- Literature Review
Legumes are staple foods for many people in different parts of the world (Ndidi et al., 2014). The seeds have twice as much protein as cereals by percentage and usually contain more balanced profile of essential amino acids (Vijayakumri et al., 1997). They are a good and inexpensive source of dietary proteins, carbohydrates, vitamins and minerals. The prevalence of malnutrition has increased in some developing countries, likely because of the increase in population and inadequate supply of protein (Adebowale, 2007). This is mainly due to the consumption of cereal-based porridge which is bulky, with low energy, low nutrient density and high antinutrient content (Michaelsen and Henrik, 1998). However, legumes can play an important role in tackling the problem of malnutrition due to their high content of proteins, which ranges from 20-40% (Buadoin and Maquet, 1999). Plants provide 70% of the world supply of protein (Nergiz and Gokgoz, 2007) and legumes provide a significant amount of this class of proteins required by human.
Legumes are fruits or seeds of pod-bearing plants belonging to the family Leguminosae (Singh et al., 2004). Migration, trade and wars enabled species like common bean (Phaseolus vulgaris), Soyabean (Glycine max), Pea (Pisium sativum), Faba bean (Vicia faba) to spread to different regions of the world. Legumes play a unique role in agriculture worldwide due to their ability to fix atmospheric nitrogen (N2) and to their high protein content (Messina, 1999). Legumes are used for food or feed crops. Grain legumes are common food throughout the world. They are second only to cereals in providing food crops (Duranti, 2006). The importance of legumes as food lies primarily in their high protein content. Most dry legumes provide 20-40% protein (Buadoin and Maquet, 1999). On a worldwide scale, legumes provide 22% protein, 32% fat and oil and 7% carbohydrates in terms of human nutrition. Grain legumes play an essential role in human nutrition, balancing the deficiencies of the basically cereal-based diet (Messina, 1999). Cereal proteins are deficient in certain essential amino acids, particularly lysine (Anjum et al., 2005), whereas legumes contains adequate amount of lysine (Sai-Ut et al., 2009).
Legumes have a higher percentage of dietary fiber, protein and resistant starch than cereal grains. Both can be attacked by bacteria, converting them to butyrate- a short chain fatty acid which has cancer preventing properties and aid metabolism in diabetics (Fahey et al., 1997). Legumes seeds are also valuable sources of dietary fibre, vitamins and minerals including folate, thiamine and riboflavin (Messina, 1999). Grain legumes are often considered as meat substitutes for people in less developed countries (Amarteifio and Maholo, 1998).
1.1.2 Importance of Legumes
Legume proteins contribute energy and amino acids, which are essential for growth and maintenance. According to Liao et al. (2007), consumption of legume proteins could reduce low density lipoproteins and help in reducing weight. Singh et al. (2008) have reported that soy protein lowers blood cholesterol. Apart from being a valuable source of protein, consumption of legumes has also been linked to reduced risk of diabetes and obesity, coronary heart disease (Bazzano et al., 2001), colon cancer and gastrointestinal disorders. Legume starch causes less change in plasma glucose and insulin upon ingestion (Philips et al., 2003). Legume starch also have low glycaemic index (Wang et al., 2008) and therefore, it is a very good source of nourishment for controlling diabetes.
Consumption of legumes may also have protective effect on prostate cancer in humans. The phenolic compounds present in these legumes are known to exhibit strong antioxidant, anti-mutagenic, and anti-genotoxic activities (Khattak, et al., 2007; Xu and Chang, 2008).
Furthermore, isoflavones, a class of phytochemicals found in soyabeans helps to prevent cancer. Soybean is an alternative source of protein for people who are allergic to milk protein and are also highly digestible (92 to 100%). It contains all essential amino acids. Although relatively low in methionine, it is a good source of lysine. Soy-protein products contain a high concentration of isoflavones, up to 1g/kg (Singh et al., 2008).
The legume seed generally, contains about 17-25% protein except soybean which contains about 40% protein (Enwere, 1998). They are good sources of minerals like phosphorus, calcium and iron. They are generally low in fat and oil, except for soybean and groundnut which contain about 18% and 48% respectively.
Cereal proteins are deficient in certain essential amino acids particularly lysine (Amjad et al., 2003). On the other hand, legumes have been reported to contain adequate amounts of lysine, but are deficient in sulphur-containing amino acids (methionine and cysteine), (Farazana and Khalil, 1999).
