable for use on small farms. On the one hand the processes are not yet mature, and on the other the essential financial and labour inputs are too high to encounter sufficient acceptance.
4.7.7 Control of insects damaging stored produce
Measures to control insects causing damage within yam stores basically have two purposes: firstly the damage caused by insects (eating and loss of quality) are to be avoided or at least reduced. Secondly, control measures are to avoid secondary damage caused by rot pathogens which can penetrate the tuber through the injuries to the epidermis caused by insects.
As precautionary measures, separate storage of infected and healthy tubers can be considered. In some cases, e.g. with the yam moth, this is difficult since infestation cannot always be observed externally. For hygienic reasons, all parts of me tuber which are infested by insects should be burned and not kept in the proximity of the store (WILSON, undated).
The types of storage also have an influence on me infestation of the produce by insects. SAUPHANOR and RATNADASS (1985) report that tubers stored in trench silos are not infested by moms or pyralid moms which can cause great damage in storage above ground. As storage structures are also selected on the basis of other criteria no particular type of storage can be recommended at this point to reduce storage losses due to insects.
The control of scale insects can be carried out with pyrimiphos-methyl in a concentration of 25 g per litre water. The tubers remain in this solution for 10 minutes and are subsequently dried (SAUPHANOR and RATNADASS, 1985).
Deltamethrin is recommended for the control of tineid and pyralid moths. The product is applied in concentration of 2.5 grammes per 100 litres water as a dip with a duration of 10 minutes. If infestation occurs this should possibly be repeated. For economic reasons, the agent can also be sprayed onto the stored produce (ibid.).
The statements on chemical control of insect pests on stored yams in the literature available are very limited. Further investigations are necessary or should be repeated to define suitable insecticides, concentrations, application techniques and me time of application.
Farmers tend to take me problem into their own hands when a certain intensity of damage is evident. When this happens, chemical products which are not appropriate are often used, and which can lead to food poisoning in some cases if the yams treated are eaten. The products are often improperly concentrated and applied. In view of possible mistakes in application but also in view of the possible economic damage the insects can cause to yam storage, clear recommendations should be compiled for application during chemical control.
The matter of biological control of pests has hardly been mentioned so far. According to SAUPHANOR and RATNADASS (1985), Phanerotoma leucobasis Kriech is a natural enemy of E. vapidella whose eggs it eats. In how far there is a basis for biological control here will have to be a question for future research.
4.7.8 Measures for protection from mammals
To protect stored produce from mammals, measures depending on the species and on me type of storage have to be undertaken.
Domestic animals can mostly be kept away from stores by fences. Stores erected on stilts, e.g. a platform store, due to their construction provide good protection from domestic animals which could damage the stored produce. These stores can be quite easily protected from rats. Metal funnels are mounted on me stilts with the wide end downwards at a height of approx. 100 cm. Rats and other rodents are not able to get past these obstacles.
4.7.9 The improved traditional yam barn
In view of reducing losses and long storage, the yarn barn shows the best results in comparison to other storage systems widespread in West Africa. This is one of me reasons for the yam barn frequently being selected as the basis for improvement measures to traditional systems of storage on which this work is based. Without changing the type of storage some measures and extensions to the construction can be carried out and can lead to a considerable improvement of me barn.
Improved storage of fresh yam tubers begins during harvesting. Injuries should be avoided as much as possible as these constitute doors for rot viruses. For this reason, harvesting, transport and storage have to be carried out with as much care as possible (NWANKITI et al., 1989). When transporting over longer distances, the tubers should not be piled up too high or this will quickly lead to injury to the epidermis and me formation of bruises.
Immediately after harvesting, the tubers should be subjected to curing (cf. Chapter 4.7.2). Bruises and lesions on the tubers should be cut out as smooth wounds heal better. For hygienic reasons the soil clinging to the end of the tubers should be removed. In how far treatment of wounds with ash or other traditional means improves storage ability will have to be clarified by further experiments. Prior to storage, the remains of the previous year's harvest should be removed and burned as this can constitute a source of infection.
The traditional yam barn has some disadvantages. Consequently, the following improvements should be made.
- A roof construction similar to a hut and made out of local
materials like straw, palm leaves etc. should cover the barn. A roof made
of plant materials not only provides sufficient protection from sunlight
or rain but also regulates temperature fluctuations due to its insulation
features. The roof should have a height of at least 2.50 metres so that
ventilation of me barn is not restricted (FAO, 1990).
- The barn should be made safe from rodents and domestic animals. There are
several possibilities here. It can be surrounded by a fence made of oil barrels
which have been cut open. Possible would also be a wall which, however, would
have to be at least one metre high. As rodents can easily overcome a wall
(in contrast to an oil barrel barrier) the space between the top of the wall
and the roof should be protected with fine wire mesh. It is important that
me barn is fitted with a door which closes well and will also prevent
theft.
- In the modified yam barn the tubers are stored on multi-level shelves.
The shelves can be constructed of various locally available materials as
far as these provide sufficient support. The lowest board should be about
50 cm above me ground so mat no moisture is taken up from the ground. The
shelves should be arranged so that a visual control of the tubers is possible
quickly and all around. This is facilitated by me tubers only being stored
in two or three layers on each shelf. It will also prevent too much weight
exerting pressure on individual tubers and thus reduces the risk of
bruising.
- The selection of me site is very important in making use of me advantages
for the system. This should be chosen so mat natural air movements can be
used for ventilation. The store should be set sideways to the main wind direction
so that the natural movement of me air can be used to its full effect. Existing
natural sources of shade, e.g. evergreens, should also be taken into account
during selection of me site as me temperature in me interior of me barn can
be considerably reduced by these.
The natural shade and its temperature reducing effect can mean too strong ventilation during the day. Consequently it must be ascertained mat not too much hot air enters the store as ventilation during the day.
The size of the store can be adapted to individual needs. There is no documented experience on costs for construction and maintenance of the yam barn. As local materials are mostly used, the extra financial means necessary should be limited in comparison to me traditional yam barn.
The use of germination inhibiting agents like gibberellic acid, treating the tubes with fungicides and insecticides can be considered as complementary means of improving storage systems. The lack of practical experience in me application of these prevent any concrete recommendations on the use of such products at this stage.
Regular inspection of me stored products is important for the success of storage systems. Rotting tubers must be sorted out and removed. Germs have to be removed regularly. The INPT (1988) recommends removing germs when these are approx. 50 cm long. Removing the germs too frequently induces the tuber to produce more germs.
According to investigations by NWANKITI et al. (1988), the improved yam barn can contribute considerably to reducing losses. The weight losses observed after six months storage in the traditional yam barn were 41.7%, in the improved yam barn these were 13.3% and with me improved yam barn with extra protection from rodents, 10.8%.
The results of investigations by NWANKITI et al. (1988) indicate that even simple improvements to the traditional yam barn can substantially reduce losses. For this, not all of the improvements mentioned above have to be carried out. Also individual improvements can clearly reduce losses. This means that improvements can be oriented to particular local conditions and requirements of farmers.
Considered macroeconomically, the improved yam barn leads to an increase in the supply of foodstuffs which can be produced on me domestic market. A contribution can be made to me balance of trade if the foodstuffs produced substitute food imports.
For me farmer, improved storage means an increase in subsistence security. At the same time he gains larger scope for decisions on selling and is better able to take advantage of price movements to improve his income.
