Since the beginning of life, plants have been utilized for human benefits, providing food, therapeutics, wood, fibers, and many others. Moreover, plants were considered the bioproduction system for valuable substances and provide many primary and secondary metabolites having therapeutic effects. Primary metabolites (protein, carbohydrates, fats, and nucleic acid) are the building blocks of life. Besides these, the secondary metabolites are produced by plants to protect them from predators and pathogens, cope with environmental stress, attract pollinators, and work as their defense system (1). Proteins are molecules with great complexity and diversity that play an important role in maintaining the structure and function of the living form (2). Therefore, it is being used for many applications such as medicine, food, and feed.

By 2050, the world's total population is expected to grow or might exceed 9 billion, and, hence, the demand for food, feed, and fiber around the globe is expected to increase by 70% (3). To meet this increasing demand, new sources must be explored. Nowadays, food derived from plants plays a vital role in the human diet as an important source of bioactive components, such as vitamins, phenolic compounds, or bioactive peptides. Hence, these components benefit human health and protect against various disease conditions (4). For meeting protein requirements, generally, animals are considered perfect. However, due to many diseases in animals, their consumption is not safer for human health. Also, it replaces animal-based proteins with plant-based proteins due to various limitations, such as increased cost, limited supply of nutrients, hazard for human health, freshwater depletion, and susceptibility to climate change (5–7). Plant-based proteins are considered vegan food, provide an ample number of amino acids, are directly absorbed by the body, and help in treating various disease ailments. Moreover, the proteins derived from plant-based foods are rich in fiber, polyunsaturated fatty acids, oligosaccharides, and carbohydrates. Hence, they are mainly associated with a reduction in cardiovascular diseases, low-density lipoprotein (LDL) cholesterol, obesity, and type II diabetes mellitus (8). Different sources of plant-based protein that include cereals (wheat, rice, millet, maize, barley, and sorghum), legumes (pea, soybean, bean, faba bean, lupin, chickpea, and cowpea), pseudocereals (buckwheat, quinoa, and amaranth), nuts, almonds, and seeds (flaxseed, chia, pumpkin, sesame, and sunflower) were well-explored (5, 9–11) (Figure 1). However, the demand for the supply of protein is continuously increasing with the rise of the global population (12), hence the need to search for new sources.

It is hard and expensive to extract an adequate amount of animal proteins; therefore, an alternative for improving the nutritional status of humans is mainly received from plant proteins. Hence, attention has been paid to evaluating the nutritional quality of proteins from different plant species. The best way to increase the supply of proteins is to improve the protein expression and efficiency of protein production in natural resources. The advancement of recombinant technologies of protein production, such as engineering of expression hosts, upstream cultivation optimization (e.g., nutritional, bioreactor design, and physical parameters), and development of methods of protein extraction, as well as purification, supported the growth of the market (13). Also, improving the protein functionality in foods through modification, enhancing the plant proteins proportion in human diets, and improving the bioavailability and digestibility of food proteins in the digestion process (14, 15) could be helpful to increase the overall utilization of plant-based protein.

Along with providing amino acids in food, proteins play a significant role in food formulations due to their diverse properties, such as emulsification, gelling, thickening ability, water holding, foaming, and fat absorption capacity (16, 17). Therefore, several thermal techniques (such as cooking, autoclaving, microwave heating, irradiation, germination, fermentation, extrusion, and drying) used during food processing could be optimized to improve the quality of plant proteins (2). Also, they can be isolated from sustainable and cheap sources such as plant-derived wastes from agriculture and by-products of crop and oil industries, which can also regulate food waste reduction (2, 7, 18).

To provide an overview of plant-based protein that helps sustain a better life for humans and the nutritional quality of plant proteins, this review mainly focuses on the current state of using plants to produce proteins for human health. It mainly focuses on various sources and their alternatives with high-quality protein, factors affecting the nutritional value of plant-based protein, bioactivity and functionality, and its modifications. Also, the information on the nutritional quality of proteins derived from plants and potential health issues linked with plant protein will be elaborated. Finally, the issues and challenges of plant-based proteins from availability, consumption, processing, and functionality will be elaborated, and recommendations were made for sustainable production and better utilization of plant-based proteins for meeting human health requirements.

