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11. Root and tuber storage

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Harvesting is a most important function. Unless this operation is carried out with maximum efficiency, later PFL (prevention of food loss) activities may be a waste of time. If, for example, roots and tubers are bruised or otherwise damaged during harvesting, consideration of improved handling or packaging is not likely to be worthwhile, since an early infestation with moulds and viruses will occur and rotting will have started. If harvesting operations are correctly undertaken there is greater scope for later introduction of improved methods. Provision of the proper tools and equipment for harvesting and training workers in their correct use should be a priority PFL activity.

11.1 Yams

In most parts of West Africa, for example, the main crop of yams is harvested between September and November. Some will be consumed or sold immediately, but the bulk will be stored for a maximum period of six months for later use, either as food or for planting.

There are several traditional methods of yam storage:

(a) one of the most popular, particularly in the forest areas, is a bamboo enclosure (yam barn) containing a vertical framework onto which yams are tied individually with rope or bush twine. Shade is always provided, generally by trees;

(b) in the savannah zone less elaborate storages are generally found, such as shelters built of sorghum stalks with yams stacked inside;

(c) using clamps or underground storage has been successful.

Losses in these types of traditional storage are very high; a conservative estimate would be 25 percent. They are the result of a number of factors:

As a general rule, storage losses in the savannah zone are fewer than in the forest zone. This is probably because of the lower incidence of the Scutellonerna nematode and to the tendency for yams grown in drier environments to have a higher dry-matter content. This appears to be important in storage, as is the fact that the cultivars grown in the ssavannah have a longer endogenous dormancy and do not sprout as early during the storage period.

Although it is recognized that storage is one of the critical problems limiting yam production, most discussions on the improvement of yam storage have failed to produce any new ideas in the area of intermediate technology. There appears to be no advantage in practice between the best traditional storage methods now used by the farmers and the large-scale advanced technology methods, which in any case would not be applicable in many parts of the rural world at present.

The following are important in reducing losses in traditional storage:

(a) it should be recognized that there are differences in storability between species of yams. As a general rule, water yam (D. alata) stores better than white yam (D. rotundata), and white yam better than yellow yam (D. cayenensis). Also, there are differences between varieties within a species. Varieties which are inherently better storers are those with (i) a long dormancy, (ii) good healing capacity and (iii) shapes easily dug from the soil with little injury. Poor storing varieties should be sold or consumed first and only varieties known to be good storers should be kept for long periods;

(b) the condition of the yam tubers when they are placed into storage is very important in determining storage life. Only sound yams will store; this means tubers which are free of nematodes, rots and physical damage. To reduce physical damage tubers should be handled carefully during all harvesting and transporting operations. Any injury, such as scraping, bruising or deep cuts from the cutlasses or hoes used for digging, will predispose the tuber to pathogen attack and subsequent rotting in storage. Tubers, especially those freshly harvested, should not be exposed for long periods to strong sunlight, as this may lead to injury and rotting;

(c) stored tubers should be checked routinely and tubers which are beginning to deteriorate should be marked for immediate consumption;

(d) if tubers are to be used only for food and not for planting, sprouting can be delayed by breaking off the head of the yam before tying in the barn or placing in storage. If planting sets begin to sprout before planting time it is not wise to break off the heads, but the sprout should be cut off as soon as they begin to elongate and be kept cut until planting time. This prevents the sprouts from exhausting the set before it is planted;

(e) storage areas, whether barns, cribs or pits, must be cleaned before any new yams are brought in. Old yams and debris should be removed from the store and surrounding areas. This avoids contamination by the pathogens carried in old decaying tubers. Piles of logs or pots, machinery, tools, etc. where rats and other rodents may hide should be cleared away. If trees are used for shading, they should be trimmed three months before storage begins so that sunlight can penetrate to the barn floor and sanitize it. Early trimming also enables the trees to regain the necessary degree of shading before the storage season begins. In yam barns insecticide dust may be sprinkled at the base of all vertical supports to control ants and other crawling insects, but great care must be taken to ensure that food yams never become contaminated with insecticide;

(f) it may be possible to modify the structure of traditional storages so that rat guards can be added.