1.1.3 Nutrients in Legumes
Based on nutritional composition, legumes can provide a high proportion of proteins, fats, carbohydrates, dietary fibers, B-group vitamins (thiamin, riboflavin, niacin), and minerals (Prodanov et al., 2004). This composition can vary according to cultivar, location of growth, climate, environmental factors, and soil type in which legumes are grown ( Bishnoi and Khetarpaul, 1993).
Proteins in legumes represent about 20 % (dry weight) in pea and beans, and up to 38 % – 40 % in soybean and lupin (Guegen and Cerletti, 1994). Consumption of legume proteins could reduce low density lipoproteins and help in reducing weight (Singh et al., 2008). Proteins in legumes are usually classified into two major classes; globulins and albumins. Furthermore, prolamin and gluatamin fractions have been noted in very low amounts (Adebowale et al., 2007). Globulins are salt soluble, albumins are water soluble (Duranti, 2006), prolamins are soluble in ethanol/water solutions (Duranti, 2006) while glutalins are soluble in sodium hydroxide. The most abundant class of storage protein in grain legumes are the globulins. The amount of protein in grain legumes varies considerably, but even the lowest concentrations are about three times that of maize. Generally, the protein content of grain legumes provides between 20% and 30% energy. Soybeans are particularly high in protein, over 35% (Messina, 1999). Proteins in food supply the amino acid from which the body produces its own proteins. Grain legumes contain mostly essential amino acids which the body do not produce or do not produce in sufficient quantity (Rolfes et al., 2009); but they generally do not supply the sulphur-containing amino acids, methionine and cysteine. Soya bean is an exception having higher methionine (0.5 g) and cysteine (0.7 g) per 100 g edible portion than staple foods. The protein quality of soya beans equals that of animal proteins (Mateos-Aparicio et al., 2008). Generally, plant proteins have been reported to be less susceptible to proteolytic breakdown in vivo than animal proteins (Friedman, 1996).
Most legumes are very low in fat, generally containing approximately 5% of energy as fat. The primary exceptions are chickpeas and soybeans, which contain approximately 15% and 47% fat, respectively. The predominant fatty acid in beans is linoleic acid, although beans also contain the n−3 fatty acid, α-linolenic acid. However, because the overall fat content of most legumes is so low, the dietary contribution of beans to α-linolenic acid intake is minimal.
About 50% of energy in grain legumes is provided through carbohydrates except for groundnuts and soya beans. Carbohydrates can be divided into water-soluble components such as sugars (monosaccharides) and insoluble ones such as oligosaccharides (3 to 10 monosaccharides) and starch (polysacharides). Legumes have a low glycaemic index (GI). Low glycaemic index (GI) foods release glucose into the bloodstream less rapidly than foods with higher GI. This means that legumes are excellent sources of carbohydrate for people with diabetes as well as for the general population (Rizkalla, et al., 2002). Sucrose is the major sugar in grain legumes. Unlike cereals, grain legumes contain appreciable quantities of oligosaccharides. These are not digested by human enzymes and therefore are involved in the production of flatus from grain legumes.
All grain legumes are high in dietary fibre providing about 10% of energy. Dietary fibre is the skeletal remains of plant cells that are resistant to hydrolysis by the enzymes of man (AyKroyd and Doughty, 1982). Dietary fibre consists of indigestible complex carbohydrate of cell wall in plants. High dietary fibre in the diets can have some beneficial biological effects such as laxative effects on gastrointestinal tract, increased feacal bulk and reduction in plasma cholesterol level (Okoye, 1992). Legumes contain resistant starch which positively affects digestive health by functioning as a prebiotic (Jenkins et al., 2000). Prebiotics are a good source for probiotics, they encourage growth and protection of beneficial bacteria in the bowel, while suppressing harmful bacteria.
