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4. Bulb onions and garlic (Allium cepa, A. sativum and other species)
Onions and garlic of different types are grown worldwide for the flavour they contribute to food. They are also commonly regarded as having medicinal properties. In many countries onions are used in the immature green state. In others, where the crop is seasonal, cultivars which produce bulbs that can be stored in a dry state are grown.
Maturity for harvesting
When the bulbs developing from the leaf bases of both garlic and onions are fully formed, the leafy green tops begin to yellow and eventually collapse at a point a little above the top of the bulb, leaving an upright short neck. When the tops "go down" in this way, the bulbs are ready for harvesting. Because all the onions or garlic in a crop do not mature at the same time, large-scale commercial growers harvest them when about half the tops have gone down.
Small-scale growers can, if they wish, harvest their crops progressively as the tops go down, especially so if they intend to store the dry onions for sale or use at a later date.
Since onion bulbs are normally formed at the soil surface, it is sometimes possible in sandy soils to pull the mature bulbs by hand. Where conditions make hand-pulling impossible, and with garlic where the bulbs develop below ground, harvesting is done by loosening the bulbs with a fork or hoe before lifting them, in a manner similar to that described for root crops in Figure CP4.1.
In dry, sunny weather the harvested crop is left in windrows in the field for a few days until the tops are dry. Where the harvested bulbs are exposed to high-intensity sunlight (e.g. at high altitudes in the tropics), the windrows should be made so that the green tops cover the bulbs to protect them from sunburn.
Selection and grading
All damaged or decaying onion and garlic bulbs should be discarded. Onions with thick necks should be put aside for immediate use because they will not store well.
Market requirements will determine whether onions need to be size graded or not. Retailers in local markets will normally do their own grading when making up lots for sale.
Figure CP4.1 Damage in harvesting roots, tubers and other underground crops is more easily avoided if crops are grown in mounds or raised beds
If the onions or garlic are to be made up into strings for storage or sale, as described below, it is an advantage to separate them into sizes so that the bulbs will be more or less uniform in size in any string. This makes the stringing operation easier and gives a better appearance to the finished product.
The only post-harvest treatment required for the long storage of bulb onions is a thorough curing of the bulbs. Curing is a drying process intended to dry off the necks and outer scale leaves of the bulbs to prevent the loss of moisture and the attack by decay during storage. It can be carried out in the field under dry conditions by windrowing the bulbs as described above under Harvesting.
The essentials for curing are heat and good ventilation, preferably with low humidity. This dries out the neck and the two or three outer layers of the bulb. The outermost layer, which may be contaminated with soil, usually falls away easily when the bulbs are cured, exposing the dry under-layer, which should have an attractive appearance.
If onions cannot be dried in the field, they can be collected in trays, which are then stacked in a warm, covered area with good ventilation.
In cool, damp climates, onions in bulk ventilated stores are dried with artificial heat blown through the bulk at a duct temperature of 30 degrees Celsius.
Garlic and onions can also be cured by tying the tops of the bulbs in bunches and hanging them on a horizontal pole in a well-ventilated situation.
For bulk marketing, the tops of onions are removed when they are thoroughly cured and the necks are quite dry. The tops of garlic are cut off I cm above the bulb and only the loose outer skin rubbed off.
Both onions and garlic may be made up into strings. These are of 2 kg for garlic or 5 kg to 10 kg for onions. This is, however, a labour-intensive operation suited to small-scale production using family labour. It is not cost-effective on a commercial scale.
The first requirement for successful storage of dry bulb onions is that the cultivar chosen should have the right characteristics for long-term storage. The principal needs are:
The storage environment must be dry and well-ventilated. Optimum storage temperatures are 0 or 24-30 degrees Celsius under ambient tropical conditions. At temperatures between these, onions will sprout in storage (Figure CP4.2).
Onions stored in a damp atmosphere will develop roots (Figure CP4.3). Onions can be stored in bulk in insulated stores, with fans for cooling the onions using cold night air. This method is used where large tonnages are to be stored. Small-scale growers can use naturally ventilated stores made from local materials. The onions can be stacked in trays or in layers on slatted shelves.