Recent experiments indicate that the curing of yam tubers, using relatively high temperatures and humidities, can improve storage by healing wounds and toughening skins. Temperatures of 30-40C and 70-90 percent humidity for one to four days are effective in reducing storage losses. These conditions can be achieved by various means; one of the easiest is to cover tubers with a tarpaulin.

In curing, timing is important. Curing should be undertaken immediately after harvest to heal the wounds inflicted during harvesting and subsequent transportation to the barn. Any handling after curing must be carried out with utmost care to avoid new injuries.

The advantages of curing are greatest when tubers are stored in reduced temperatures, and less when they are put into traditional barns or cribs. The reason is that traditional structures allow the yam tubers to cure during the early part of the storage period.

Two large-scale advanced technology improvements have been studied. Although they are not appropriate for storage on the farm, they may have a future application as part of the marketing system for bulk storage at collection points. These include the following methods:

(a) controlled temperature storages used throughout the world for a wide variety of perishable products may be used to prolong the life of yam tubers. Temperatures of around 20C have proved effective in inhibiting sprouting and slowing down respiration. However, yams should not be stored below 15C, or chilling damage will occur;

(b) a second method, the use of gamma irradiation, has also been tested. Dr Adesuyi of the Nigerian Stored Products Research Institute has shown that if tubers are irradiated before being placed into storage, sprouting is inhibited for up to six months.

These are methods of the future, but they are mentioned to indicate that research on yam storage is not being neglected.

12. Processing of cereals (other than rice)

The harvesting and threshing of cereals are just as important as the harvesting of roots and tubers. Unless the operations are carried out efficiently other PFL (prevention of food loss) activities may prove of little value. For example, the husks of grains should not be broken in harvesting; otherwise, insect attack and infestation will develop more quickly. Provision of the proper equipment for harvesting and threshing, and training in its correct use, are essential PFL activities.

The principal operations in cereal processing are:


12.1 Threshing

The process of threshing separates the kernels from the stalks or panicles on which they grow. Threshing may take place in the field, or at the homestead or village; it may be carried out manually with the aid of animals, or with machinery. A simple method consists of beating the cereal heads against a wall or the ground; animals or humans can also trample the panicles on a hard surface, or animals can draw a machine or sledge over the grain.

Threshing machines may be powered by humans or animals or, in more sophisticated forms, by internal combustion engines. Many designs have been field-tested and found to operate satisfactorily.

Maize grains must be separated from the cob after the husk has been removed. A variety of manual and powered systems are available for this operation.


12.2 Grading

Grading consists of separating the sound kernels from chaff and impurities, and may be achieved by sieving or winnowing.

Figure 12.1 Manual sieving


12.2.1 Sieving. Impurities are separated on the basis of their differences in size from the kernels. Hand sieves are usually used singly. The simpler machines will have two sieves: one with oversized holes (which retain large impurities and let the grain kernel pass through), and one with undersized holes (which retain the kernels but allow smaller impurities to pass through).

12.2.2 Winnowing. In this process impurities are separated on the principle that their density differs from that of the grain kernels. The operation depends on air movement to remove the lighter fractions. The simplest method is to drop a basket of kernels and impurities in a thin stream onto a clean surface through a slight natural breeze. This is a slow and laborious process but it is still widely practiced.

Winnowing machines operate on the same principle, but air movement is created by a fan.

12.2.3 Selective picking. An internally indented cylinder will remove impurities which are smaller than the grain kernels by carrying them beyond the point at which the sound kernels are ejected and into a trough as the cylinder is rotated.

Figure 12.2 Three-sieve separator

Sophisticated machinery (SORTEX) is available for sorting individual kernels according to their colour, but these are expensive and are used only in specialized applications.

Hand-picking is an effective but tedious operation that is nevertheless widely practiced by farmers.

Figure 12.3 Simple aspirator


12.3 Milling

Milling involves the production of flour from the endosperm of cereal grains. In most cereals, including maize, the seed coat is first removed by stripping (either by hand-pounding after soaking or in a hulling mill) before being milled to make flour. Milling may be by hand-pounding in a mortar, by forcing the grain between two stones, or by using mechanically powered hammer, plate or roller mills.

Figure 12.4 Two-sieve cleaner with head aspirator


12.3.1 Milling equipment. The most widely used mills at village level are the plate mill and the hammer mill; for commercial operations, the roller mill is most common.