In general, grain legumes contain concentrations of different vitamins. Among the fat-soluble vitamins, most grains contain only small amounts of carotenoids (provitamin A), vitamin E and vitamin K. As regards water-soluble vitamins, the thiamin (vitamin B1) content of grain legumes is equivalent to or slightly exceeds that of whole grains. Beans are an excellent source of folate, which in addition to being an essential nutrient is thought to reduce the risk of neural tube defects (Daly et al., 1995). One serving of beans provides more than half of the current recommended daily allowance (RDA) for folate (NRC, 1989). Grain legumes contain little riboflavin (vitamin B2), but are good sources of niacin (vitamin B3). There is considerable variation of vitamin content between varieties. They also contain pantothenic acid (vitamin B5) and folic acid. However, with the exception of germinated seeds, grain legumes as consumed are almost devoid of ascorbic acid (vitamin C)
Legumes are also high in iron; 1 serving provides approximately 2 mg. This compares favorably with the recommended daily allowance (RDA) of 10 and 15 mg for adult men and premenopausal women, respectively (NRC, 1989). However, iron bioavailability from legumes is poor and thus their value as a source of iron is diminished.
In contrast to iron bioavailability, zinc bioavailability from legumes is relatively good at approximately 25%. Also, many beans are good sources of calcium, providing on average approximately 50 mg Ca/serving, although there are variations among the legumes. Calcium bioavailability from beans in general is approximately 20%, which is lower than that from milk and green leafy vegetables but is still reasonably good (Weaver et al., 1993). Calcium bioavailability from soybeans and soyfoods is quite good, equivalent to calcium bioavailability in milk, despite the fact that soybeans are high in phytate and oxalate (Weaver and Plawecki, 1994).
Antinutrients are compounds or substances which act to reduce nutrient intake, digestion, absorption and utilization of nutrients and may produce other adverse effects in the body (Akande et al., 2010) even though they may exert beneficial health effects at low concentrations (Gemede and Ratta 2014). Their presence in food gives rise to genuine concern for human health in that they prevent digestion and absorption of essential nutrients (Mohamed et al., 2011). Antinutrients are structurally different compounds broadly divided into two categories;
- Proteins (such as lecithin and protease inhibitors)
- Others such as phytate, tannins or proanthocyannins, oligosaccharides, saponins and alkaloids. The presence, distribution and ingestion of antinutrients in grain legumes have intensively been reported (Grant, 1991).
In general, raw legumes contain far higher levels of antinutritional factors than their processed forms. Hence, processing is necessary before the incorporation of these grains into food or animal diets (Hajos and Osagie, 2004). Common antinutrients in legumes are protease inhibitors, α-amylase inhibitors, hemaglutinin, saponins, goitrogens, cyanogenic glycosides, antivitamin factors, metal binding constituents, oestrogenic factors, flatulence factors, toxic amino acids, urease, unidentified growth inhibitors, lathyrogens and favism causing agents. Unless these antinutrients are destroyed by heat or other treatments, they exert adverse physiological effects when ingested by man and animals (Liener, 1994). However, some antinutrients may exert beneficial health effects at low concentrations (Gemede and Ratta, 2014).
1.4.1 Phytic acid/Phytate
Seeds, such as nuts, edible seeds, beans/legumes, and grains, store phosphorus as phytic acid. When phytic acid is bound to a mineral in the seed, it is known as phytate. In legumes, phytate can be found in the protein bodies of the cotyledon (Schlemmer et al., 2009). Phytic acid has a strong affinity to minerals such as calcium, magnesium, iron, copper and zinc. This results in precipitation making these minerals unavailable for absorption in the intestine. They also affect bioavailability, solubility, functionality and digestibility of proteins and carbohydrates (Salunkhe et al., 1990). The greatest effect of phytic acid on human nutrition is its reduction of zinc bioavailability. However, consumption of phyate has some favourable effects. These include anti-carcinogenic (Shamsuddin, 2002), anti-oxidant (Minihane and Rimbach, 2002) and reduced blood glucose response (Thompson, 1993). Others include reducing cholesterol and triglycerides, prevention of renal stone development, removal of traces of heavy metal ions and even inhibition of HIV-1 replication (Kaumar et al., 2010).
Tannins are oligomers of flavan-3-ols and flavan-3,4-diols. Tannin-protein complexes may cause inactivation of digestive enzymes and reduce protein digestibility by interaction of protein substrate with ionizable iron (Salunkhe et al., 1990). Tannins are known to interact with proteins forming complexes which in turn decrease the solubility of proteins and make protein complexes less susceptible to proteolytic attack than the same proteins alone (Carbonaro et al., 2005). They also impair starch and disaccharide assimilation (Carmona et al., 1996). Other toxic effects of tannin include reduction of food intake, inhibition of digestive enzymes, increased excretion of endogenous protein, digestive tract malfunctions and toxicity of absorbed tannin or its metabolites (Jansman and Longstaff, 1993).
