Grain legumes

Introduction

legumes.jpg
Examples of Legume Seeds


Grain legumes, of the family Leguminosae (legume), have been utilized as important crops and food sources for several thousand years. Legume plants are able fix atmospheric nitrogen, and this ability of legume can reduce the damage of fertilizer on farm soil and reduce costs for farmers who grow legumes. Generally, legume plants are divided into six classes: forage, grain, blooms, pharmaceutical/industrial, fallow/green manure, and timber species. Grain legumes are the most important legume plants, which are cultivated for their seeds, including beans, lentils, peas and peanuts. Recently, more and more nutritionists have paid attention to the bioactivity of grain legumes and their products. Grain legume seeds have positive effects on human health, which can treat some diseases, such as cardiovascular disease, diabetes, overweight and obesity. Dietary fats play a very important role in human lipid metabolism and health. Generally, healthy diet should be of low fat diet firstly. Legume seeds contain low total oil content, in which there are low level of saturated fatty acids and high content of unsaturated fatty acids. Isoflavones are found in most legume seeds in high concentrations. Many studies have confirmed that isoflavones are involved in cancer prevention and body weight control. High protein content is another positive factor of legumes seeds. In India, legume seeds are also named as “poor man’s meat”. In addition, some researchers found that some specific subunits of protein from legume seeds, such as α′ chain and 7S globulin of soy protein, are beneficial in reducing serum cholesterol levels. High fibre content is the fourth positive factor of legume seeds, associated with the prevention of bowel cancer and body weight control.

Legumes for food



Grain legumes, as main human food sources, provide humans with high quality protein. In addition, grain legume seeds contain other nutrients, such as mineral and carbohydrates, which are also beneficial to our health. Beans, soybean, lentils, and peas are all grain legume seeds. Every year, total value of grain legume crops can be up to two hundred billion dollars in the world. At present, the main edible grain legume seeds are divided in to two groups: pulses and non pulses (Table.1). “Pluses are annual leguminous crops yielding from one to 12 grains or seeds of variable size, shape and colour within a pod”, which was defined by The Food and Agricultural Organisation (FAO) of the United Nations (Duranti, 2006). The excluded grain legumes are non-pluses, including soybean and peanut, which are main oil crops.

Table.1. List of pulses and their production (Duranti, 2006)

Main edible legume seeds (grain legumes)
Latin name
World crop produciton,
metric tons*1000
Pulses:

3819
1. Dry beans

1162
Kidney bean, haricot bean, pinto bean, navy bean
Phaseolus vulgaris

Lima bean, butter bean
Vigna lunatus

Adzuki bean
Vigna angularis

Mung bean, golden gram, green gram
Vigna radiata

Black gram, urd
Vigna mungo

Scarlet ruuner bean
Phaseolus coccineus

Rice bean
Vigna umbellata

Moth bean
Vigna acontifolia

Tepary bean
Phaseolus acutifolius

2. Dry broad beans (Vicia faba)


Horse bean
Vicia faba
255
Broad bean
Vicia faba

Field bean
Vicia faba

3. Dry peas (Pisum spp.)

892
Garden pea
Pisum sativum var. sativum

Protein pea
Pisum sativum var. arvense

4. Chickpea
Cicer arietinum
478
5. Dry cowpea, blackeye pea, blackeye bean
Vigna unguiculata
ssp. dekindtiana
350
6. Pigeon pea, cajan pea, congo bean
Cajanus cajan
103
7. Lentil
Lens culinaris
199
8. Bambara groundnut, earth pea
Vigna subterranea

9. Vetch, common vetch
Vicia sativa
99
10. Lupins
Lupinus spp.
45
11. Minor pulses:


Lablab, hyacinth bean
Lablab purpureus

Jack bean, sword bean
Canvalia ensiformis, gladiata

Winged bean
Psophocarpus teragonolobus

Velvet bean, cowitch
Mucuna pruiens var. utilis

Yam bean
Pachyrrizus erosus

Non-pulses (oil-crops):


