Lentil (Lens culinaris Medik) as nutrient‐rich and versatile food legume: A review

Lentil is one of the most important food legumes consumed widely throughout the world. Lentils are produced in diverse agroecological regions, such as Asia, North and South America, Africa, and Oceania. During the last two decades (2001–2020), world production of lentils increased by 107%, from 3.15 to 6.54 million metric tons. Canada leads lentil producing countries (with 44% share of the global output), followed by India and Australia having 18% and 8% share, respectively. Being a rich source of protein, complex carbohydrates, dietary fiber, and folic acid, lentils are considered a healthy food nutritionally. Lentils also contain a number of bioactive phytochemicals, such as flavonoids, total phenolics, phytate, saponins, and tannins. Dehulling and splitting of lentils are the most‐commonly used basic processing methods. Additional value‐added operations include milling of whole or dehulled lentils and isolating fractions that are rich in protein and starch that can be used as ingredients in diverse food applications. Lentils are aligned well with the changing consumer trends towards meat alternatives, plant‐based diets, and healthy food options. Furthermore, due to increased environmental concerns for the production of meat, consumers are minimizing or even excluding meat consumption and opting for non‐meat foods produced in a sustainable manner. This review article provides an overview of lentil's production/trade, consumption trends, nutritional profile, value‐added processing, emerging research and development trends, and the role of lentil production in environmental sustainability.

are termed as Adas (Middle-East), Mercimek (Turkey), Masoor (Pakistan and India), Messer (Ethiopia), and Heramame (Japan). Lentils are grown in over 40 countries, with Canada, India, Australia, Turkey, and the United States being the leading producers (FAO, 2021). In the early 1980s, lentil production was introduced to North America as a complementary new pulse to existing crop rotations. In recent years, lentil has become a major food legume crop in both Canada and the United States (Siva et al., 2017).
Lentils are a nutrient-dense food legume, having high protein content, complex carbohydrates (consisting of slowly-digestible starch and less crude fiber), essential minerals, vitamins, and high energy value (Dhull et al., 2022;Joshi et al., 2017). Moreover, lentils also have significant content of bioactive phytochemicals, for example, antioxidants and phytoestrogens (Siva et al., 2017). Podder et al. (2021) reported on the potentiality of lentil in providing Fe and Zn requirement to satisfy a major portion of the recommended Fe and Zn dietary allowances (RDAs) in deficient populations. Lentil protein isolates have various functional properties allowing their use as emulsifier and foaming agent (Khamidova et al., 2022). Moreover, lentil flours and fractions significantly improve the ability of food systems to retain water and fat. It is noteworthy that, even with extensive processing, lentil grain proteins retain their native conformation (Khamidova et al., 2022). Besides a variety of nutritional benefits, Patterson et al. (2017) noted that lentils contain some antinutritional factors (ANFs) including lectins, trypsin enzyme inhibitors, phytates, saponins, and flatulence-causing oligosaccharides. These ANFs are known to impair the digestive enzymes' activities and sequester essential nutrients, which lowers their bioavailability thereby rendering them unavailable for digestion and absorption (Dhull et al., 2022;Nosworthy et al., 2018). As a result, appropriate processing or cooking methods are required to significantly reduce or eliminate ANFs in lentils (Campos-Vega et al., 2010).
The traditional utilization of lentils is primarily in home prepared cuisines as well as commercially processed fried and extruded snacks.
The usage of pulses, such as lentils, is gaining increasing recognition as ingredients for diverse food applications. This trend aligns well with the increased concerns about the sustainable agriculture, negative impact on the environment for the production of meats, and other animal-based foods. Thus, consumers are notably decreasing or excluding meat use and moving to sustainably produced plant foods and non-meat protein alternates (Hill, 2022;Marette & Roosen, 2022). Lentils fit well with these changing dynamics as lentil crop plays an important part in the maintenance and improvement of soil fertility due to its high organic matter content and symbiotic nitrogen fixing (Matny, 2015). This decreases the need for inorganic fertilizers, thereby increasing productivity in terms of yield of the subsequent crop or cropping system as a whole (Reddy, 2004). Generally, the environmental concerns, along with documented health problems from consuming meat, have resulted in the increased acceptance of plant-derived foods, which serve to diverse consumers well beyond those vegans and vegetarians (Dhull et al., 2022;Siddiq et al., 2022).
These combinations create important ecological and economic synergies. Canada's Agri-Food Innovation Council (AFIC, 2019) has projected that by the year 2024, plant-based proteins will constitute about 33% of the food proteins' global market. This article provides an overview of lentil production/trade, consumption trends, processing methods, nutritional profile, and emerging research and development trends. Further, the role of lentils in environmentally sustainable agriculture systems is also discussed.
Canada ranked first in lentil producing countries, with 2.87 MMT, thus accounting for about 44% of total global production in 2020.