Saponins are steroids or triterpenoid glycosides common in a large number of plants and plant products that are important in human and animal nutrition (Francis et al., 2002). The major sources of dietary saponins are legumes, and many types of saponins can be present in the same bean. Saponins are very poorly absorbed. Most saponins form insoluble complexes with 3-β-hydroxysteroids and are known to interact with and form large, mixed micelles with bile acids and cholesterol. Although saponins were shown to lower cholesterol in some animal species, the hypocholesterolemic effects of saponins in humans are rather speculative (Milgate and Roberts, 1995). Saponins may have anticancer properties, as suggested by a rodent study in which it was found that a saponin-containing diet (3% by wt) inhibited about two-thirds the development of azoxymethane-induced preneoplastic lesions in the colon (Koratkar and Rao, 1997). Saponins have been reported to alter cell wall permeability and produce some toxic effect when ingested (Belmar et al., 1999).
A salt formed from oxalic acid is oxalate. Strong bonds are formed between oxalic acid and other minerals such as calcium, magnesium, sodium and potassium. This chemical combination results in the formation of oxalate salts. Some oxalate salts such as sodium and potassium are soluble, whereas calcium oxalate salts are insoluble. The insoluble calcium oxalate has the tendency to precipitate (or solidify) in the kidneys or in the urinary tract, thus forming sharp-edged calcium oxalate crystals when the levels are high enough. These crystals play a role in the formation of kidney stones in the urinary tract, where the acid is excreted in the urine. Calcium oxalate adversely affects the absorption and utilization of calcium in the body (Olomu, 1995). Because it is so reactive, oxalate also interferes with the duties of many other positively charged ions like copper, iron, magnesium, manganese and zinc. Oxalate specifically impairs iron’s intracellular release and interferes with the whole class of vitamin B7 (biotin) dependent enzymes called carboxylases. These disruptions of cell chemistry only happen when oxalates are not bound to calcium. The human body can also produce oxalates, especially when certain enzymes are unbalanced in their activity because of genetic differences or when someone has deficiencies in enzyme cofactors such as vitamin B1, vitamin B6 or magnesium.
Although numerous toxic compounds and antinutritional factors are found in legumes, they can be detoxified and eliminated by proper heat treatment and processing for effective use of the legumes as food to avoid the problems of toxicity and to enhance the quality of the nutrients (Liener, 1994).
1.5 Effect of Boiling on legumes
Legumes have been recognized as hard-to-cook and this could be a particular factor that discourages the use of legumes. Boiling improves the nutritional value of legumes (Ghavdiel and Prakash, 2007) though excessive cooking can decrease nutritive value (Taiwo et al., 1997). During cooking of legumes, two simultaneous processes occur inside and outside the cotyledon cells. Gelatinization of intracellular starch and denaturation of seed protein causes softening of the seeds. These result in the plasticization or partial solubilization of the middle lamella, leading to the separation of individual cotyledon cell (Klamczynska et al., 2001). The effects of processing vary, notably depending on the techniques and conditions, including time, temperature, moisture content and pH (Habiba, 2002). Most food preparation processes reduce the amount of nutrients in food. In particular, processes that expose foods to high levels of heat, light, and/or oxygen cause the greatest nutrient loss. Loses in protein can be attributed to partial removal of certain amino acids along with other nitrogenous compounds on heating (Rehman and Salariya, 2005). Nutrients can also be “washed out” of foods by fluids that are introduced during cooking process. Legumes are usually cooked before being used in the human diet. Losses of nutrients during normal cooking can be controlled by the amount of cooking water and its damage (Attia et al., 1994)
Combined soaking and boiling is an effective method of reducing most of antinutritional factors. It improves the protein quality by destruction or inactivation of the heat labile anti-nutritional factors (Vijayakumari et al., 1998). Cooking, however, produces denaturalization of proteins and their diffusion to the liquid phase (Haytowitz and Mathews, 1983) and decrease of phytic acid (Khalil and Mansoura, 1995). Cooking causes considerable losses in soluble solids, especially vitamins and minerals (Barampa and Simmard, 1995) but can increase the bioavailability of some minerals such as iron (Lee and Clydesdale, 1981). Earlier works suggests that different cooking methods improve the nutritional quality of food legumes to various extents (Chi-Fai et al., 1997).