Chemical Composition

Pulses (i.e., dry peas, lentils, and chickpeas) are some of the most widely available, inexpensive, and nutritionally complete staple foods in the world. Offering a balanced proportion of proteins, starches, fiber, and minerals, they are a valuable feature of a healthy human diet.

Over the previous two decades, non-nutritional bioactive factors, such as phytates, tannins, alkaloids, saponins, and oligosaccharides, have been linked with health-promoting properties. These compounds are increasingly considered natural bioactive substances and are credited with playing an important role in the prevention of heart disease and some types of cancer (Champ, 2002; Dills and Trichopoulou, 2009). The amount of protein, starch, fiber, fat, vitamins, and minerals vary in proportion and importance depending on the pulse. For example, significant genetic variability in seed protein composition offer the possibility of breeding for improved protein nutritional value. A better understanding of the fundamental aspects of assimilate uptake, transport, partitioning, and metabolism in pulse seeds, and the genetic factors that regulate all of these processes, will help contribute to the successful breeding of improved pulse seed quality over time. Chickpeas have one of the highest nutritional compositions of any dry edible legumes. The average nutritional content of chickpea is 22 % protein, 67 % total carbohydrates, 47 % starch, 5 % fat, 8 % crude fiber, and 3.6 % ash. The lipid fraction is high in unsaturated fatty acids, primarily linoleic and oleic acids. The content of the amino acid lysine is adequate, while the sulfur-containing amino acids, methionine and cysteine, are the first limiting amino acids. Chickpeas help reduce cholesterol due to their unsaturated fatty acid and fiber content, and they are also unique in their ability to moderate the rise in postprandial plasma glucose. In addition, the chickpea mineral component includes generous amounts of potassium, phosphorous, iron, and magnesium. Apart from being eaten as a vegetable, grains are also a source of raw material for the processing industry. Fractionated pulses can be used as simple ingredients or as additives. Pulse starch and fiber both have useful functional properties and can be readily used in food products. Starch, protein, and fiber can be extracted, using wet or dry process from a variety of pulses and used as ingredients for food. See chapter 5 for more information on fractionated products. Alpha-galacto-oligosaccharides including raffinose, stachyose, and verbascose, can be isolated during wet processes from the soluble extract. Due to their high fermentability, alpha-galacto-oligosaccharides induce the production of gases responsible for the digestive discomfort (i.e., bloating) related to pulse consumption. These oligosaccharides are characteristic of legumes and are present in all species to one degree or another.

The characteristics of the extracted fiber depend on its origin. Inner fiber produced by finely ground outer hull of pulses whereas inner fiber is separated out from cotyledons. As a result, inner fibers are often used as texturing agents especially in the extrusion process, while outer fibers are most commonly used in bakery and extruded products to increase fiber content of the food.

Health Benefits of Pulses

Pulses are considered an excellent source of protein, dietary fiber, and vitamins and minerals such as folate, iron and zinc (Oomah et al. 2011). A nutritional analysis of dry split pea, lentil and chickpeas are provided in figure 2. They remain as a critical part of the diet in many parts of the world, especially on the Indian subcontinent. Mixing lentils with grains, such as rice and wheat has been practiced as to supplement protein quality.

The benefit of a high fiber diet goes beyond regularity and prevention of hemorrhoids and diverticulitis. Health benefits have been ascribed to 3 main components found in pulses; fibers, proteins, and starches. There is an extensive amount of literature on the nutritional aspects of pulses, including the digestibility of main nutrients (i.e., protein, starch, and dietary fiber) (Dona et al. 2010; Hoover et al. 2010; Chitra et al. 1995), colonic fermentation (Tosh and Yada, 2010), and post-prandial glycemia and insulinemia (Jenkins et al. 1983; Marinangeli et al. 2009). Complex nature of pulse carbohydrates may help to protect against diabetes, cardiovascular disease, and even some cancers (Rizkalla, et al. 2002). It is thought that regular consumption of pulses can help lower blood cholesterol concentration. At the same time, pulses provide the amino acids necessary to build and repair muscles and assure proper muscle development. They also nourish muscles with a range of other nutrients, including folic acid and minerals. They boast a low allergenic capacity compared with some other sources of protein.

Pulses are highlighted as key daily ingredients in the Mediterranean Diet. The product of more than 50 years of scientific research into the eating habits of those living along the Mediterranean Sea, the diet is today considered to be the “gold standard” for healthy eating.

Peas offer more than 1/3 of the recommended daily value for folate, a nutrient that plays a critical role in the prevention of birth defects. In addition to the high protein content, lentils are high in folate, manganese, phosphorous and thiamin. Lentil is one of the best vegetable sources of iron, and high in tannins. As a phytochemical in plants, tannins have antimicrobial properties. In the body, tannins can act as an antioxidant and may reduce blood pressure, lower cholesterol, and help regulate the immune response.

