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Grain legumes: Processing and storage problems

Introduction

Grain legumes or pulses occupy an important place in the world food and nutrition economy. Present world production is estimated to be 50 million metric tons and, at an average price of $400 per metric ton, its total value would amount to $2 billion. They are important constituents in the diets of a very large number of people, especially in the developing countries, and are good sources of protein which help to supplement cereal diets, improving their protein nutritive value. They also provide substantial quantities of minerals and vitamins to the diet. Although most legumes are consumed as dry grains, immature green pods or green seeds are also used as vegetables.

The availability of grain legumes over the last few years has dropped because their production has not been very profitable compared with that of other crops. The consumer demand for legumes has, however, not fallen and prices have increased considerably. Improved conservation and processing to reduce post-harvest losses, and the manufacture of economically priced products based on grain legumes will help to increase the supplies. The development of this industry would provide additional rural employment, improve nutrition standards, bring a better price to the grower, and ensure supplies at lower prices to the consumer.

In Asia and Africa, a substantial portion of the grain legumes is consumed after having been milled for removal of the husk and splitting, or after some form of processing. However, most of the commercial technologies available for this purpose are either obsolete or inadequate and result in heavy losses due to breakage and powdering of the grain. Successful efforts have been made to develop improved technologies to reduce milling losses and improve product quality. Similarly, there is need for development and utilization of improved technologies for the manufacture of products based on grain legumes.

Grain legumes are more difficult to store than cereals and they suffer much greater damage from insects and microorganisms. This not only results in quantitative losses, but also in qualitative reduction of the nutritive value because of vitamin loss and deterioration of protein quality. The milling losses in insect-damaged grain are even higher as more breakage and powdering occur with such grain.

Although many species and sub-species of legumes are known, only about a dozen of them are important as commercial food crops. (The common legumes and their botanical names are given in Table 1. The contributions of different legumes to the total world production and the major producing countries are listed in Table 2.) Beans and peas each account for about 25 per cent of the total production of legume crops. Chickpea and broad beans rank next in importance. Some of the legumes, however, are of only regional or local importance.

TABLE 1 COMMON LEGUMES AND THEIR SCIENTIFIC NAMES

Processing

Legumes go through several primary processes-hulling (husking), puffing, grinding, splitting, etc.-before they are used in different food preparations. The primary processing methods followed in different countries are summarized below.

Hulling, practiced widely in Asia and Africa either on a home-scale or as a cottage industry, produces refined cotyIedons with good appearance, texture, and cooking qualities. Husked grains are easily digested and efficiently utilized by the body. As the husk tightly envelops the endosperm, usually through a thin layer of gums and mucilages, the primary step in hulling involves laborious procedures.

The oldest and most common home-scale technique for hulling grain legumes is to pound them in a mortar with a pestle, either after spreading the grains in the sun for a few hours, or after mixing them with a little water. The husk is then winnowed off to get the clean cotyledons. Methods followed in the home or village industry or in commercial mills are usually similar in principle, but differ in the use of techniques for better yield, operational efficiency, and large-scale application.

Home-scale hulling consists generally of two steps: (a) loosening of the husk by wet or dry methods, and (b) removal of the husk and cleaning. In South Asia, the first step is achieved by sun-drying the raw mature grains as such, or after they have been treated with oil and/or water. In some areas, grain is steeped in water for two to eight hours prior to sun-drying. Grain varieties whose husks are tightly attached to the cotyledons are soaked and then treated with red-earth paste before being sun-dried. The steeping technique to loosen the husk is also practiced in several Southeast Asian and African countries.

Dry-method husking is accomplished by pounding the grain in a mortar with a pestle, or grinding in a hand-operated wooden or stone sheller. The husk is then separated by winnowing. This is common practice where dry cotyledons or grain-legume flours are used in food preparations. When a batter or dough is to be prepared, the soaked grain is either rubbed by hand to remove the husk, after which it is separated by flotation, or wet-ground in a stone grinder. In several Southeast Asian countries this technique is used for extracting starch from mung beans, the husk being removed by straining a dilute slurry through coarse muslin or other cloth. Home-scale methods are generally laborious and not always hygienic.