The plate mill consists of two circular cast-iron plates with surface burrs mounted on the same horizontal axis, so that the plates are vertical. One plate is held stationary and is attached to the frame of the mill; the other is mounted on a driving shaft and can be adjusted to alter the clearance or separation from the fixed plate. In operation, grain is introduced through the centre of the stationary plate and is ground when it passes between the plates to their edge. Here the flour is collected and discharged from the outlet spout. Some models have three plates, the two outer ones stationary and the central one rotating.

Figure 12.5 Three-sieve cleaner with head and tail aspirator

Great care is needed to ensure that the plates do not make contact. They must be separated, using the hand-wheel, when the mill is running empty. A feeding device is necessary to provide an even flow of material to the intake point, which is usually located at the centre of the fixed plate. The plate separation is adjusted by a hand-wheel which moves the driving shaft and its rotating plate in its plain bearings against a compression spring.

The shaft rotates at low speed-usually less than 1 500 rpm-and the output of a typical mill with a 5-hp motor and 270-mm diameter plates is approximately 250 kg/in. The plate mill will grind wet cereals, which would block a hammer mill. The fineness of grinding in a plate mill depends on the following:

Figure 12.6 Winnowing

Small diameter plate mills may be hand-powered.

The original hammer mill was developed from the hand-pounding system. The hand-held pestle was replaced by a heavier wooden hammer fixed at the end of a lever pivoted near its centre. When at rest the hammer rested in a hollow (often cut from rock) into which grain was placed. Pounding was achieved by pressing down on the other end of the lever, which raised the hammer, and then releasing it to fall under its own weight.

Modern hammer mills consist of a set of fixed or swinging hammers mounted on a rotating shaft and surrounded by a perforated metal screen. The shaft is rotated at up to 6 000 rpm, depending on the design and diameter of the hammers, which usually have a speed of 75-100 m/s measured at the tip of the hammers. The grain is introduced into the path of the rotating hammers through a slot in the screen and the ground material is discharged through the screen.

A paddle-bladed fan draws air through the screen and delivers the flour to the outlet spout, where it is separated from the air flow by a cyclone and/or woventextile filter bags.

The modern hammer mill is excellent for the fine grinding of dry cereals. It is not damaged by running empty and can be powered easily from high-speed internal combustion engines or electric motors. Output is generally about 60 kg/in per kW of power input. Up to 25 percent of the power is consumed by the fan, which not only removes the ground meal but provides the necessary flow of air through the screen.

The fineness of grinding depends almost entirely on the size and shape of holes in the perforated screens which partially or completely surround the hammers. The grinding action results from friction of the grains being repeatedly forced against the screen and against each other and, particularly with brittle materials, from impact with the hammers. The hammers are usually reversible to compensate for wear.

The roller mill is a more sophisticated form of mill than the plate or hammer mill and is used for producing high-quality fine flour, generally from wheat but also from maize and sorghum.

The precision-cast steel rolls have fluted surfaces and rotate in opposite directions at slightly different speeds. The clearance between the rolls can be set precisely, so that when fed with a single layer of carefully graded grain a small predetermined amount from each grain's surface is removed as it passes vertically downward between the rolls. The whole grinding operation consists of passing the grain through a series of such mills in succession, in up to ten possible stages. The output from each stage is sifted and the operation allows the various constituent parts of grain, such as germ and bran, to be collected separately. These mills have a high output and usually produce flour for the urban population.

13. Small-scale rice milling

13.1 Introduction

The production of white rice from paddy is complex and involves many operations. In large-scale plants the machinery and equipment used are very specialized, with each item only carrying out perhaps a single operation of the 20 or more that may be required for commercial rice milling. Large-scale plants must operate at high capacity to justify the investment in equipment.

In small-scale rice milling, with capacities up to 500 kg/in, a piece of machinery will carry out several of the operations in producing white rice from paddy, either in a single pass through the machine, or in several passes, with machine adjustments being made between each pass. Two or more identical machines may be used in successive stages of the process, each being adjusted to perform a specified task.


13.2 Stages in rice processing

The various stages in rice processing are shown in Figure 13.1 covering the operations from harvest of the panicle to the production of graded, polished white rice.