| QUALITY GRADES OF LENTIL
According to Saskatchewan Pulse Growers, the basic quality criteria and parameters for red lentils are "dictated by milling requirements and by color, size, condition of dehulled split seeds, or the condition of dehulled whole seeds having both cotyledons attached/intact." Whereas, for green (seed coat color) lentils, the basic quality criteria are "seed diameter, seed thickness and uniformity, color uniformity, and intact green seed coats without wrinkling or staining" (SPG, 2012). It is noteworthy that the color of the lentil grain is an important quality parameter because it influences acceptance by consumers and hence value of the lentil product (Shahin & Symons, 2001). The lentil seeds' color can vary from light tan to brown or darker brown, with dark brown lentils deemed as low quality (Opoku et al., 2009). Jackson et al. (2021) noted that the color loss may be associated with a loss of nutrients as well as secondary metabolites, for example, polyphenols. It is advised that producers should not mix lentils from successive years, to avoid having the entire batch downgraded. Because lentils with green seed coat are prone to discoloration during extended storage, green lentils should not be stored for more than one year to minimize or avoid excessive discoloration and downgrading their marketability (SPG, 2012).
It is also recommended to harvest lentils from the field at a relatively higher moisture levels (16-20% wb) in order to reduce field shattering quantity losses during harvesting and dried postharvest to 13-14% to extend shelf-life and minimize resulting mechanical damage and susceptibility to breakage during handling, storage, and postharvest processing (Opoku et al., 2009). Uebersax (2022, personal communication) recommended that it is very important to minimize impact damage and seed coat abrasion at every phase of handling/ transfer because the quality deterioration is cumulative and cannot be corrected or reversed. Damaged seed coats directly affect seed appearance and are generally indicative of overall quality due to asso-

| LENTIL CONSUMPTION AND CULINARY TRENDS
Lentils are relatively quick and simple to prepare when compared to most other pulses and dry beans. Most of the lentil consumption continues to be in the form of traditional cooking and processed products.
However, there are wide variations in consumption trends across different countries/regions. Some of these variations are based on the type of lentil consumed rather than the culinary method or products prepared. As shown in Table 3, Red and Green lentils are the widely used lentil types in most countries with regular lentil intake, whereas Yellow and Spanish Brown lentils are consumed in relatively few countries (Siva et al., 2017;Thavarajah et al., 2008). Typically, green lentils are eaten as whole seeds or as dehulled-split form, whereas red lentils are generally dehulled before cooking and consumption (Siva et al., 2017).
Lentils are incorporated into many regional cuisines throughout the world. For example, besides being commonly used as curry that is consumed with wheat chapati or flat bread, lentils are frequently mixed with cereal grains, such as rice-based Khitchri, a South Asian popular dish, whereas Koshari and Mjaddara are the common dishes in Egypt and Syria. Moreover, dehulled split lentil soup, topped off with olive oil and lemon juice, is another most commonly consumed type of lentils across many Middle Eastern and North African countries (Dagher, 1991;Faris & Attlee, 2017). In Creole and Cajun cuisine in Southern United States, Cajun Lentils and Rice is made in the same manner as red beans and rice; though, red beans are replaced with red lentils and tomatoes. The intensity of aromatic heat flavor is enhanced by adding different herbs/spices, for example, garlic, green chilies, lemon juice, and occasionally Indian curry powder. Brown rice is used in this dish, and, typically, mushrooms are also added to give some texture (Amin & Borchgrevink, 2022).