In village industries, the techniques employed for loosening the husk are the following: (a) prolonged sun-drying until the husk is loosened; (b) application of small quantities of oil, followed by several hours or even days of sun-drying and tempering; (c) soaking in water for several hours, followed by coating with red-earth slurry and sun-drying; (d) soaking in water for several hours to loosen the husk before manufacture of food products; or (e) a combination of these techniques. There are no standard procedures developed for any specific variety of legume.

Removal of the loosened husks from the grain in the dry-milling technique is commonly done in small machines. Hand-or power-operated under-runner disc-shellers or grinders with emery or stone contact surfaces are used. A plate mill with a blunt contact surface is sometimes used both to husk and split soaked and dried grains. After aspirating or winnowing off the husk, the split cotyledons are separated by sieving. Remaining unsplit whole grains are similarly processed until almost all the grain is husked. In certain parts of India, oil-treated and sun-dried grains are husked in an Engelberg-type rice-huller after being mixed with 2 - 3 per cent stone powder. Sound kernels are removed by sieving, while the husk, powder, and small brokers remain in the stone powder. The husk of the velvet bean (soaked and cooked), used for the manufacture of tempeh in Indonesia, is removed by trampling the beans in wooden vats and then separating the husk by flotation. Wet-grinding of soaked whole grain, followed by the straining of the diluted slurry through cloth, is practiced in Thailand, the Philippines, and other Southeast Asian countries as a means for removing the husk from mung bean for the extraction of starch and the manufacture of noodles. In some West African countries, cowpeas, either whole or broken into coarse grits by stone grinding, are soaked in water and agitated until the husk separates and floats in the soak water, from which it is sieved off.

TABLE 2 PRODUCTION OF DIFFERENT LEGUMES AND MAJOR PRODUCING COUNTRIES*

* Monthly Bulletin of Agriculture, Economics & Statistics, 1976, 25(1), 1.
** Production Year Book, 1973 and 1974, FAO, Rome.

Hulling methods are not one-step operations. About 50 per cent removal is achieved in the first operation. After separation of the husked, split cotyledons (dhal), the process is repeated several times until almost all the grain is converted into dhal. In the process, excessive breakage with powdering of grains occurs because of repeated splitting and husking operations. Complete removal of the husk from the grain is not always achieved in cottage-scale processing, particularly with some varieties; hence, unpolished (split and unhusked) dhal is sometimes put on the market.

The cooking quality of dhal prepared by wet methods is usually poor. This is especially true of the pigeon pea for which cooking time increases with duration of soaking. However, such dhal has an attractive appearance and a more desirable flavour. The wet method may be useful where the pulse is to be ground to a paste for further processing. If it is to be dried as dhal the wet method is laborious and entails the loss of solubles in the soak-water.

Hulling of legumes on a commercial scale is generally based on dry-processing techniques. Many of the operations, particularly husking and splitting, are mechanized. The drying is done in large yards, and is completely dependent on sunshine. Legumes such as pigeon pea, black gram, and mung bean, which are more difficult to husk, require more of the oil or water treatments followed by prolonged sun-drying; (pre-milling treatments), while grains such as chickpea, lentil, peas, Lathyrus sativus, etc., with more easily removable husks, require short periods of sun-drying and fewer oil or water treatments. Sometimes these grains are given an initial "pitting" in the roller mill to crack the husk and improve the absorption of oil or water. In the case of black gram, the coating of wax and dust is removed by initial scouring in a roller mill, facilitating absorption of water or oil. Husking and splitting are done either in a single operation or, more advantageously, as independent operations. Moisture addition adversely affects husking, but it helps to split the grain. Addition of water prior to husking helps to induce simultaneous splitting, but this often leaves patches of husk on the split cotyledons (dhal) that have to be removed by scouring in polishing machines.

As the husk forms 10 - 16 per cent of the grain, a maximum theoretical yield of 84 - 90 per cent of kernels should be possible during hulling. In practice, the yields vary from 68 to 76 per cent as a result of breakage, powdering, and other milling losses. During splitting, the germ, which forms about 2 - 5 per cent, is also lost. In wet methods, water-soluble nutrients are also lost in the soak-water.