The moisture content of harvested paddy will usually be in excess of 20percent mcwb. This must be reduced to 12- to 14-percent mcwb for efficient hulling and processing operations. Paddy can be hulled outside this moisture content range but the performance of the machines is poor. The normal prehulling operations are as follows:

(a) parboiling. This is a process which involves soaking the paddy, then steaming and drying it. Parboiling improves the nutritional quality of the rice, makes the hulling operation much easier, and gives a greater proportion of whole-grain white rice. Parboiled paddy must be dried before milling. Rice milled from parboiled paddy stores better than non-parboiled rice, and has a different taste, colour and cooking properties. Parboiling is a costly operation but its benefits generally outweigh its cost;

(b) there are two main methods of drying. The prevailing local method is sundrying. The paddy is spread out on a clean surface (tarpaulin, concrete slab or even smooth, clean earth) and regularly turned by hand. Excessively rapid drying results in the development of hairline cracks in the endosperm of the paddy grain (sunchecking). These cracks enlarge and produce a higher proportion of broken grains during subsequent operations. The incidence of cracks is reduced by a slower rate of drying which, in turn, can be achieved by increasing the thickness of the layer of paddy during sun-drying up to 150 mm, and by frequent stirring.

Figure 13.1 Stages in rice processing

If artificial drying is employed the manufacturer's instructions should be followed. With very wet paddy, and particularly after parboiling, it is common practice to dry in two stages separated by a resting period during which the paddy is aerated.

Cleaning is an important operation; small stones and pieces of metal can damage the huller, while pieces of straw may cause an uneven flow of paddy to the huller. All impurities should be removed before the paddy is hulled. A combination of sieving and aspiration is commonly employed to separate the light impurities and a de-stoner is used to remove denser impurities.

If the paddy is to be parboiled before hulling, it should be washed and drained before being soaked, in order to remove soluble impurities which may otherwise discolour the grains.

13.2.1 Hulling operations. During this operation the hull (or husk, or shell) is removed from the paddy grain to produce brown rice. The husks have no nutritional value but may be used as fuel, perhaps in the parboiling operation (see Fig. 13.2). The ash can be used as a source of pure carbon for steel-making.

Figure 13.2 Diagrammatic enlarged vertical section of a paddy grain

Figure 13.3 Engleberg-type steel roll huller

Figure 13.4 Rubber roll huller

Figure 13.5 Engleberg-type steel roll huller and polisher

Figure 13.6 Indented cylinder cleaner

The Engleberg (Grant, Planter) huller represents an old design which is still widely used at the village level, and may also be used for processing maize. It consists of a fluted steel shaft operating inside a perforated steel screen, which also carries a projecting strip of steel whose distance from the shaft can be varied. To operate satisfactorily the huller must be full. The degree of hulling is regulated by the clearance between the steel strip and the shaft, and by the rate at which the mixture of rice, husks and undulled paddy is allowed to discharge from the working chamber. An adjustable slide controls the discharge rate.

Various accessories may be added to the Engleberg huller. The common attachments are a polisher, consisting of rotating cylinders fitted with leather strips which press the rice against a perforated housing, and a simple husk aspirator.

In the rubber roll huller the paddy is passed in a single layer between rubbercovered rollers rotating in opposite directions and with different surface speeds; as the paddy passes between the rolls it is subjected to a shearing action which removes the hull. Its action is far more gentle than that of the steel shaft huller, resulting in a greater yield of unbroken rice.

The rubber roll huller is frequently supplied with a husk-aspirating attachment. This separates the hulls. and immature paddy grains from the brownrice fraction.

Disc hullers are not generally used for small-scale rice milling operations.

13.2.2 Post-hulling operations. The main post-hulling processing operations are whitening, polishing and grading. In large-scale processing plants, these operations are multi-stage and utilize a succession of specialized machinery. In small-scale processing some operations may be omitted (e.g. grading); they may be unnecessary (e.g. the steel roll mill also removes the bran layers), or may be carried out by a second hulling machine that is appropriately adjusted.

Whitening refers to the removal of the bran layers as a separate operation after hulling. These layers closely adhere to the endosperm and have to be removed by rubbing against an abrasive surface and against other grains. The Engleberg-type huller can remove husk and bran in one operation.