Figure CP4.4 Onions can be strung for storage as shown above. They then can be hung as shown right, in a cool, shady place
Where small amounts are to be stored, the stringing of onions in 5 kg or 10 kg lots and the hanging of the strings in a well-ventilated dry location is a very effective storage method. The tops of the onions should not be cut off but left so that they can be fixed to a double string (Figure CP4.4) by weaving the dried top of each onion through the strings in a figure-8 fashion. Alternatively onions can be tied by their dried tops in bunches, and the bunches can hang on a horizontal line or pole in the shade.
Garlic may be stored in trays, strings or bunches in the same manner as onions, except that with garlic strings are made by plaiting the dry top leaves of the garlic.
5. Leafy vegetables and Immature flower heads (Brassica spp, Beta sp., Spinacea sp., Aplum sp., Lactuca sp., Alllium)
Include cabbage, Chinese cabbage, kale, rape, mustard, broccoli, chard, spinach beet, spinach, lettuce, celery, green onions.
Maturity for harvesting
All are harvested in the immature state before the plant has developed to the point of seed production. The older parts of these commodities become fibrous or woody.
The parts of the plant harvested vary with the crop:
Those crops forming a head, such as cabbage, are cut with a sharp knife. Young shoots and leaves are broken off by hand.
Celery and green onions are either pulled by hand or dug from the soil. They should be harvested under dry conditions when soil can be readily shaken from the roots. The roots are then trimmed with a sharp knife.
All these commodities are damaged easily if subjected to pressure. They should be packed loosely in field containers, which must not be overfilled or the produce will be damaged when the containers are stacked.
The harvested produce must be kept free from contamination by soil. Leafy vegetables and immature flower heads deteriorate very quickly after harvest because they lose water fast and produce a great deal of heat. The following care is necessary to keep losses to a minimum:
Selection and grading
All produce which is damaged, decaying, wilted or infested by insects or other pests must be discarded. Size-grading is not normally necessary for local and internal marketing.
It is essential to keep these commodities free from contamination by soil or decaying plant material. Do not wash them. Washing them may remove gross soil contamination, but it will also spread any decay through the whole bulk and result in heavy losses. Shading the produce and keeping it in a moist atmosphere helps to keep it cool, reduces water loss, and delays wilting and yellowing of leaves.
Chemical treatments to control decay are not acceptable because they are not very effective and they leave high residue levels because of the characteristic high surface area of these products in relation to their volume.
For local rural markets traditional containers are likely to remain in use. It is important, however, that containers should not be too large to be carried by one person. Rough handling of heavy packages results in damage to produce.
Leafy vegetables and immature flower heads have a very short post-harvest life, especially under ambient conditions. Even under refrigeration most remain in good condition only up to two weeks. Ideally, they should reach the consumer within two days of harvest.
6. Tomatoes (Lycopersicon esculentum)
Maturity for harvest
If they are to be used in the ripe condition, tomatoes should be picked at the earliest when they are at least mature green. Immature tomatoes do not ripen after harvest. The actual stage at which they should be picked depends upon local preference and custom in each country.
Tomatoes have reached the mature-green condition when they are fully rounded and have changed from dark to medium or light green, and the skin develops a waxy gloss. As ripening is initiated, the fruit shows a pale pink or yellow tinge, which develops through a definite pink to full red.
Most tomatoes are harvested at the early ripening or pink stage, depending on market preference and the time they take to reach the retailer. Tomatoes to be consumed immediately can be harvested when fully ripe.
Tomato fruit stalks have a natural break-point. Mature fruit readily breaks away from the cluster when pressure is placed on this point while lifting the fruit upwards (Figure CP6.1). Tomatoes are best harvested into plastic buckets (pails) and transferred if necessary to plastic field crates holding not more than 25 kg weight.
Selection and grading
Figure CP6.1 A natural break-point occurs on many mature fruits at the junction of stem and stalk. At harvest time, thumb pressure applied there should be accompanied by lifting, pulling and turning the fruit
All decaying, damaged, undersized and sunburned tomatoes should be discarded. Size-grading for the local market is normally done by retailers. Internal urban markets, including supermarkets, may have differential prices for size grades as against ungraded fruit. Catering and institutional buyers do not normally demand size-graded fruit.