Improved technologies for hulling of grain legumes

The improved method developed at CFTRI for hulling grain legumes commonly consumed as dhal or split-grain legumes in India, Pakistan, Bangladesh, Nepal, and some other countries involves a conditioning technique of moisture adjustment of the grain to a critical level in order to loosen the husk. In this process, the grain is exposed to heated air, at a specific temperature appropriate for each variety, for a predetermined time and equilibrated to the critical moisture level with gradual aeration in tempering bins. The husk is removed in an improved abrasion-type hulling machine that accomplishes almost complete removal in a single pass with the least possible scouring or breakage of the endosperm. The whole husked grain can be split, if desired, in a splitting machine after suitable conditioning for which the technology and equipment have already been developed. The method is independent of the influences of climate and variations among varieties. This improved technology has been shown to increase the yield by 5 - 10 per cent.

The time taken for processing is also less and the cost of operation is lower. Small commercial models of complete automatic milling plants of 0.5 - 2-metric-ton-per-hour capacity have been designed and put into operation. The process originally developed for pigeon pea has been adapted, with suitable modifications, for processing other legumes such as chickpea, mung bean, black gram, lentil, pea, soya and some other beans.

In the Prairie Regional Laboratory, Saskatoon, Canada, a Hill threshing unit consisting of carborundum discs has been successfully used for dehulling cowpea. Initial findings indicate that the unit is capable of mechanically hulling brown Nigerian cowpeas that can subsequently be converted to flour. The husk is removed by the abrasive action of rotating discs, the amount removed being dependent upon the through-put and retention time. An acceptable product would necessitate removal of about 27 per cent as polishings from the cowpea, whose husk content is 3 - 5 per cent (22 - 25 per cent loss of kernel). This unit is being put into rural-scale operation for processing cowpeas in some African countries.

Factors affecting the hulling characteristics of grain legumes

The hulling characteristics of grain legumes are influenced by the variety, season when harvested, and region of cultivation. Larger or bold-grain varieties are easier to hull, give a higher yield, and are preferred by millers, while the smaller varieties require repeated and severe pre-hulling treatments and complex procedures Freshly harvested grain and winter crops are more difficult to process, possibly because of their higher moisture content. Such grain is either stored for some time to reduce moisture, or treated with lime water or a solution of sodium carbonate to loosen the husk

Wet and dry grinding

Whole legumes or husked splits are either ground dry into a flour or ground wet into a batter for a number of sweet and savoury preparations, either alone or in combination with cereal and millet. The eating quality of many of these products, particularly the texture, depends on the composition of the flour, degree of fineness of grinding, relative proportion of particles of different mesh grades, and cooking conditions. Chickpea, peas, black gram and cowpea are the common grain legumes ground wet or dry.

Puffed grain legumes

Puffing of legumes by subjecting them to high temperatures for a short time has been practiced in Asia, Africa, and Latin America for many years.

On a home-scale, the grain is first heated gently to about 80°C and moistened with 2 per cent water, which is allowed to be absorbed overnight. The grain is then toasted with hot sand in a shallow pan at 250 - 300 °C. The cotyledons expand and split the husk. The roasted legume is gently rubbed against a coarse surface to separate the husk. Puffed legumes are manufactured by similar techniques as a cottage industry, using husk-fired furnaces and large toasting Pans. Either several people are employed or mechanical processes are used.

Although the chickpea is the traditionally puffed legume, peas and cowpeas are also being puffed in many countries. Yields of puffed products are 65 - 70 per cent by weight. The puffing expansion of the grain legumes is low (1.5 times) compared with cereals (8 - 10 times) Exploratory studies with the chickpea shows that moisture conditioning or moisture addition prior to heating helps to improve puffing and certain hardening agents such as calcium phosphate, egg white, gums, calcium, and sodium caseinate are also effective. It will be useful to investigate varietal differences, if any, to see whether they affect the puffing characteristics, and to find out whether puffing expansion can be improved by pre-processing or conditioning the legume prior to puffing.

Milling for manufacture of gums

Many grain legumes have a layer of gums between the husk and the endosperm, and these vary in quality and quantity. Industrial-grade gums are economically extracted from some legumes such as guar. Guar bean contains about 40 per cent gum (45 per cent kernel and 15 percent husk). About 28 - 30 per cent of the gums are extracted by a dry-milling technique. For food purposes, this industrial-grade gum is further refined to remove the last traces of the husk and to give a final gum yield of about 25 per cent. A by-product consisting mostly of kernels containing 62 per cent protein is used as a cattle feed after detoxification.

Storage of Legumes

Grain legumes can remain in edible condition for several years if properly stored. However, they are susceptible to infestation, both in the field and during storage, by weevils, which are prolific, breed rapidly, and cause serious deterioration in the nutritive value of the grain. Damage ranging from 30 to 70 per cent of the grain has been reported in various publications.