Soybean
Glycine max
6209
Peanut
Arachis hypogaea
1874


Positive factors of legume seeds on human health

Low oil content



Low lipid intake is important for good health. The total oil content in legume seeds was the lowest, compared with that in seeds and grains. Except chick peas (5 g/100g), the total oil contents of other legumes were less than 1.6 g/100g (Table.2). The levels of saturated fatty acids are also very low in all types of legumes. Moreover, there were high levels of unsaturated fatty acids (70% of total oil). The content of unsaturated fatty acids in chickpeas was up to 4.3 g/100g. Low saturated fatty acids are identified as an important standard for evaluation of the quality of food. Legume seeds contain low levels of total oil and saturated fatty acids, as well as high content of unsaturated fatty acids, therefore increase intake of legumes can be beneficial to human health (Ryan, Galvin, Connor, Maguire & Brien, 2007).

Table.2. Total oil content (g/100 g) and fatty acid
composition (% of total) of various seeds, grains,
and legumes (Ryan et al, 2007).

Sample
Total oil
SFA
MUFA
PUFA
Linseed
29.3
12.5
24.3
36.2
Mustard
15.2
4.9
74.8
18.8
Poppy
39.5
13.7
22.9
61.2
Pumpkin
42.3
22.7
36.4
41.0
Sesame
40.5
15.9
43.3
41.1
Barley
1.3
22.5
15.2
62.4
Buckwheat
2.7
21.9
41.9
36.2
Maize
1.6
15.4
30.0
54.6
Millet
4.0
10.8
23.7
65.5
Quinoa
6.3
11.2
32.8
56.1
Rye
1.3
16.4
18.1
65.5
Spelt
2.0
17.2
22.2
60.8
Butter Bean
0.9
28.7
10.5
60.8
Chick Peas
5.0
13.7
34.2
52.1
Kidney Bean
1.2
16.5
12.1
71.7
Letils
1.4
16.7
23.7
58.8
Peas
1.5
14.7
28.4
56.9


Isoflavones



Isoflavones are found in the most legume seeds. Thesehave many health benefits.For example, isoflavones have been utilised as an indirect therapy for some hormone-dependent diseases, and have also been used to prevent chronic disease such as cancer and cardiovascular disease. Soybean seeds are the most common edible legume seeds with a high of isoflavone content . There are two main soybean isoflavones: daidzein (40,7-dihydroxyisoflavone) and genistein (40,5,7-trihydroxyisoflavone), which are similar in structure. Some studies have indicated that daidzein and genistein have similar functions in the prevention of cancer. Isoflavone is very similar to human estrogen in chemical structure (Figure). There are phenolic rings in both of the structures of isoflavone metabolite equol and estradiol, which can bind to the estrogen receptor (ER). Estrogen binding to ERs induces cell proliferation in breast tumour cell lines. Therefore, the presence of estrogen is the risk factor for breast cancer. The role of isoflavone isof both an agonist and antagonist of estrogen, according to different affinities binding to ER. Isoflavone binding to ER with strong affinity plays the similar role of estrogen, which induces tumour cell proliferation. However, more evidence has suggested that the main role of isoflavone is as the antagonist of estrogen in breast cells, which binds to ER with low affinities, inhibiting cell proliferation (Davis, Díaz-Cruz, Landini, Kim, & Brueggemeier, 2008; Lo, Mak & Leung, 2007; Setchell & Cassidy, 1999).


A good protein source



Protein is main component of human tissues, most of which is from dietary meat. The range of protein in legumes is from 20% in peas and beans to 40% in soybean, so legume seeds are suggested as a good protein source in human diet. The food and drug administration (FDA) of United States suggested intake ≥ 6.5 g/d of soybean protein could reduce the risk of cardiovascular diseases and intake of 25 g/d was beneficial for human health. 7S globulin and α′ chain are soybean protein-specific subunits. There were significant reductions in plasma cholesterol concentration and plasma triacylglycerols levels by feeding α′ chain and 7S globulin in rats (Figure). α′ chain is one of subunit of 7S globulin, which involves in the up-regulation of low density lipoprotein (LDL) receptors (Duranti, 2006). The results revealed that intake of soybean protein might stimulate LDL receptors and degrade LDL, which also could reduce the risk of heart disease. In addition, soybean protein is beneficial to control body weight, which is capable to affect appetite, resulting in reduction of food intake. There were significant decreases in the weight of epididymal and perirenal white adipose tissues by replacement of animal protein with soy bean protein. There is no information to date on the effects of soybean protein on the liver (Takahashi & Ide, 2008).



