Contents - Previous - Next

This is the old United Nations University website. Visit the new site at http://unu.edu

Food science

Faba beans (Vicia faba L.) and their role in the human diet

A. Askar
Department of Food Science and Technology, Faculty of Agriculture, Suez Canal University, Ismailia, Egypt

INTRODUCTION

Faba beans, formerly know as broad beans, horse beans, and field beans, are among the oldest crops in the world. The Chinese used them for food almost 5,000 years ago, and they were cultivated by the Egyptians 3,000 years ago, by the Hebrews in biblical times, and a little later by the Greeks and Romans. Today, faba beans are a major crop in many countries including China, Ethiopia, and Egypt, and are widely grown for human food throughout the Mediterranean region and in parts of Latin America (table 1). In the United States and northern Europe, however, faba beans are not grown in large quantities and are used almost exclusively for livestock pasturage, hay, and silage.

People living in the Mediterranean region and China may depend upon faba beans to supply much of their dietary protein. About 75 per cent of daily per capita protein intake in Egypt is of vegetable origin, mostly cereals and beans Egyptians consume about 14 g per capita per day of faba beans, which accounts for about 3 g of protein. In the Mediterranean region almost all of the broad bean crop is dried, and only small amounts are consumed fresh. Higher meat prices during recent years and the need for protein rich foods have led people in most less developed countries to shift their consumption to certain grain legumes, especially Vicia faba.

TABLE 1. Leading faba-bean-producing countries of the world

Source: FAO, Production Yearbook 1982.

Faba beans, like other beans, are a good source of calories, protein, carbohydrates, and fibre (tables 2 and 3). They are also rich in phosphorus, iron, potassium, and vitamin B complex [16, 20] . However, the ratio of calcium to phosphorus is low, so the calcium may not be well utilized unless other foods rich in calcium and somewhat lower in phosphorus (such as dairy products and green leafy vegetables) are consumed in addition to the beans.

The percentage of solids in the mature seeds is over three times that of the solids in the immature seeds; hence, the mature seeds are much higher in calories, protein, minerals, and starch, although the immature seeds are a much better source of vitamins A and C.

POPULAR WAYS OF PREPARING FABA BEANS IN THE MIDDLE EAST

Stewed Faba Bean (Fool Medames)

Most people eat stewed faba beans for breakfast and supper as well as in sandwiches at any time of the day. Many ad" ditives such as oil (cottonseed oil), salad, spices (pepper and cumin), lemon juice and eggs are used, while cheese is a usual adjunct of faba bean dishes. The sandwiches of stewed faba beans consist of almost 80 per cent bread(prepared from wheat flour) and about 20 per cent cooked faba beans mixed with oil, salt, spices, and salad.

TABLE 2. Chemical composition of faba beans (percentages)

a. FAO, Food Composition Table for Use in East Asia (1972).

TABLE 3. Chemical properties of Canadian and Egyptian faba bean samples (percentages)^a

a. Means of 20 samples of faba beans (10 each from Canada and Egypt).
Source: Youssef et al. [71].

The dried seeds are stewed in about twice their volume of boiled water, on a flame which is only high enough to keep the content gently boiling for 10-12 hours until the beans become soft. Soaking the beans in water before cooking shortens the cooking time. The cooking time for dried beans may also be shortened by pressure cooking, in which case fat must be added to prevent excessive foaming, which could prove dangerous by clogging the vent of the pressure cooker. Stewed beans, cooked under pressure, cook down to less volume than beans cooked gently. The green immature pods of faba beans or stewed faba beans can also be cooked in tomato sauce prepared from fried onion, fat, spices, and tomato juice.

Bean Cakes (Falafel or Taamia)

The most common way of preparing faba beans is in the form of dried cakes, which are widely used in the Middle east as a breakfast and supper delicacy, usually in sand-wiches containing 70 per cent wheat bread, 20 per cent falafel and 10 per cent green salad. The decorticated dried beans are soaked in water for 12 hours and then the water is drained off. Small amounts of garlic, spring onion, parsley, dill, and spices (salt, pepper, and cumin) are added for flavouring. The mixture is then crushed into a thick paste. When ready, the mass is removed from the mortar and allowed to stand for some time. The paste is finally shaped into small, round pieces and deep-fried in boiling cottonseed oil until the surface turns from green to a uniform brown.

Germinated Beans (Fool Nabet)

Germinated and cooked faba bean is a traditional meal in Egypt. The dried seeds are soaked in water for about 12 hours then germinated for three days. The germinated beans are then cooked in boiling water for about one hour and served after adding spices, fried garlic or onion, and pieces of dried bread (Nabet soup).