At 30° C and 70 per cent relative humidity ( R.H.), some species of bruchids take only a few weeks to develop from egg to pupa. Higher humidities are conducive to more rapid proliferation of all species.

Losses due to insect infestation

An evaluation of the damage caused by one species of bean weevil showed that nearly 28 per cent of over 240 market samples contained damaged cowpeas exceeding 5 per cent. Fifty-six days of storage resulted in infestation and damage of 69 per cent of market samples. High temperature, high humidity, softness, and high nutritive quality, as well as storage in small quantities, are all conducive to insect damage. Losses may be aggravated by protracted storage, unhygienic warehouse conditions, and left-over, infested sweepings. Damage during harvest may increase the vulnerability of some crops to insect attack.

When grain legumes are husked and split into dhal they become vulnerable to infestation by other stored-product insects and moths.

Effect on germination

Seeds meant for sowing usually have to be preserved for a period of at least six months, and in the tropics this period is long enough for the grain to become seriously damaged. Many of the important pests which attack cereals and legumes do not attack the germ at the initial stages but live in the cotyledons and thus a high proportion of viable seeds is not greatly affected until the insect population becomes high. The loss of germination capacity of from 80 per cent to zero in field beans and 75 per cent to 24 per cent in black gram during a storage interval of two to six months has been observed. In case of high moisture (R.H. 85 - 90 per cent), stored seeds may lose their germination capacity due to the pathogenic action of moulds. Germination may also be severely affected by the complex changes caused by heating and a rise in grain moisture as a result of insect respiration.

Prevention and control of infestation

Selection by breeding of resistant varieties is one method of pest control. Early-ripening, large-seeded varieties of field beans (Vicia faba) are reported to be more heavily infested by certain species of bean weevils than are late-ripening, small-seeded varieties. The physical characteristic of the seed coat and the starch composition of the cotyledons are considered to be factors in determining infestation.

Pre-harvest prophylaxis is the insecticidal treatment of the mature grain before harvest in order to prevent carry over of field infestation to the warehouse. Dusting of the crops with appropriate insecticides when the first ripe pods appear, followed by a second application ten days before harvest, has also been suggested as a prophylactic control measure.

Post harvest conservation

(a) Physical treatment such as exposure of grains to temperature -10 °C or heating the grain at 55 - 60 °C for 12 minutes after spreading it in a layer of 4 - 5 cm has been tried. Employment of X-rays and gamma rays is another prospective control measure.

A control method based on restricting the available space between the grain legumes by plugging the holes with smaller grains (0.4 - 2 mm) such as ragi (Eleusine coracana), which would "dissuade" the weevils from ovipositing, has been proposed. Steaming or parboiling of the grain to bring about hardening by gelatinization of the starch has also been advocated. Pearling also makes the grain unattractive to bruchids.

(b) "Non-toxic" grain protectants Inert dusts have been employed to control different types of insect pests. Of several clays, e.g., diatomite, bentonite, kaolin, and talc, tested for effectiveness, diatomite has proved most successful. Rock phosphate at 1 per cent level is also used to control insects in horse bean (V. faba), cowpea (V. unguiculata), and lentils. Kaolinic clay, when activated and transformed into metahydrogen halloysite, killed 100 per cent of the insects within 24 hours. Cowpeas treated with activated clay at 10g-per-kg level remained completely free from infestation for up to 225 days. The mortality is attributed to the loss of moisture from the insect's body after the treated clay had absorbed the epicuticular wax from the beetles. The practice of mixing paddy-husk ash and fine sand with the grain or packing the top exposed surface of grain legumes in bamboo containers with wet mud are some of the common protective measures in use in Indian villages. Tricalcium phosphate at level of 3 per cent is also found useful for insect control.

Lemon oil was found to be quite a promising protector against cowpea weevils in black-eyed peas. Limonene, a constituent of citrus oil, and other terpenes such as pinene, cineole, carvone, phellandrene, and terpineol also possess insecticidal properties. Ground-nut oil mixed with cowpeas at 5 - 10 ml per kg of seed offers protection for more than six months. Castor, mustard, and sesame oils at a level of 0.3 per cent also inhibit weevil multiplication in green gram. Coconut and ground-nut oils are also reported to be effective at a level of 0.5 per cent. Use of the toxic extracts of various plants as contact insecticides is also being investigated.