Dietary fibre


Legume seeds contain high level of dietary fibre. Fibre, the indigestible material of plant cell walls, plays an important role in the human digestive system. Fibre is a combination of various substrates, such as cellulose, hemi-celluloses, pectins, gums, mucilages, resistant starch, other polysaccharides and lignin. Generally, there are two groups of dietary fibre: insoluble dietary fibre (IDF) and soluble dietary fibre (SDF). Plant cell wall is the main source of IDF, and consists of the complicated mixture of cellulose and non-celluloses. It is confirmed that IDF can prevent bowel cancer. IDF increases faecal bulk and therefore dilutes faecal content, thereby decreasing interactions between the intestinal mucosa and any carcinogens. Pectins, gums, mucilages and some hemi-celluloses fragment are soluble in water, which are also identified as soluble dietary fibre (SDF). SDF is able to delay gastric emptying, and this is beneficial in reducing the risk of insulin resistance. SDF also participates in hind gut microbial fermentation, which also prevents colo-rectal cancer. Additionally, SDF can reduce the level of plasma cholesterol and LDL by changing cholesterol metabolism. Bile salts are inhibited from taking part in micelle formation by SDF, which inhibits cholesterol and other lipid absorption, decreasing cholesterol concentration in the serum (Tharanathan & Mahadevamma, 2003).


Negative feathers



Although grain legume seeds have a lot of benefits to human health, there are also some of deleterious effects. These include sulfur-containing amino acids deficiency and the presence of anti-nutritional compounds (ANCs): hydrolase inhibitors and lectins (Duranti, 2006).


Certain amino acids deficiency



Many studies indicated that there were low contents in sulfur-containing amino acids, methionine, cysteine, and tryptophan in legume protein, but high content of another essential amino acid, lysine. It is not suggested that legume is the only protein stable of human. Hereby, intake of legumes should choose cereal which includes high content of sulfur-containing amino acids, as dietary supplement (Duranti, 2006; Duranti & Gius, 1997; Sathe & Venkatachalam, 2007).



Hydrolase inhibitorslectin.jpg



Hydrolase inhibitors, the major family of protein ANCs, are found in many legumes, including peas, lentil, beans and soybean, which are able to inhibit various digestive enzymes, including trypsin, chymotrypsin, and amylase. The best characterized protease inhibitors are trypsin/chymotrypsin inhibitors of both the Bowman-Birk and Kunitz type and α-amylase inhibitors, which have been identified to play an important role in protein digestibility. Interestingly, the negative effects of these inhibitors are only exhibited when legume seeds or legume flour are consumed uncooked. Legume cooking results in inactivation of these inhibitors, which plays a positive nutritional role, due to their high content of sulfur-containing amino acids, compared with the majority of the seed proteins (Duranti, 2006).


Lectins



Lectins are another major family of protein ANCs, which are specific sugar-binding proteins exhibiting reversible carbohydrate binding activities. Lectins are similar to antibodies in their ability to agglutinate red blood cells; however, lectins are not the product of immune system. The toxicity of lectins have been identified by consumption of food with high content of lectins, which can lead to diarrhoea, nausea, bloating and vomiting. Many legume seeds have been proven to contain high lectin activity, termed as hemagglutinating activity. Soybean is the most important grain legume crop, the seeds of which contain high activity of soybean lectins (soybean agglutinin or SBA). SBA is able to disrupt small intestinal metabolism and damage small intestinal villi via the ability of lectins to bind with brush border surfaces in the distal part of small intestine. Heat processing can reduce the toxicity of lectins, but low temperature or insufficient cooking may not completely eliminate their toxicity as some plant lectins are resistant to heat. In addition, lectins can result in irritation and over secretion of mucus in the intestines, causing impaired absorptive capacity of the intestinal wall (Ayyagari, Narasinga Rao, & Roy, 1989; Duranti, 2006; Francis, Makkar, & Becker, 2001).



References

Written by Feng Shi (April 2008)
M.Sc. student
Food and Wine Sciences Group, Lincoln University