If only those tomatoes which are in good condition are marketed, there should be no need for any post-harvest treatments. Tomatoes produced on a large commercial scale may be subjected to artificial ripening; but in countries where production is mostly on a small scale, this is not necessary since tomatoes are normally harvested at maturity and ripen naturally.
For local markets tomatoes can be packed in baskets or other traditional containers assuring careful handling, i.e. rigid enough to protect the contents from being crushed.
For urban markets cardboard telescopic boxes or trays, or wooden trays with capacities of not more than 15 kg, should be used. Size-graded tomatoes can be pattern-packed in layers to make best use of the box.
Ungraded tomatoes are jumble-packed to a given weight.
Tomatoes have relatively poor storage capability. Green mature fruit can be held for up to two weeks at 13-18 degrees Celsius, but for less time under ambient tropical temperatures.
Fully ripe tomatoes have only two to six days' storage life, depending on ambient temperatures.
7. Potatoes (Solanum tuberosum)
These are also called Irish or white potatoes. Although most of the world potato production is in temperate regions, the crop is becoming more important as a food source in the tropics and subtropics.
Maturity for harvest
Potatoes can be harvested for immediate consumption in an immature state, usually from the time they reach full flowering. At this time the skin is thin and soft, and the potatoes cannot be stored. Main-crop potatoes which may need to be stored should not be harvested until at least two weeks after the plant tops have died off, by which time the skin of the tubers is fully developed and they are mature. They are then less susceptible to damage than immature potatoes.
Potato harvesting is best done when the soil is slightly moist. Where they are produced on a small scale, harvesting is carried out with hand-tools (Figure CP4.1). The tubers must be lifted carefully to avoid damage, and shaken free of soil. They are left to dry in the field, after which they are collected in field containers and placed in a cool, shady place. Potatoes for food must not be exposed to the light for more than a few hours after harvest or they will turn green, develop an unpleasant taste, and may become toxic.
Selection and grading
All potatoes showing greening, decay or severe damage owing to harvesting or pest attack should be discarded at harvest. Immature tubers and those showing minor damage or wetted by rain should be put aside for immediate consumption. Potatoes to be stored for food or seed should be fully mature and free from any visible damage or decay. Size-grading requirements will depend on market demand. In most cases there will be only minimum-size standards, sometimes maximum-size also. Local specialists should be consulted on the subject.
Potatoes which are to be stored need to be cured to repair any skin damage which may be present. The principles of curing root and tuber crops are discussed in Chapter 9 of this manual. Curing is best carried out after the potatoes have been placed in store. It involves reducing ventilation to allow a buildup of the temperature and humidity needed to promote curing. The potatoes in store should be covered with straw, and the store should be well insulated in order to prevent the condensation of free water on the potatoes.
The storage conditions suitable for curing potatoes are:
|Temperature||13 to 20 degrees Celsius|
|Relative humidity||85% or more|
|Curing time||7 to 15 days|
The highest temperature requires the shortest time. At the end of the curing time, full ventilation should be restored to the store.
Only sound potatoes with no apparent damage or decay should be stored. Potatoes to be used for food or for processing must be kept in the dark to prevent greening. Seed potatoes are stored in diffuse light to promote the development of several strong shoots on each tuber.
On-the-farm storage can be carried out using low-cost structures employing local skills and materials. Where climatic conditions are suitable, potatoes can be left in the field some weeks after maturity but in general it is preferable to collect them in a structure where some measure of control over storage conditions can be achieved.
Clamps constructed as shown in Figure 10.2 are used in cool climates for storage of six months or more. They can be effective under warmer conditions provided they have adequate ventilation and are in a well-drained situation.
Low-cost, small-scale pole and thatch stores holding up to two tonnes of potatoes can be constructed in the field; they are particularly suitable for seed potatoes held in diffuse light conditions (Figure 10.1). Potatoes are held in these stores in open trays or on well-ventilated shelves.
Existing buildings may sometimes be modified for storing up to 20 tonnes of potatoes under natural or assisted ventilation. Whatever the type of store, it is necessary to keep the potatoes dry and as cool as possible by having an insulated structure with good ventilation.