(c) Biological methods to prevent and control infestation are also being thoroughly investigated. Use of other predacious parasites, hormone analogs that block embryonic development, chemosterilants, and chemical pesticides are all under active study.

Various types of fumigants have been tested and found very useful for control of insect infestations. Some of these are carbon disulphide, ethylene dibromide (EDB), aluminum phosphide, methyl bromide (MB), and methyl iodide (Ml). Treatment schedules and precautionary measures for fumigants such as thylene dibromide, ethylene dichloride, carbon tetrachloride, methyl bromide, and phosphine are also available.

General observations on storage

In the tropics, cereal and leguminous grains need continuous protection against insect attack at all stages, as fieldtostorage infestation is common. Inadequate storage methods immediately after harvest and before processing add to the problem, and infestation continues to increase during transportation and long-term seasonal storage before processing, causing an estimated overall loss of over 30 per cent. The types of structures and containers used for drying and storing are as important as the infestation and insecticide treatments (including fumigation) employed to counteract pests. The storage facilities of traders are usually better than those of subsistence farmers. Warehouses for large-scale, long-term storage have generally proved superior to those available to, and used by, the trade.

Utilization

Grain legumes, processed and converted into primary products, are further utilized in various ways after cooking, or are processed to make different products in the home or in commercial catering establishments. Problems relating to such utilization are discussed here.

Effects of cooking on quality of legumes

Food legumes, either as whole grains or as husked and split dhal, are consumed in the form of a soft, cooked product. Cooking imparts a soft texture, improves palatability, enhances digestibility, and also increases the nutritive value by destroying certain antigrowth factors and enzyme inhibitors. However, legumes require a very long cooking time, ranging from one to four hours in the case of whole grains and 30 to 90 minutes for dhal, in order to reach an acceptable soft consistency. The reasons why such long cooking times are necessary are not fully understood. The reduction of cooking time is, therefore, a primary problem for pulse-consuming countries, especially in view of the prevalent fuel shortage.

The husk is relatively impermeable to water. Scarifying the husk or its complete removal improves the hydration rate and shortens the cooking time. Storage results in hardening of the husk and adversely affects the cooking quality of beans. Presoaking of beans in water overnight or in hot water for short periods has been practiced in the industry as well as in households in order to reduce the cooking time. The chemical composition of peas and pigeon peas with respect to their calcium, magnesium, pectin, and phytin contents is also known to affect cooking requirements. Conversion of the calcium and magnesium ions into an insoluble form, or treatment with agents that chelate with the calcium and magnesium, has been known to improve the cooking quality.

Addition of alkaline salts or other chemicals to remove the interference from calcium or magnesium compounds present in the bean has been employed to reduce cooking time. The use of baking soda for this purpose is also well known. Its concentration should be regulated so as not to raise the pH beyond 8, otherwise it imparts an alkaline flavour and also destroys some of the vitamins.

Alkaline salts such as trisodium phosphate, sodium hexametaphosphate, or sodium citrate can be used in small quantities without appreciably raising the pH or altering the taste of pulses. These chemicals could either be added to the cooking water, or incorporated into, or coated upon, the surface of the dhal as a final step in milling.

Cooking under pressure is probably the best method for reducing cooking time. Reduction of the thickness of the dhal by flaking has also been shown to reduce the cooking time. Flaking, however, enhances the pastiness of the cooked product, and overimbibition of water imparts an insipid taste. A change of flavour caused by enzyme action is also caused by flaking. Flaking can be very useful in preparing quick-cooking pulses if the thickness of the flakes is controlled and the material is heat-treated to destroy enzymes and to decrease viscosity.

While cooking is known to destroy the antinutritional factors in legumes, the long cooking times needed for softening of beans may lower the nutritional value through destruction of vitamins, the binding of Iysine, or by leaching out the water-soluble nutrients. It is desirable to soften the legumes prior to cooking to lessen the cooking time to about 30 - 45 minutes, which also saves energy and lowers the cost. Technologies have been developed to provide quick-cooking, convenient legume products for immediate use.

Correction

Food and Nutrition Bulletin, Volume 1, Number 1, October 1978, page 47.

The name of the director of the Institute of Nutrition and Food Technology (INTA) should be changed from Dr. Fernando Monckeberg to Dr. Sergio Oxman.

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