In the preparation of stewed faba beans (medames), the cooking time required to reach the optimum eating quality varies among cultivars. The so-called "soft" cultivars which cook in a shorter time are preferred over the "hard" cultivars. The cooking quality of faba beans can be evaluated by the following parameters.

— Cookability index [71]: After cooking at 115.5^oC for two hours, the penetration readings (Precision Universal Penetrometer) for 100 seeds were determined and the average value recorded as the cookability index (between 2.00 and 9.00 mm).

— Percentage of seed coat (between 11 and 20 per cent).

— Thousand seed weight (between 200 and 950 g).

— Hydration coefficients [30], calculated by weighing the beans before and after cooking (1:4 beans/water at 120 ^oC for two hours) under specified conditions (between 200 and 300 per cent).

Statistical analyses showed highly significant correlations between cookability index and hydration coefficient and percentage seed coat for the Egyptian faba bean samples (high seed coat content, mean 15.0 per cent). For the Canadian samples (low seed content, mean 12.8 per cent), highly significant correlations were obtained between the cookability index and thousand seed weight [71]. The study of Youssef et al. showed that the length of cooking time in faba beans is controlled by characteristics of both the seed coat and the cotyledons, and is not a "hard shell" problem only. The starch properties affect the cookability; the gelatinization and phase transition temperatures were lower for the hard-cooking samples. It seems that there is a critical temperature at which gelatinization plays an active role in terms of the cookability of faba beans. This point deserves further study [71] .

Jackson and Varriano-Marston [33] suggested that there are two types of "hard shell," one related to seed coat impermeability and the other to cotyledon impermeability.

In general, the cooking time of faba beans can be shortened by soaking. The higher the moisture content after soaking, the shorter the cooking time. The soaking and cooking time for dried broad beans may be shortened by hot soaking and/or pressure cooking.

Faba beans consumed in the home are often prepared through hydration and cooking to achieve the desired palatability. This method of preparation is very time- and energy-consuming. A canned product that simplifies or eliminates the preparation process in the home and offers long storage life may be of some value in the market-place.

Experience has shown that quality changes such as unfavorable colour development and water-soluble vitamin losses will take place during canning. These changes are further complicated by the additives, such as the disodium salt of ethylenediamine tetraacetic acid (Na EDTA) and sodium bicarbonate (NaHCO₃), used during processing. Soaking of faba beans for 12 hours in ethylenediamine tetraacetic acid solutions (50 ppm, 100 ppm and 150 ppm) only caused a slightly lighter bean colour; soaking in bicarbonate solution (0.5 per cent, 1.0 per cent, and 1.5 per cent) increased both the drained weight and the softness of the cooked beans, and it also made the bean colour darker. The mechanism of loss for riboflavin and thiamine during the processing of faba beans was mainly leaching; very little thermal destruction was observed. None of the soaking treatments affected the retention of these vitamins [42] .

PROTEIN AND AMINO ACIDS

The faba bean has a high protein content (table 3). Over five years, Picard [56] found the variation among populations of different origins to be mostly between 26 and 41 per cent. Protein content does not seem to be greatly affected by either seed weight—as thousand grain weight (TGW)—or seed yield per plant [3]. One hundred grams of cooked faba beans supply about 9 g protein. The protein quality, like that of most other leguminous proteins, is lower than that of most animal foods because it is moderately deficient in the amino acid methionine (table 4) [26]; however, the faba bean has, in contrast to cereals, a high content of lysine and also of arginine [61] . According to Sjoedin [61], an increase in the protein level involves a certain decrease in the level of sulphur-amino acids as well as of lysine, which means that there is a negative correlation between sulphur-amino acid content and protein percentage in the seeds. This negative correlation is probably explained by the fact that storage proteins, which are relatively low in sulphur-amino acids, constitute a higher proportion of the protein fraction of the seeds [12]. This problem can be overcome as it is possible to complement the diet with other proteins in which the sulphur-amino acids are not the limiting factor, such as wheat products (bread or macaroni). For example, cooked faba beans, salad, and bread is a popular dish in Egypt, as is faba with macaroni in certain parts of Italy.

TABLE 4. Protein content (percentage on dry basis) and amino acid composition (g/16N) over 10 Vicia faba L varieties

Source: Lattanzio et al. 1361.

The crude protein content of the seeds varies widely depending on many factors, for example variety, fertilizer application, and location of growth [10, 71]. The seed coat may contain less protein and more carbohydrate in comparison to cotyledons and whole seeds (table 3).