Although baskets or wooden boxes may be used to market potatoes, sacks are cheaper and more commonly used. In temperate climates potatoes are commonly packed for distribution in 25 kg multiwall paper sacks or woven synthetic fibre (polythene or polypropylene) sacks. The use of paper sacks is not recommended under warmer conditions because they lack adequate ventilation. Woven jute sacks are preferred for potatoes in the tropics. They are usually of 50 kg capacity and provide good ventilation. Woven synthetic fibre (polythene or polypropylene) sacks are also used, but they are so smooth that they slide easily against each other and make stacking them very difficult.
8. Sweet potato (lpomoea batatas)
Sweet potatoes are grown widely throughout the tropics as a basic or subsidiary staple food crop in subsistence economies. They are also widely used as an animal feed and in some countries as an industrial raw material.
Maturity for harvest
Sweet potatoes are considered ready for harvest when the leaves begin to yellow. A further test of readiness to harvest is said to be that when mature tubers are cut, the cut surface does not discolour. In some countries experienced growers harvest at a specified time after planting. This has to be based on careful observation and long experience since there is a difference in the maturity period of the various cultivars.
Harvesting is carried out either progressively or all at one time. Subsistence growers tend to harvest progressively, often from the same plants over a long period. Sweet potato crops grown for sale are usually harvested all at one time.
The preferred harvesting tools for most small-scale producers are pointed wooden sticks or metal bars, or machetes (cutlasses, bolos, pangas), especially where progressive harvesting is practiced. These tools are said to cause less damage to the roots (Figure CP4.1) and enable a few roots to be harvested from a plant on each occasion. When the whole crop is harvested at one time, growers tend to use pronged rakes, hoes or digging forks.
On no account should the roots be thrown, whether into field and storage containers or at any other time during their handling. Great care must be taken to avoid damage to the skin of sweet potato roots since they are very subject to post-harvest decay under tropical conditions. For this reason it is recommended that the harvested roots be gathered into baskets, boxes or crates in which they can remain throughout their post-harvest life without disturbance, through curing and storage if necessary.
Harvested tubers which have damp soil adhering to them at harvest may be left in the field for an hour or so to dry, but not long enough to suffer sun scorch. The soil can then be carefully removed.
Selection and grading
All decaying roots should be discarded. Slightly damaged roots can be used for immediate consumption, and those which are undersize or badly damaged may be fed to animals. Tubers which are to be stored should be fully mature and free from visible injury. Most sweet potatoes are sizegraded by the retailer if necessary.
Curing of those roots which are to be stored after harvest is the only treatment necessary for sweet potatoes. This curing has to be carried out according to the principles laid down in Chapter 9.
The roots should remain in the containers into which they were harvested and in which they will be stored. The containers can be placed in the storage structure and covered with straw. Ventilation should be restricted to allow a buildup of heat and moisture in the store, to give the correct conditions for storage, which are:
|Temperature||27-34 degrees Celsius|
|Curing time||5-20 days|
Curing is a process of healing by the formation of new skin on damaged areas of sweet potatoes, and also of the maturing and hardening of the whole skin of the roots. The length of time required for curing cannot be forecast since it has been shown to vary even under identical environmental conditions. Indications of maturity are said to be thus: when the skin can no longer be rubbed off easily from a sample root and when small buds appear on the roots.
Sweet potatoes are subject to very rapid deterioration after harvest at ambient tropical temperatures. There are reports in the literature of storage of sweet potatoes for four months or more. In most reported instances of successful storage for such a time the storage temperature has been in the low range of 10-18 degrees Celsius. Even at the higher end of this range sprouting of the roots has been a problem. At temperatures below 10 degrees Celsius sweet potatoes suffer chill damage.
The storage structures used have been either custom-built ventilated stores, with or without refrigeration, or sunken or underground chambers, protected by a building above. Conditions required for successful storage are as follows:
If there is any indication of free water on the roots or in the store, more ventilation should be provided to remove the excess moisture. If the air gets too dry the floor of the store can be lightly sprinkled with water.
These conditions can be achieved at higher altitudes in the tropics at those times when night temperatures fall to within the required range. In a well-insulated ventilated store, the tubers can be cooled at night by full ventilation and heat rise can be slowed during the day by closing the store. It is unlikely that sweet potatoes can be stored at ambient tropical temperatures for more than three weeks without heavy losses from decay and sprouting.