Polishing is the final, more gentle stage consisting of cleaning bran particles and dust from the white rice and smoothing its surface so that it looks better.

Whitening and polishing operations are combined in the steel roll mill with polisher attached.

Grading of the polished rice into whole grains and broken grains if necesary if the white rice is to be sold, or stored for more than a few days. Broken grains deteriorate more rapidly than whole grains and whole kernels usually command a higher price.

Grading is carried out by sorting machines on the basis of grain size (using sieves or an indented cylinder cleaner), grain density (using aspiration) or a combination thereof.

14. Sociological, economic and institutional implications of the prevention of post-harvest food losses

Physical losses occur at various stages after a crop has matured and before the food is consumed. Losses may be reduced at any stage of the post-harvest system by improved harvesting, drying, storage, processing or handling methods. The processes and operations, however, are interrelated and are subject to climatological, sociological, economic, agronomic, cultural and ecological conditions imposed by the environment in which they take place.

The effectiveness of any action undertaken to reduce losses must be economically justifiable, and also practical within the prevailing post-harvest system. In attempting to reduce or even assess post-harvest losses, it is essential that the functioning of the system in any particular environment be fully understood and analysed. Only then may the constraints, problems and eventual solutions or improvements be identified.

In Sierra Leone, for example, parboiling of rice is practiced because such rice has a higher nutritional content for which consumers are willing to pay more. The milling of parboiled rice is also easier, as despite poor milling equipment fewer broken grains are produced and less loss incurred. Whereas in Sierra Leone the provision of parboiling equipment is appreciated, in Malaysia, parboiled rice is regarded as a food for the lower classes and many Malaysians are willing to pay extra for well-milled white raw rice with a low percentage of broken grains. Hence, extending the practice of parboiling here would not be generally acceptable.

Earlier sections of this manual have dealt with loss assessment, as well as the main technological and biological aspects of the post-harvest system. As already noted, the system must also be considered in its entirety before the adoption of innovations. It is therefore necessary to take into account such factors as cost-effectiveness, the institutional framework (including the marketing system), labour availability and consumer preferences.


14.1 Economic justification

Post-harvest treatment of any commodity is only undertaken where it will result in a reward to the owner. In a subsistence economy the activity may be the storage of grain or tubers, where the benefit arises from the longer period over which consumption of the commodity may be enjoyed. Harvests usually occur at the same time, leading to a glut of produce that cannot be consumed immediately. Part must therefore be stored if it is not to be lost.

In a mixed subsistence and cash economy or where a crop is produced solely for cash sale, producers will only make those changes in post-harvest procedures which they consider will increase their revenue. It is hoped that PFL (prevention of food loss) activities will lead to increased cash returns, but they will only be adopted if the cost/benefit relationship of the operation is favourable, and when the markets are able to absorb the increased supplies at a price which is profitable to the producer.

The cost of PFL activities depends on many factors. PFL project activities are normally concerned with the introduction of techniques to reduce physical losses and improve the incomes of small-scale farmers. They are concerned with improved handling, storage and primary processing of grains, pulses and roots and tubers, and with techniques to maintain the quality of fruits and vegetables. Activities have included providing farm and vi]lage storage structures, designing and constructing warehouses, providing small-scale driers, improving processing facilities (from threshing equipment for rice to field-grading and packing of fruit and vegetables), improving rodent and insect control measures and undertaking training activities on all aspects of loss reduction during the post-harvest period.

It is important that the initial cost-benefit analysis be positive. According to some reports a cost-benefit ratio of 1:1.5 is insufficient to persuade individual farmers to take the risk of introducing a change in a post-harvest activity, but a ratio of 1:2 is likely to provide the necessary incentive. This may be taken as an important guideline by both planners of activities to reduce postharvest losses and those who have the responsibility for project implementation and for training in these matters.

For example, the provision of bin storage made of sheet metal to a farm or village would undoubtedly reduce losses in grain, but the initial cost could be so high in relation to the extra grain saved in the short run that other farmers would not be interested. On the other hand, where the cost is small a development will be repeated, such as using a straw and mud-coated container with a little malathion insecticide. In this case, only the malathion has to be purchased, while the straw and mud can be gathered and the container made with family labour. In the Scarcies area of Sierra Leone, for example, rice is stored in large wooden boxes, which many houses in the area have. The boxes are about 2 x 1.5 x 1.5 m and are made from planks of hardwood which were once readily available in that country. They are impervious to rodents and are often an integral part of the house. The initial cost was negligible and the boxes have endured for many years. This specific example is given to demonstrate the utility of using locally available, inexpensive materials and methods.