Figure 2. Nutrient content of dry split pea (left), lentil (middle), and chickpeas (right)

Quality

Moisture

Dry peas are adapted to grow during the cool season when evapotranspiration (i.e., the sum of evaporation and plant transpiration into the atmosphere) is minimal and rely on stored soil moisture for a large part of their growth cycle. In years of warm and wet springs and cool and wet summers, desiccant herbicides are important tools for promoting faster drying to avoid threats like pod shattering and sprouting, and seed coat slough and bleaching.

Despite having moisture requirements similar to those of cereal grains, dry peas have a lower tolerance to saline and waterlogged soil conditions. Peas commonly die after 24 to 48 hours in a waterlogged condition, making poorly drained and saline or alkaline soils a hazard when growing peas. Maintaining seed-to-soil moisture contact is critical. So seeding peas well into moisture is important, with a half inch being the minimum and 1 to 3 inches the preferred depth.

Excessive tillage in the spring is also to be avoided to prevent drying out the seedbed. By comparison to cereal grains, pea seed requires considerably higher amounts of moisture for germination. It is their relatively shallow root system and high water use efficiency that make them an excellent rotational crop with small grains, especially in arid areas where soil moisture conservation is key.

Dry peas can be harvested when seed moisture is less than 15 %. Harvesting at very low moisture levels will cause cracking and splitting, which can result in lower quality for food grade. Harvesting at too high a moisture content will require immediate aeration or drying. Ideal threshing and separation occur when the crop is below 13 % seed moisture, while the harvest starts in late July when pods are dry and seed moisture is less than 13.5 %.

Like dry peas, lentils have been adapted to grow during the cool season when evapotranspiration is minimal. They usually rely on stored soil moisture for a large part of their growth cycle. Proper packing after seeding is also very important to prevent moisture loss, and to make the ground smooth and even for harvest.

Also similar to dry peas, as long as moisture is available, lentils will continue to flower and set pods. If growers wait for the crop to dry naturally under such high-moisture conditions, they risk compromising the integrity and value of the crop. Weeds can also mechanically impair the harvest of the crop. Swathing improves the moisture uniformity of the lentil seed and reduces the amount of seed discoloration, helping protect seed quality and value.

In each case, the level of moisture can impact herbicide effectiveness and pest control. The density and injury wrought by the wireworm, the larvae of the click beetle that feeds on plant roots, are directly related to soil moisture. Wireworms are generally low in years of average or below average precipitation, and high and more destructive in years of above-average precipitation.

Due to a deep tap-root, the chickpea can use water from greater depths than other pulse crops. But because of its indeterminate nature, and the fact that it continues growing into the fall, the chickpea can deplete subsoil moisture of a field. If it is a dry fall and there is limited winter precipitation, this can undermine the cereal crop yield the following year.

Chickpeas are seeded at a depth of 1 inch below moisture for the Desi type and up to 2 inches below moisture for the Kabuli type. Kabulis may be planted to a depth of 4 inches to use available soil moisture for germination. To protect this necessary level of moisture, it is often recommended with chickpeas to minimize soil tillage to reduce moisture loss. This is especially important for the large-seeded Kabuli chickpeas. Chickpeas can be harvested at 18 % moisture.

Storage

The moisture content of the pulses provides a major impact on how long that crop can be stored and remain nutritious and edible. This is because the outside of the seed can host thousands of fungi spores and bacteria. Though natural, such bacteria, if left to propagate in an environment of higher than recommended moisture, can promote disease and seriously compromise seed quality.

Seed moisture must, therefore, be carefully monitored when storing pulses. Peas can be safely stored at 15 % moisture, chickpeas and lentils at 14 % moisture content. Moisture is tested several times during the first few weeks of storage to maintain proper levels and to prevent seed tendering or sweating (i.e., the balancing of inner and outer seed moisture levels). If moisture level is too high, grain dryers are often used, though extreme caution should be used as they can cause mechanical and thermal damage to pulse crops. Aeration is used to cool and dry the seed and to avoid storage complications. Managing storage moisture levels for chickpeas can be especially challenging. When a chickpea seed is harvested, the outside seed coat normally has a lower moisture level than the inside of the seed. As it sits in the bin, moisture migration may cause the overall moisture level to rise.

This can result in higher moisture level than the recommended 14 % within a week regardless of safe moisture level of the crop at harvest. For this reason, chickpeas, like lentils, tend to be stored in a hopper-bottomed bin that has aeration, which, when left on, can bring the moisture down to prescribed levels.