The protein's nutritional quality can be identified in different ways, but, in a simple way, it is the balance and relative amounts of essential amino acids, as well as digestibility, bioavailability, and bioactivity, which mainly identify its nutritional value. Compared with animal-based protein, the proteins derived from plants are easier to produce; however, when utilized as dietary sources for human consumption, most of the plant proteins are deficient in essential amino acids and are, therefore, nutritionally incomplete. For example, some cereal proteins are low in tryptophan, lysine, and threonine content, while vegetable proteins and legumes have a lower amount of sulfur-containing amino acids, such as methionine and cysteine (58). Due to this deficiency, these essential amino acids become the limiting factor in legumes and cereals. Practically, neither legumes nor cereals can compensate for the deficiency of amino acids for other crops, and, hence, diet feeding regularly provides supplementary amino acids. There are also other factors that affect the nutritional quality of crops, including soil condition, crop maturity, postharvest handling, storage, use of fertilizers and pesticides, crop variety, and climatic conditions (Figure 2).

It is important in terms of nutritional as well as economic value to increase the essential amino acids content in plant-based proteins (59). In the past decades, plant breeders and geneticists have done much research for the improvement of the quality and characteristics of plant proteins. For instance, natural mutations, like the high content of lysine in barley and corn, have been recognized and made as elite genotypes (60). But, unfortunately, undesirable characters, like lower yields and susceptibility to pests and diseases, were also linked with these types of natural mutations. Nowadays, the techniques of modern biotechnology as alternative methods help to solve these problems. The method known as the protein digestibility-corrected amino acid score (PDCAAS) is an effective tool for the quality evaluation of protein (49, 61). One of the new methods recommended by FAO in 2013, digestible indispensable amino acid score (DIAAS), has also been used to evaluate protein quality, and, in terms of scientific knowledge, it is considered more accurate than PDCAAS (62).

Dietary proteins could be derived from animals and plants. Animal protein, although higher in demand, is generally considered less environmentally sustainable. A gradual transition from animal to plant-based protein food may be desirable to maintain environmental stability, ethical reasons, food affordability, greater food safety, fulfilling higher consumer demand, and combating of protein-energy malnutrition. Since the last 20 years, among the alternative sources of protein, the scientific research team and private companies have mainly focused on algae, earthworm or earthworm meal, insects, and other invertebrates (52, 53). Nowadays, food derived from plants plays a vital role in the human diet as an important source of bioactive components, such as vitamins, phenolic compounds, or bioactive peptides. Hence, these components are very helpful to human health and protect against various pathogens (4). Instead of animals, plants were considered the bioproduction system for useful substances, especially in medicine, which usually provide a large number of secondary metabolites having therapeutic effects. These substances produced by plants mainly help protect from predators and pathogens, attract pollinators, and have properties like anti-inflammatory, wound healing, anti-microbial, psychoactive, etc. (1), and hence utilized for protecting and maintaining human and animal health.

The proteins derived from plant-based foods are increasingly used as a health-promoting and economical alternative source in place of animal proteins in human nutrition. However, various limitations, such as increased cost, limited supply, biodiversity loss, hazard for human health in different diseases, freshwater depletion, and susceptibility to climate change, replace animal-based proteins (5–7). Moreover, it is hard and expensive to extract an adequate amount of animal proteins; therefore, an alternative for improving the nutritional status of humans is mainly received from plant proteins.

Globally, protein is produced from both plants (80%), such as cereal grains, beans, soy, pulses, nuts, vegetables, and fruits, as well as animals (~20%) in the form of meats, milk, eggs, fish, yogurt, and cheese (50). Compared to animal-based proteins, the proteins derived from plant-based foods are rich in fiber, polyunsaturated fatty acids, oligosaccharides, and carbohydrates. Therefore, they reduce the cardiovascular diseases and type II diabetes (8). Increased urbanization and economic development have led to various transitions in dietary patterns in the population of low- and middle-income countries, especially the demand for foods derived from animals, which was seen in developing countries.