In a cost-benefit assessment, another factor to take into is whether the commodity is for home consumption or for cash sale. If an improvement of quality only is related to home consumption, producers will be reluctant to pay cash for the innovation. The introduction of simple crop driers has been of interest where the crop is consumed by the family, although discolouring and offflavours may in fact develop. The situation is different when the crop is for cash sale, particularly if the sale prices are appreciably different for varying levels of moisture content or admixture content. Normally, the producer will wish to take steps to reduce imperfections in order to attract the highest prices, but price differentials may not be sufficiently wide to provide an incentive for the producer to improve quality. Those responsible for price-fixing policy in grain procurement authorities should note that a sufficient price incentive for welldried grain (usually 14-percent maximum moisture) will remove the burden of drying from the authority and encourage efficient drying on the farm. This ensures that drying is done more quickly, thus promoting a viable PFL activity for the producer, reducing physical losses, and, at the same time, greatly reducing the authority's operating costs.

An important consideration in the cost-benefit ratio is provision for replacement of the capital assets. Tools, machinery or storage provided under such a PFL activity need repair and maintenance and will eventually wear out and require replacement. These factors should be taken into account in the initial costing estimates for the activity.

In analysing cost-benefit ratios, it is important to be as accurate as possible. It is easier to arrive at costs than to quantify benefits. Costs may arise which were not anticipated, and they should therefore be estimated on the higher side. Benefits will usually be based on estimates of future selling prices unless sales are being made to an agency, such as a marketing board, that has already declared its buying price for the next season.

The following example shows how a cost-benefit analysis should be made. The example is hypothetical, but the costs and prices are based on actual figures recorded in Indonesia in 1983.

In a village producing cassava in Southeast Asia, it is proposed to introduce a solar drier as a PFL activity. The product, cassava chips, is destined for sale to a processing factory which will pelletize the chips for export. One problem facing the processing factory is that normal sun-dried cassava chips are contaminated with dirt and fungal growths, are discoloured and may have acquired off-flavours. Animals and people walk over the drying material, there are showers of rain, and the wind blows dust onto the drying chips. For this product the processing factory will pay Rp 40/kg. For clean, uncontaminated cassava chips, the factory will pay Rp 45/kg. Use of a solar drier prevents this contamination. All the materials for constructing a solar drier are available in the locality or may be purchased in the village or the local "kecematan" (county) town. The capacity of the drier is 1 tonne of cassava chips, and the drying time is three days. Cassava production and processing is a year-round activity, which means that about 120 tonnes of chips could be dried annually. Assuming that only 50 tonnes will be dried in the year, the completed drier should last for several years, subject to repair and maintenance; but the initial cost will be covered within one year. It is also assumed that the piece of ground in the village on which the drier is built will be provided free, since many villagers will use the facility. Also, producers will provide their own labour to fill and empty the drier, as they do in spreading chips on the floor in the traditional way. Other costs such as handling, packing, sacks and transport are the same, whether drying traditionally or using the solar drier.

Cost of construction of the solar drier (1983, Rp 970 = US$ 1)

48 bamboo poles each 6 m long Rp 250 each Rp 12 000
110 m white plastic sheeting, 0.10mm thick   200 m 22 000
2.5 m black plastic sheeting, 0.10 mm thick   200 m 500
1 kg nails   200 kg 200
1 roll yarn   200 each 200
9 pieces wood board 2 m x 25 cm   1 000 each 9 000
8 pieces wire netting, No.18 gauge 2m wide   300 each 2 400
1.5 q charcoal   10 kg 1 500
41 tar   500 tin 500
Labour, 7days (1 carpenter, 21labourers)   5 000 day 35000
  Rp 83 300
  or, approximately Rp 90 000
Normal return for 50 tonnes of cassava chips at Rp 40/kg Rp 2 000 000
50 tonnes of clean cassava chips at Rp 45/kg   2 250 000
Additional return from using solar drier Rp 250 000

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