Legumes or are the fruits or pod of plant belonging to the Fabaceae (Leguminosae) family. The seeds within such pods can be harvested and dried, and the resultant product is called a pulse or grain legume. Because they are not dried but instead harvested for their oil, peanuts and soybeans, although grains from legume seed pods, are not referred to a pulses. A number of legumes are grown commercially and these include clover, alfalfa, peas, peanuts, beans, chickpeas, lupin, and lentils. Legumes and pulses are useful agriculturally because their root nodules can fix nitrogen and this allows them to be used in crop rotation to maintain the nitrogen content of the soil, increasing yields of other crops involved in the rotation. In nutritional terms pulses and legumes are also interesting because they have wide ranging health effects in humans. In particular, pulses and legumes show beneficial effects against weight gain and have been shown to favourably affect postprandial blood sugar and insulin levels.
The beneficial health effects of legumes and pulses relates to their nutritional composition. The composition of a wide variety of legumes has been well studied and well characterised in the nutritional literature. The nutritional profile of legumes is unique amongst vegetables and their ability to affect health relates to the presence of a number of nutrients and anti-nutrients. For example, pulses contain probiotic substances that are fermented by the gut bacteria in the colon, and may contribute to gut health. In particular pulses are rich in oligosaccharides (also called α-galactosides) that are structurally chains of sucrose with α-1-6-linked galactosyl units. Raffinose (1 galactosyl unit), stachyose (2 galactosyl units) and verbascose (3 galactosyl units) are the common names of some of the smaller oligosaccharides. The α-galactosidase group of enzymes can digest the galactosyl units leaving sucrose which can then be digested by α-glucosidase enzymes to fructose and glucose.
Pulses contain around 20 to 45 % carbohydrate, the lower value being more common in the oilseed producing pulses. The starch in pulses is in the form of both linear chain amylose composed of α-1,4-linked glucose (molecular weight in the range of 105-106 Da) and branched amylopectin composed of α-1-4-linked chains interspersed with β-1-6-linked branching points (molecular weight in the range of 107 to 109 Da). The amylopectin in pulses is arranged in clusters made up of branching points and short linear chains, which are linked by longer chains. Amylose and amylopectin gelatinise upon contact with water and heat, a process that causes water molecules to interact with hydrogen bonds on the starches. Gelatinisation causes a loss of the crystalline structure of the starch to a more amorphous viscous gel. Upon cooling, the amylose and amylopectin retrogrades and the starch molecules rearrange themselves into more crystallised structures once more. Retrograded starch is less digestible that non-retrograded starch and beans have a high capacity for retrogradation, explaining their slow digestion rates.
The slow digestion rates of pulses may also relate to their cellular structures. Cellular and cotyledonous material may inhibit the access of digestive enzymes such as α-glucosidases to the starch contained within cells. Amylose also possesses a lower surface area than amylopectin, and so the high amylose content relative to amylopectin may inhibit digestion rates further. Removing the native macrostructures of the grains does increased digestive rates significantly suggesting that they do play a pivotal role at inhibiting digestion rates of the starch. Pulses also contain 15 to 35 % dietary fibre of which around one third is insoluble and around two thirds soluble. The soluble fibre may form a viscous gel in the stomach which inhibits gastric emptying rates and may also act as a physical barrier to the absorption of glucose across the enterocytes. In this way the soluble fibre component of pulses may contribute significantly to the slow rate of digestion and absorption of glucose that is observed following pulse consumption.
The protein component of pulses represent around 15 to 35 % of the total dry weight. Pulses contain both water soluble albumin proteins and salt soluble globulin proteins. Globulin is pulses are storage proteins and include vicillin and legumin. In contrast, the albumins represent a heterogenous group of enzymes, lectins and amylase inhibitors. Pulses have been shown to produce a greater degree of satiety compared to cereal grains and this may relate to their high content of total protein. Protein can slow gastric emptying rates because undigested protein can cause the release of cholecystokinin from I cells in the small intestine and this then causes contraction of the pyloric sphincter of the stomach. The protein in most legumes in low in methionine, and this means that it is not a complete protein. Combining pulses with cereal grains which are high in methionine can however form a complete protein. Soybeans are considered a complete protein as they possess enough methionine for human requirements.
Pulses also contain a number of anti-nutrients, including amylase and protease inhibitors. These inhibitors are broken down to some extent by the heat of cooking. Protein inhibitors act on the serine proteases trypsin and chymotrypsin and inhibit their activity. The amylase inhibitors reduce the digestibility of starch by inhibiting α-amylase. Amylase inhibitors may therefore contribute to the beneficial glycaemic effects of pulse by inhibition of one of the main starch digesting enzymes. Pulses also contain a number of phenolic substances that may provide antioxidant capacity. Those pulses that possess dark skins tend to have higher concentrations of phenolic compounds which can include tannins, phenolic acids and flavonoids. Studies show that the antioxidant capacity of pulses is related to their phenolic component. Certain phenolic compounds may also interfere with α-glucosides enzymes further reducing digestion rates. Phytic acid (myo-inositol hexaphosphate) is a storage form of phosphate in plants, and pulses contain phytic acid which may also contribute to reductions in starch digestion rates.