Recently, plant-based sources of protein have dominated the supply of proteins throughout the world (57%), with the remaining 43% consisting of dairy products (10%), shellfish and fish (6%), meat (18%), and other products from animals (9%) (3, 19). Generally, the daily intake of protein is provided by animal-based foods. However, changes in the consumers' requirement led to adoption of alternative sources of proteins for human consumption. Therefore, emerging factors for animal proteins like growth of world population, climate change, and production of protein sources that are economically and environmentally sustainable need more research focus, and that is mainly dedicated to proteins from plants with high content, resilient to changing of climate and providing balance nutrition in humans' diet.

Compared with animal-based protein, the proteins derived from plants are easier to produce. Still, when utilized as dietary sources for human consumption, most plant proteins are deficient in essential amino acids and are, therefore, nutritionally incomplete. For example, some cereal proteins are low in tryptophan, lysine, and threonine content. In contrast, vegetable proteins and legumes have lower sulfur-containing amino acids, such as methionine and cysteine (58). Due to this deficiency, these essential amino acids become the limiting factor in legumes and cereals. Practically, neither legumes nor cereals can compensate for the deficiency of amino acids for other crops, and, hence, feed diets regularly provide supplementary amino acids.

Many studies have been done on the potential impact of dietary proteins derived from plants and serve as reducing cardio-metabolic risk factors. The first study for the synthesis and intake of plant proteins as an alternative to animal protein was reported (15). However, the regulation of protein intake is critical to many physiological development and functions. Therefore, enhancing the proteins derived from plants in adolescent diets as a substitute for animal-based proteins help in controlling obesity and other cardio-metabolic factors (85). Although plant-based diets are mainly linked with reducing the risk of diabetes (91), it is not clear that substituting the plant-based proteins for animal proteins helps in reducing the risk of diabetes in the population. After studying and analyzing the Nurses' Health Study II dataset, Malik et al. (92) observed that 5% substitution of vegetable protein for animal protein was linked with the 23% reduction of type 2 diabetes risk. Another interesting area of research to examine the benefits of the intake of plant proteins instead of animal protein is reducing cancer risk factors (96).

Protein modification is the process of alteration of the chemical groups or molecular structure of a protein by specific methods for improving the bioactivity and functionality of proteins. The modification approaches for plant-based proteins help them to make multifunctional food products. The modification of proteins can be classified into physical (18, 62, 69–82), chemical (125–130), biological (131, 132), and other novel methods (133–137) as briefly described in Tables 1–3. The physical modification approaches include heat treatment (such as conventional thermal treatment, ohmic heating, microwave heating, radio frequency treatment, infrared irradiation), gamma irradiation, electron beam irradiation, ultraviolet radiation, pulsed-electric field, high-pressure treatment (such as high hydrostatic pressure, dynamic high-pressure fluidization), sonication, extrusion, ball mill treatment, cold atmospheric plasma processing, and ultrafiltration. The chemical modification approaches include glycation, phosphorylation, acylation, deamidation, cationization, and pH shifting treatment. The biological modification approaches include enzymatic modification and fermentation. Instead of physical, chemical, and biological modifications, various other modification approaches have been identified, which include complexation (such as protein-polysaccharide, protein-protein, protein-phenolic, and protein-surfactant) and amyloid fibrillization (Tables 1–3) (138).

The advancement in recombinant technologies of protein production, such as engineering of expression hosts, upstream cultivation optimization (e.g., nutritional, bioreactor design, and physical parameters), and development of protein extraction methods supported the growth of the market (7, 13). The use of protein extraction technologies can help improve the yield of extracted protein and its nutritional and functional properties. Hence, a suitable type of protein extraction method should be selected (Figure 4).