T A B L E 3 Common lentil market classes and consuming countries
Seed size/ utilization Red lentils Green lentils Yellow lentils

Spanish brown
Size (100-seed weight) •Extra small (2.9-3.2 g) •Extra small (2.9-3.2 g) •Extra small (2.9-3.2 g) •Small Hill (2022) noted that the popularity of plant-based protein sources, as meat alternatives, has been increasing in recent years among consumers in developed countries. Lentils are being used in a number of bakery and extruded products, but the long-term success of these expanded uses depends on evaluation and understanding lentil flours functional properties and their effects on the sensory attributes of the end-products (Sidhu et al., 2022). Similarly, lentil flour substitution levels need to be optimized for dough properties (i.e., optimal rheological characteristics) in those products that are traditionally produced using wheat flour. Amin and Borchgrevink (2022) suggested that the compositional and techno-functional properties of food legumes, like lentils, present them as a suitable option for pasta making and other wheat flour products. By replacing wheat flour with that from pulses flours or flour blends, the percentage of protein and dietary fiber of such products can be significantly increased as compared with traditional 100% wheat-based products.

| LENTIL NUTRITION AND HEALTH BENEFITS
Lentils possess a nutritional profile that is not only nutrient-dense but also offers a better balance between protein and carbohydrates, when compared with other pulses (selected sampling) and wheat ( Figure 2).
Protein, carbohydrate, and dietary fiber content of lentils is higher than that in wheat and selected other pulses, whereas, by contrast, the ash (minerals) content is comparatively higher. These nutritional advantages of lentils are important attributes of lentils from a human health perspective. It is to be noted that, owing to a substantial number of cultivated varieties, considerable differences exist in the composition and nutritional profile of lentils (Sidhu et al., 2022;Zia-ul-Haq et al., 2011). The amino acid profile of lentil seeds is presented in Table 5, which shows the presence of the most essential amino acids. However, similar to most other pulses, lentils lack methionine-a sulfurcontaining amino acid (Joshi et al., 2017;Yadav et al., 2007). When combined with cereal-based foods, which are high in methionine (e.g., chapati or flat wheat bread, rice), lentils contribute complete proteins with enough amounts of most essential amino acids (Sathe, 2012). Uebersax (2022, personal communication) indicated that the assessment of protein quality (in terms of bioavailability) is a rather complex process, but complimentary blending or mixing of proteins can lower the limiting amino acids content. Perera et al. (2010) reported that lentils are considered a rich source of resistant starch (RS), which varies from 11.4% to 14.9% in different cultivars. From a digestion rate perspective, starches are classified into different groups, namely rapidly digestible starch (RDS), slowly digestible starch (SDS), and RS. The RDS or "bad" starch is digested in the small intestine thereby eliciting a high glycemic response. In contrast, the SDS is digested at a slower rate, which results in a relatively low glycemic response. By contrast, the RS is not digested in the small intestine and is therefore classified as a dietary fiber to undergo fermentation by the microorganisms in the large intestine, which produces short-chain fatty acids (Englyst et al., 1992). Tovar and Melito (1996) reported that regardless of processing methods used, the RS content in processed lentils still remains at a F I G U R E 2 Protein, carbohydrate, total lipids, and ash (minerals) content of lentil compared to selected cereal grains. Source: Based on USDA Food Central data (USDA, 2022) relatively high level when compared to other food sources, especially cereal and potato, both of which upon processing show generally reduced RS content.