Desi chickpeas require a specialized seed coat removal process if used for human food. The process, called decortication, involves adjusting the moisture level of the seeds to facilitate the mechanical removal of the thick seed coat.

Protein content

The history of pulses is intertwined with that of human civilization. During times when meat was not available, pulses were an important staple by providing essential supplementing protein, as well as key vitamins and minerals. Protein was the major reason for the development of pulses, especially in Europe. It remains a signature feature of the diet of millions of people around the world, often combined with cereal crops to provide energy. Average protein values of pulses, of 2015 crops for example, ranges from 25% to 24.2% to 21% for yellow peas, lentils and chickpeas, respectively (U.S. Pulse Quality Survey, 2015). Pulses are also an important part of vegetarian diets because they are rich in an essential amino acid, lysine, and complements the amino acid profile to be more balanced when combined with a source of the amino acid methionine, such as cereals.

The high protein content of the legume seed is thought to be due in part to the additional nitrogen that pulses receive through nitrogen-fixation symbiosis. In round-seeded peas, high protein content is also often associated with increased legumin (i.e., a globulin found in legume seeds) content. The amount of protein varies in proportion and importance, depending on the species.

Numerous studies have been published suggesting the environment is the primary factor affecting seed protein content. Yield may influence protein content. When the yield is low, it is possible to have low protein content if the nitrogen nutrition is deficient at flowering time and after. It is also possible to have a high yield and a high protein content at the same time. Similarly, no connection has been made between protein content and crop height at harvest (i.e., the standing ability).

In the search of new food protein resources, commercial facilities have begun focusing on extracting protein concentrates from pulses by air classification or wet milling techniques. Legume seeds can be fractionated (i.e., the separation out of component ingredients) to obtain the desired protein concentrates and isolates. More information is found on chapter 5.


Starch content

Starch is the main carbohydrate reserve found in plants, accounting for 22 % to 45 % in the pea seed. It is also a ma and animals, and an important raw material for industry.

Starch content varies between genera, from negligible amounts to half the dry seed weight in a wild-type, round-seeded pea. Mutations that affect the activities of enzymes of the starch can dramatically affect not only the starch content but also its composition.

Cooking pulses can significantly increase the rapidly digestible starch and decrease the resistant starch. Known as a prebiotic, resistant starch passes through the stomach and small intestine undigested. In the colon, it is digested along with dietary fiber to stimulate the growth of “good bacteria” and produce fatty acids that have anti-cancer properties.

It is thought that certain pulse genes that affect starch synthesis might enable pulse starch to be used for a wide range of food and non-food applications. Part of the interest is due to the beneficial health effects offered by pulse starch. Their low glycemic index is, for example, credited with contributing to the prevention of diseases related to insulin resistance.

Pulse starch has unique properties, including a good stability at high temperature and high point viscosity compared with cereal or tuber starches. These properties can be further improved by starch processing, including a use of chemical and biotechnological methods. As with protein, pulses can be fractionated to capture the desired starch concentrates and isolates. Pea starch, for example, is usually made available as a byproduct of protein extraction. This makes it a relatively cheap source of starch compared to other popular grains such as corn, wheat, and potato.

Pea starch is an integral part of noodle manufacturing in China. To date, starch from peas is used in deep-frozen dishes, dressings, extruded bakery products, cookies, crackers, sauces, instant soups, and puddings. They are often incorporated to modify food texture, which is important both for processing and consumer acceptance.

The isolation of starches and protein from peas is a difficult process. Wet and dry processes are used depending on the product specifications and applications. Dry fractionation is a two-step process that includes pin milling and air classification. Air classification is the most commonly used commercial method. Separating starch granules from protein requires a great degree of particle size reduction, which could be achieved by certain milling equipment such as pin milling. A major result from the air classification process is a low-protein starch, which is separated from the fine protein during the process. The concentrate is about 65 % starch, which is called starch concentrate.

Researchers have also developed a process to extract starch from wrinkled peas via wet fractionation process. This involves the steeping of wrinkled pea seeds in warm water, separating hulls from cotyledons via hulling with rubber rollers, gentle particle size reduction of the cotyledons, and high pressure disintegration of the screened-out protein/starch in the water. This facilitates the extraction of up to 90 % of the starch present in the wrinkled peas.

​Starch has also been isolated from chickpeas, including one particular variety that seems to have similarities with native maize starch. The production of pulse starches remains small compared to the major starch such as corn, potato, and wheat. However, due to preferred characteristics of pulse starches, especially their amylose content, food processors and manufacturers see a great potential for new applications targeted at industrial uses and human nutrition.