The best form of packaging for long transport is either wooden crates or cardboard boxes holding not more than 25 kg. The roots should be packed firmly to prevent movement within the boxes or crates during handling and transport. Sweet potatoes should not be packed in 50 kg sacks, which are difficult to handle and, when dropped, cause heavy damage to the roots.
9. Yams (Dioscorea spp.)
Yams are grown principally as a subsistence crop and for internal marketing. The main types are:
Maturity for harvest
Yams are ready for harvest when the above-ground parts of the plants have died off. The greater and white yams can be left in the ground for a time after maturity. Yellow yams, which have a very short dormant period, should be lifted as soon as mature.
Yams are normally harvested by carefully scraping the soil away from the tubers in order to avoid damaging them. Wooden digging sticks or spades are less likely to cause damage to the tubers than are metal forks or hoes.
Selection and grading
Heavily damaged or decaying yams should be discarded. Those which are slightly damaged may be consumed immediately or subjected to a curing process before storage. Size-grading is not always practiced. It is mainly done when there is an advantage to be gained in the packaging for marketing.
Where yams are cut or deeply injured, a new skin can be formed on the damaged surfaces by curing the tubers at high temperature and humidity. Curing has been shown to be effective in yellow and white yams, but its effectiveness in other types is not known. Injuries caused by skin abrasion or bruising tend to dry out rather than form replacement skin. Curing is carried out in accordance with the method referred to in Chapter 9 of this manual.
A method recommended in West Africa for curing yams which are to be stored is illustrated in Figure CP9.1. This provides the necessary conditions for raising the temperature and moisture content of the air to suitable levels by restricting ventilation.
The conditions found to be effective in promoting the curing of greater and white yams are:
|Temperature||32-40 degrees Celsius|
|Relative humidity||90% or above|
|Curing time||1-7 days|
Curing should be carried out immediately after harvest at the location where the yams are to be stored.
Figure CP9.1 Yams stacked in this method to cure skin damage should be covered with grass to keep the canvas or jute cover from touching the yams. The curing pile should not be exposed to direct sunlight and the cover should be removed after four days
Figure CP9.2 In humid areas of West Africa yams may be stored in 'barns" like this whose side poles have taken root and are growing leaves to provide shade (right). Inside walls of yam barn are vertical frames to which yams are tied (above). (Figures are reproduced from Careful storage of yams: some basic principles to reduce losses, Commonwealth Secretariat. London.)
Yams being sent to local markets may be carried in bulk by vehicle or in ordinary baskets. When they are carried in bulk, the floor and sides of the vehicle should be padded with sacks loosely packed with straw, or with grass mats or plastic foam covered with polythene sheet. Whether the yams are carried in bulk or in baskets, the vehicle must not be overloaded and should be driven with care. For internal urban markets the tubers are best packed in wooden crates or ventilated cardboard boxes. These containers should not be overpacked and must be handled and transported carefully.
Greater and white yams in good condition can be stored for several months under appropriate conditions. Yellow yams have poor storage potential due to their very short dormancy period. Although yams may keep in storage for several months, they shrink over such a period owing to water loss and to natural living processes which use up stored dry matter (starch). There may also be additional losses because of decay caused by moulds.
There are many different storage practices in various countries. Owing to the generally non-commercial nature of yam production and limited resources of growers, most storage uses low-cost methods. Yams are generally stored during the hot dry part of the year when the provision of ventilation and other conditions which help to reduce their temperature are key factors.
Yams kept in the ground and harvested progressively when needed are subject to attack by insects and other pests. They are also exposed to attack by moulds. Yams kept undug may also tie up limited land resources.
The tubers can be piled in small numbers in shaded situations or in well ventilated huts built of local materials, in which case they are best stored on racks or shelves.
In West Africa, yam "barns" are a common method of storage. They are vertical frames to which individual yams are tied (Figure CP9.2). The uprights supporting the frames are bush poles up to two or more metres in height. The use of poles which will take root and provide a protective canopy of leaves to shade the yams is of benefit. Such growing poles are also less likely to decay or be attacked by termites. The stored frames of yams may be protected by a fence to keep out rats.
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