As shown in
Lentils are rich in phenolic content compared to other legumes, which shows its relatively higher antioxidant activity. Lentils also contain a number of bioactive phytochemicals, such as flavonoids, phytic acid, phenolics, tannins, and saponins (Oomah et al., 2011;Paranavitana et al., 2021). However, some of these compounds are classified as antinutrients because they are known to impede the absorption of minerals (Sathe, 2012;Thavarajah et al., 2010).

| Health benefits of lentils
Regular intake of lentils, similar to other pulses, offers a variety of health benefits. For example, benefits of consuming lentils have been reported in several other biochemical processes, that is, lowering blood cholesterol levels, inhibition of angiotensin I-converting enzyme (ACE), and working as an antioxidant (Didinger & Thompson, 2022;Johnson et al., 2020;Patterson et al., 2017). Regular intake of lentils or lentil-based products also provides a protective effect against various chronic disease conditions, for example, cardiovascular diseases, high blood pressure, high cholesterol, and cancer. Whole lentil or lentil flour of some lentil cultivars (e.g., Blaze and Laird) binds bile salts that is known to reduce blood cholesterol (Alshikh et al., 2015;Zhang et al., 2018). Predating modern research conducted during the last few decades, lentil's health benefits were also mentioned in ancient treatment remedies (Faris & Attlee, 2017;Lardos, 2006). In developing countries, both micronutrient malnutrition and protein-energy malnutrition are serious health problems particularly among children between 1 and 5 years old. Therefore, expanding lentil consumption and their incorporation in weaning foods can help ameliorate overall micronutrients and protein deficiency.
Higher protein and significant dietary fiber contents in lentils result in a slower digestion of carbohydrates and hence a lower glycemic response. This particular effect is helpful for patients suffering from diabetes mellitus and for those who may be pre-diabetic or susceptible to this wide-spread disease. One serving/cup of cooked lentils has 15.6 g of fiber, which meets about 60% daily fiber requirement. In addition to other physiological benefits, lentil's dietary fiber, which includes raffinose and stachyose, are reported to imparts positive effects to humans, for example, regulating bowel movement and stimulation of probiotics' growth (Johnson et al., 2020;Sidhu et al., 2022).
6 | VALUE-ADDED PROCESSING OF LENTILS Joshi et al. (2017) divided lentil processing into three levels: (a) Primary processing that includes some basic operation, for example, cleaning, sorting, and grading to market packaged whole lentils for retail use and for further processing; (b) Secondary processing that involves a range of unit operations, for example, dehulling and splitting of the whole and split lentils; and (c) Tertiary processing that consists of grinding or milling of whole or dehulled lentils and separation of protein and starch fractions for subsequent use in various food applications. Dhull et al. (2022) noted that lentils are cooked for direct consumption or further processed as flour and protein, starch, and fiber fractions ( Figure 3). Both dry milling or wet milling can be used to produce flour and protein-and starch-rich fractions (Dhull et al., 2022;Xing et al., 2020). The milling method employed has considerable impact on the functional properties of flours and specific fractions that must be evaluated prior to use in various food products (Joshi et al., 2017;Sozer et al., 2017). Thermally processed (canned) lentils are commercially available in some retail markets in Canada, the United States, the United Kingdom, and many European Union countries. Siddiq et al. (2022) indicated that the versatility of lentils provides multiple venues for the development of new lentil-based products, including gluten-free product development. Extrusion processing also could be used to prepare lentil flour by adjusting feed rate, moisture level, and processing temperature (Dhull et al., 2022). The use of raw lentil flour should be avoided due to potential the development of rancid off-flavors produced through lipid oxidation and also intrinsic food safety implications considering the presence of ANFs.
In addition to ensuring palatability and improving nutrients availability, various cooking or processing methods also affect the protein digestibility and level of ANFs in lentils. Soaking in water is shown to effect substantial reduction in the phytate content, while the adding of sodium bicarbonate (NaHCO 3 ) in soak water reduces phenolics and tannins (Dhull et al., 2020;Joshi et al., 2017;Martín-Cabrejas et al., 2009). Different processing methods (autoclaving, extrusion, germination or sprouting, roasting, and fermentation) have variable impacts on compositional and nutritional profile of lentils and can also reduce antinutrients' effects. Autoclaving improves digestibility due to interference of protein structure and gelatinization of starch (Dhull et al., 2022;Rehman & Shah, 2005). Extrusion processing is also reported to be effective in improving nutritional composition of lentils with optimum flavor and texture (Pasqualone et al., 2020). Ghumman et al. (2016) reported that germination/sprouting of lentil seeds has significant effect on compositional and nutritional quality, for example, an increase in crude proteins and a reduction in lipid, carbohydrates, phytic acid, and tannins contents.