Fiber content

The dietary fiber is captured as a byproduct of the process of fractionation in which protein and starch concentrates are obtained from pulses. Preparations are generally richer in dietary fiber when obtained from hulls. Cotyledons contain variable amounts of starch and protein, while the inner-fiber exhibits higher water retention capacity than outer fibers.

Legumes have more dietary fiber than any major food group. Servings of the most commonly consumed grains, fruits, and vegetables contain one to three g of dietary fiber. Fibers are classified into two types, soluble and insoluble and together are called total dietary fiber. Soluble fiber can slow the absorption of lipids and lower blood cholesterol level. It can also slow the increase of fecal bile excretion, promoting reduced intestinal absorption of fat and cholesterol. Insoluble fiber assists in maintaining regularity and helps prevent gastrointestinal problems. When pulses are part of a diet low in saturated fat and cholesterol, soluble fiber may actually reduce the risk of coronary heart disease. The exact mechanism remains unknown; yet, scientists theorize that insoluble fiber adds bulk to stool, which in turn dilutes carcinogens and expedites their passage through the lower intestines and out the body.

The typical Americans eat only about 11 g of fiber a day, according to the American Dietetic Association. Health experts recommend a minimum of 20 g to 30 g of fiber a day for most people. The Food and Drug Administration (FDA) has recognized the importance of fiber by requiring it to be listed on the Nutrition Facts panel of food labels along with other key nutrients and calories.

Dietary fiber content varies depending on species, varieties, and the processing of the legumes. It ranges from 8 to nearly 28 %, with soluble fiber in the range 3.3 % to 13.8 %. Dietary fiber content in the cotyledon of legume seeds is generally low compared to that of the testa, or outer seed coat.

The fiber from pulses boasts excellent water hydration properties that can be utilized in food products to replace fat in items including confectionery products, dressings, and meat. Such fiber provides a broad range of positive effects, both physiological and metabolic, related to the source of the fiber (from cotyledon or hull), with the nature of that benefit being dependent on the form in which the fiber is ingested.

The composition of the dietary fiber depends on its location in the seed coat (outer fiber) or in the cotyledons (inner fiber). A major difference between the inner and outer dietary fiber is the amount of cellulosic and non-cellulosic polysaccharides present. The cell walls of the cotyledons contain a range of polysaccharides, including pectic substances (about 55 %), cellulose (about 9 %), and non-starchy non-cellulosic glucans (i.e., a polysaccharide that is a polymer of glucose) (6 to 12 %). The seed coat contains large quantities of cellulose (35 to 57 %) and lower amounts of hemicelluloses (i.e., polysaccharides that are more complex than sugar and less complex than cellulose) and pectin (i.e., a water-soluble carbohydrate). In terms of application, inner fibers are generally employed in texturing or bulking of products. In many cases, they can replace food additives, offering the benefit of more favorable labeling of the product. The fiber is most commonly used in bread and baked goods, particularly biscuits, and to enrich desserts such as mousses, jellies, and beverages. The outer fiber is used primarily to enrich the fiber content of food. It is found mostly in bakery and extruded products, snacks, and cereals.

Processing can be applied to improve the functional characteristics of fiber. For example, a mixture of cellulose and appropriate enzymes has been used to enhance important characteristics like mouthfeel and smoothness. Success in this regard is influenced by the fiber dimensions, porosity, hydration, and rheological and fat-binding properties.

Micronutrient profile

Lentils, dry peas, and chickpeas are good sources of important minerals like iron, magnesium, phosphorous, and manganese. They also contain significant amount of phosphorous, and the B vitamins, which play a key role in cellular metabolism. Lentils and chickpeas provide zinc as well. While the iron aids in the prevention of anemia, zinc is one of several nutrients necessary for fending off infections. Eating the recommended portion of pulses helps avoid the low magnesium that can come from a diet too heavily weighted with refined grains and cereals. Lentils and chickpeas also boast among the highest concentrations of folate (or folic acid as it is called when used in supplements), a single cup providing 37 % of the recommended daily allowance. A form of the water-soluble vitamin B9, folate is essential nutrient especially for women at child-bearing ages, and it must be obtained from foods and supplements. FDA instituted rules in January 1, 1998, that grain products such as breads, macaroni, rice, corn meal, and enriched flours are required to be fortified with folic acid. Among many health benefits, folate is necessary for the formation and development of new and normal tissue. Because new tissue forms at a rapid pace during pregnancy, the need of body for the important nutrient nearly doubles at that time, helping prevent anemia and the risk of neural tube defects such as spina bifida. Folate also helps break down homocysteine (i.e., an amino acid associated with heart disease) in the body, improves metabolism functions as well as the immune and nervous systems, and promotes cell growth and division.