The use of legume flours (particularly soy) to improve the nutritional value of bread has attracted considerable interest. Soy flour is a valuable additive, not only because of its high protein content but also because of its high lysine content compared to wheat flour

The production of high-protein bread from wheat-faba bean composite flours was studied by McConnell et al. [51], D'Appolonia [18], Finney et al. [24], Morad et al. [54], and Hsu et al. [29] . They found that the addition of faba bean flour had deleterious effects on loaf volume and crumb grain of bread. These difficulties could be overcome and protein levels markedly increased by using faba bean concentrate prepared by air classification in place of a faba bean flour. Finney et al. produced acceptable breads using 15 per cent germinated and ungerminated faba bean flours with wheat flour [24] .

STARCH AND OLIGOSACCHARIDES

The microscopic structure of broad beans has been studied by McEwen et al. [52] and El-Shimi et al. [123] using scanning electron microscopy (SEM). They examined the cells in the cotyledon and found granules of about 25-40 µ in diameter, surrounded by irregularly shaped protein bodies of 1-5 µ in diameter. The cell walls were about 2 µ thick and had a ribbed or furrowed inner surface. Photomicrographs showed only slight differences in the appearance of cotyledon cell structure between the dry and soaked faba beans, but the size of the starch granules increased after soaking. Also, scanning electron micrographs of the cotyledons and the starch and protein fractions of the faba bean revealed changes in the surface of the cells during germination of the seed. The starch granules appeared to be very fragile and proteins began to break into small fragments after eight days of germination. There was a decrease in amylose, amylopectin, and raffinose during germination. Sucrose and fructose increased during the four day germination period, but decreased thereafter. Raffinose decreased gradually during the six-day germination period and was not detectable after eight days.

According to Lorenz [41], faba bean starch had oval or irregularly shaped granules which were larger in average particle size than those of corn starch. It also had a higher amylose content, gelatinization temperature range (61 to 69°C), and water-binding capacity, but a lower swelling power and solubility at 90°C compared with wheat and corn starch.

The fine structure of laboratory-purified faba bean starch, with an amylose content of 32 to 34 per cent, has been studied by Colonna et al. 117]. The enzymatic study showed the presence of the three chain populations (DP>60, 45, and 15) observed with other starches. The linear DP 15 and 45 chains occur in ratio of 8.5, which indicates an amylopectin similar to cereal starches. The beta-amylosis limit and intrinsic viscosity of the amylopectins confirms the cereal-like nature. The amylose components are not completely linear according to their beta-amylosis limit of 81.5 per cent, which corroborates the complex. debranching of the total starch. The physical structure, studied by X-ray diffractometry, is of C-type. Gelatinization of starch granules occurs at 44-65-86°C, with a heat of gelatinization of 3.8 calg^-1 [17] . Lineback and Ke [39] found that the gelatinization temperature range of faba bean starch was 61-63.5-70°C, the iodine affinity was 6.28 per cent, and the amylose content was about 30 per cent.

Pritchard et al. [58], Cerning et al. [15], and Lineback and Ke [39] observed sucrose, stachyose (maltotetraose), raffinose (maltotriose), verbascose (maltopentaose) and traces of other sugars. The presence of stachyose and raffinose contributes to problems involving flatulence if faba beans are consumed in large amounts.

AIR CLASSIFICATION

Crops such as faba beans are seen as an agronomically viable alternative to cereal grains. Protein production will be economic only if the starchy by-products are made profitable. Pin-milling of faba bean seeds, either whole or de-hulled, yields flours that contain two distinct populations of particles based on both their size and density. It was observed that this phenomenon could be exploited, using air-classification separation techniques, to produce protein concentrate (the light fraction) and a starchy flour (the heavy fraction) [67]. The results of Vose et al. [67] showed that the pin-milling (Alpine Pin Mill Model 250 CW/less than 325 mesh) and air classification (Alpine Air Classifier Type 132 MP) yielded a protein concentrate (60 to 70 per cent protein) and a crude starch fraction. De-hulling was not essential to this process, as the fibre was dense and was collected with the starch fraction, resulting in a low-fibre protein concentrate. The less dense fraction contained 60 to 70 per cent protein with an adequate lysine content and low methionine—a useful complement to protein derived from cereal grain. The larger fraction contained up to 80 per cent of a 32 to 34 per cent amylose starch.

The application of air-classification techniques to grain legume processing has relatively low capital requirements and obviates the need for costly effluent disposal operations [67].