The proteins derived from plants are considered as important and functional ingredients, having different roles in food formulations as gelling and thickening agents, foams and emulsion stabilizers, and binding material for water and fat (142–145). Most of the proteins have biological activities, like antihypertensive, antioxidant, antimicrobial, and stimulating characteristics (146, 147), and the protein from vegetables is also utilized for synthesizing and extraction of bioactive peptides (148, 149). However, most of the proteins from plant origin are interactable because of their susceptibility and complexity of ionic strength, pH, and temperature, and also have poor water solubility that mainly limits the applications of plant-based proteins (150). Most of the plant-based proteins, like flaxseed, soy, and pea proteins, have the combined nature of various proteins with different fractions, and, hence, they have a wide range of isoionic point (pI). Therefore, modulating the properties of plant-based proteins for improving their functions and formulation characteristics is essential. A deep understanding of the functional and physicochemical properties of proteins derived from plants is necessary for improving their utilization in food formulation and nutritional value (151–153). The presence of some particular plant residues considered as antinutrients is another challenge of plant-based proteins. These compounds are produced in plants having various biological properties, such as they protect the plants and seeds from insects, fungus, viruses, and other microbes. Therefore, some of the modification approaches discussed have been used to reduce or eliminate the adverse effects of antinutrients (18). Furthermore, some plant-based proteins have challenges in food applications due to their bitter taste, which can be masked by various modulation techniques. The methods of modification for plant-based proteins should be carefully chosen, especially in pharmaceutical and food applications, because these methods have effects on the organoleptic and functional characteristics and nutritional value of plant proteins.

The bio-efficacy of any active compounds generally depends on various factors, like digestibility, solubility, bioaccessibility, food matrix, transporters, metabolizing enzymes, and molecular structures. Therefore, identifying the bioavailability of food constituents is challenging. There are many challenges associated with sustainability and food availability that needs to be solved with different methods of protein modification. The higher amount of essential nutrients found in animal products (meat, milk, egg, etc.) was important and provided a large number of nutrients in the daily diet compared with plant-based proteins (154). Although animal-meat-based products provide a large nutrient component, however, the disease associated with animals, unhygienic conditions, and environmental impact will all provide more attention to the plant-based proteins. Because of that, consumers are also more focused on the health and environmental benefits of plant-based diets, promote the food guidelines on the basis of health and sustainability criteria, produce more attractive plant-based alternative products, and realign their fiscal policy along with environmental and efficiency criteria (155–159).

People are facing protein and mineral deficiency in their diet throughout the world, especially in developing countries. This challenge is due to lower consumption of pulses and cereals in their diets and other foods that are rich in zinc, iron, calcium, and magnesium. These foods derived from plants also contain higher levels of antinutritional factors that bind to the minerals ions and reduce bioavailability and absorption of plant minerals as well as proteins. Animal protein, although higher in demand, is generally considered less environmentally sustainable and prone to disease conditions, which negatively impact health. A gradual transition from animal- to plant-based protein may be desirable in order to maintain environmental stability, ethical reasons, affordability of food, greater food safety, fulfilling higher consumer demand, and combating of protein-energy malnutrition. Nowadays, products made with proteins from plant origin gain popularity throughout the world. Plant-based proteins have been linked with a number of health-related functionalities. Plant-based proteins are becoming innovative and fast-growing ingredients in various food application industries due to a large number of benefits over animal-derived proteins. Various technologies help in improving the functional and nutritional properties of plant-based proteins. Generally, plant-based proteins have inferior functionality as compared with animal proteins, and also various factors affect their nutrient quality; hence, modification approaches have been required. Different physical, chemical, biological, and other approaches were also mentioned for modification of proteins that induce the structural, chemical, and biophysical changes in protein from plant origins.

This review mainly focuses on the current state of using plants for the production of protein. The potential plants offering various sources and their alternative with high-quality protein demand for future consumption were discussed. Factors that affect protein consumption, bioavailability, and also protein production techniques were covered. Various bioactive and functional properties of plant-based proteins, as well as the factors affecting the nutritional quality of plant-based proteins and the future research strategies, were explained. The modification approaches, protein extraction, purification technologies, along with digestibility, absorption, and bioavailability of plant-based proteins, were discussed. Finally, it gave an idea of issues and challenges as well as future prospects in this emerging area.

SL conceived the idea. SL and FK wrote the manuscript. PY, ZD, RS, and AK edited the manuscript. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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