| Innovative processing technologies for lentils
Lentil is a somewhat least researched crop and common beans and pulses; therefore, emphasis should be on lentil value chain, especially in breeding strategy to develop varieties with high yield. The research should focus across value chain using a holistic approach that allows sustainability mechanisms. Modern tools and techniques of biotechnology should be used for developing varieties with novel traits that will make them climate smart. In the area of postharvest processing, a number of innovative technologies have been introduced for valueadded processing of food, which can be employed in grain legume processing. These technologies include high pressure processing, ultrafiltration, ionizing radiation, ultraviolet radiation, pulsed electric field, pulsed light treatment, ohmic heating, and ultrasound treatment (Ahmed, 2018;Gharibzahedi et al., 2018;Tokuşo glu & Swanson, 2014). Some of these innovative technologies have been explored in the value-added processing of lentils to enhance the shelf-life and sensory properties of lentil based products. Boye et al. (2010)  It is to be noted that the use of innovative processing technologies is mostly at research and development stages. Nonetheless, with further research on optimization of processing conditions and the quality assessment of thus produced lentil products and ingredients can help towards commercial use of such technologies. In this regard, the non-thermal innovative technologies, for example, high-pressure treatment, ultrasound application, and SC-CO 2 extrusion, offer a greater potential for lentil processing.

| ROLE OF LENTILS IN AGRICULTURAL SUSTAINABILITY
Legumes, including lentils, provide a number of numerous ecosystem benefits while also contributing to a sustainable cropping systems primarily via (1) their ability to contribute nitrogen (N) to the system by biological N 2 fixation; (2) diversification of cropping systems (including crop rotation) that results in reduced incidence of disease, pest, and weed, thus, potentially increasing biodiversity; and (3) a significantly reduced fossil energy use in plant production/protection practices (Jensen et al., 2010). Jensen et al. (2010) also reported that a legume crop's consequent contribution to the N-economy in a cropping system is achieved mainly through (1) unused or spared soil N and rhizodeposits of N remaining after plant growth, and (2) mineralized N from surface organic residues and the nodulated roots upon harvesting. González et al. (2011) noted that lentils have a high protein content and its efficient delivery with respect to energy usage or emission of greenhouse gases, in comparison with dairy and eggs, cereals, fruits, and vegetables. Warne et al. (2019) recommended the adoption of lentil in agricultural production systems along with plantbased food products is an excellent food system solution, which can provide sustainability through the environmental and socio-economic aspects.

| CONCLUSION
Lentil is among one of the first food crops to be cultivated by humans.
Lentils are an essential part of diets in many countries due to their high protein, balanced carbohydrates (including dietary fiber), and minerals content, and a variety of health-promoting bioactive compounds. In addition to traditional domestic cooking, a wide variety of processing techniques such as milling, extrusion, germination/sprouting, fermentation, roasting, and frying are commonly used to prepare a variety of end products with improved quality and consumer acceptance. Lentil flours, fractions, and isolates that are rich in protein and starch offer a potential for developing gluten-free food products. One of the major aims of cooking/processing is to improve palatability, reduce antinutrients, and improve bioavailability of proteins and mineral. Lentils can be used in a wide range of traditional cuisines and value-added food product development, including plant-based alternate to meat proteins. However, the functional and sensory attributes of lentil-based ingredients should be carefully assessed and optimized for food diverse applications for ensuring acceptance by consumers.

CONFLICT OF INTEREST
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.