The composition of the protein "light" fraction and starch "heavy" fraction obtained by passing the seeds through the pin-mill and air classifier is given in table 5. Air classification did not remove all nitrogen from the "heavy" fraction. The oil and physic acid moieties were concentrated along with the light protein fraction. No significant overall partition was observed with the soluble sugars of the faba beans. However, the verbascose and stachyose were mainly fractionated into the light protein flour [67].

TABLE 5. Percentage composition of the "light" and "heavy" fractions by air classification of dehulled, pin-milled faba beans

Source: Vose et al. 1671.

Sosulski et al. [62] studies the composition of oligosaccharides in flour of faba bean (hull-free) in pin-milled and air-classified protein and starch fractions using a cut point of about 800 mesh (15 µ diameter). Faba bean flour contained about 5 per cent sugars, with sucrose representing the main sugar. Verbascose and stachyose were the principal oligosaccharides. The protein to flour fraction of faba bean flour was higher than the flours in alphagalactosides, especially raffinose, manninotriose, stachyose, and verbascose. The starch fraction was depleted in alphagalactosides (table 6).

FLATUS-PRODUCING FACTORS

The ability of legume seeds to stimulate intestinal gas formation has been recognized for many years and is one of the main reasons why people limit their consumption of legumes. A number of investigators have demonstrated that the oligosaccharides raffinose and stachyose are the principal causes of flatulence in human and animal studies [13, 25] .

Normally, the low molecular weight sugars such as sucrose, glucose, and fructose are absorbed along the lining of the small intestine.

TABLE 6. Composition of sugars and inositols in hull-free faba bean flour, and in protein and starch fractions (percentage of dry matter)

Source: Sosulski et al. 1621.

However, the human digestive system lacks the enzyme alpha-galactosidase, and the raffinose family of oligosaccharides pass into the large intestine where they are fermented anaerobically to produce gas (hydrogen, carbon dioxide, and, to a lesser extent, methane). The oligosaccharides are related by having one or two alpha-galactopyranosyl groups attached to sucrose via alpha-1, 6-galactosidic linkages. Faba beans contain a substantial quantity of verbascose, stachyose, and raffinose [62, 67]. Flatulent test meals (with legumes) fed in the morning cause elevation of breath hydrogen concentration, flatus volume, flatus hydrogen, and carbon dioxide content about five to seven hours later [13, 37] . The flatus volume increased from 17 to 171 ml/hr with relatively more carbon dioxide [63] (table 7). The gases so produced are responsible for the characteristic features of flatulence, namely nausea, cramps, diarrhoea, abdominal rumbling, and the social discomfort associated with the ejection of rectal gas. A partial elimination of polysaccharide by soaking, boiling in water, or germination is possible [13, 25] .

PROTEASE INHIBITORS

Substances that have the ability to inhibit the protolytic activity of certain enzymes are found throughout the plant kingdom, particularly among the legumes. They are important because of their possible effect on the nutritive value of plant protein. General reviews on this subject are available [66, 2, 38, 9].

The initial observation by Wilson et al. [70] that Vicia faba contains a trypsin inhibitor was followed shortly by reports describing the isolation and characterization of this inhibitor. Several isoinhibitors were noted, two of which were purified [69]. Both inhibitors had a molecular weight of 11.00 but differed in isoelectric points [1] .

One of the features that distinguishes the Vicia faba inhibitors from most other legume inhibitors is their very broad specificity. They inhibit not only trypsin and chymotrypsin but also thrombin, pronase, papain, and several other proteinase of microbial origin [38].

TABLE 7. Composition of intestinal gas due to bean diets

Source: Steggerda and Dimmick [63].

In Vicia faba, there is a twofold greater concentration of trypsin inhibitor in the hull than in the cotyledon [46]. The trypsin inhibitor level of the whole seed was 3.2 units/g, of cotyledon 2.9 units/g, and of the hull 5.6 units/g. A comparison of trypsin inhibitor level of whole faba bean (3.2 units/g) with whole soybeans (27 units/g) shows that whole soybeans contain about a ninefold higher concentration of trypsin inhibitor.

The studies of Wilson et al. [70] also indicated that faba bean trypsin inhibitor had no effect on the growth of chicks, although pancreatic hypertrophy and reduced feed efficiency were observed in the same group of chicks. Wilson et al. [70] and Marquardt et al. [45, 46] observed that about 90 per cent of trypsin inhibitor activity of faba beans was destroyed by cooking (110^oC for 40 minutes), autoclaving (120^oC for 20 minutes), extrusion cooking (152^oC), or microwave radiation (107^oC for 30 minutes). The growth of chicks fed heated faba beans improved significantly over those fed unheated beans, although autociaving gave the best weight gain in chicks.

Continue

Contents - Previous - Next