1. Introduction

The exceptional nutritional value of quinoa relies on its balanced composition of high protein, amino acid profile, minerals, fibers, and minor compounds (such as antioxidants and vitamins)^[1]. Moreover, due to the absence of gluten, quinoa is suitable for celiac patients or gluten related disorders. Several factors may affect the nutritional composition of quinoa seeds and the yield of the plant. Genetic and environmental conditions are two factors that may affect the yield and nutritional quality of quinoa. Accordingly, quinoa cultivation altitude can range from sea level to 4000 m high, and cultivation location ranges from Colombia (2° N) to Chile (47° S) in its origins. This variability in cultivation location and altitude, as well as rainfall regimes, has led to a high biodiversity of quinoa species, given that growing conditions are different for each location and thus plant adaptation was required^[2]. Moreover, quinoa breeding programs are focused on developing high yielding varieties with desirable nutritional properties which are better environmentally adapted to several agroecological zones. Emphasis is placed on the consumer markets—namely rich westernized countries—as quinoa has gained recent attention as a ‘superfood’^[3].

2. Proximate Composition

The proximate composition of quinoa seeds, as reported in the literature, is presented in Table 1. Among the macronutrients, carbohydrates can be found mostly on the perisperm of quinoa seeds, while the endosperm and embryo are richer in protein, minerals, and fats^[2][3].

Table 1. Proximate composition of quinoa seeds cultivated in different regions.

* values for fiber are reported as total dietary fiber. ^† n= 34 for carbohydrate, 37 for protein, 37 for fat, 23 for fiber, and 37 for ash. ^‡ mean values for two growing years.

Briefly, a thorough assessment of the reported data regarding the nutritional composition of quinoa by Nowak et al. presenting the data from 27 articles (103 data lines) found considerable variation of nutrient values among different varieties from different locations^[9]. Values reported in g 100 g⁻¹ edible portion—Fresh weight basis ranged as follows: protein (9.1–15.7 g), total fat (4.0–7.6 g), and dietary fiber (8.8–14.1 g) while the moisture content of quinoa is reported to be around 15%. In their report of the data, the majority of entries (68) were from samples from South America—mainly from Peru and Bolívia (the biggest producer of quinoa in the world)—followed by data from Europe (23) and Asia and North America (six each). This reflects the traditional production of quinoa in South America but also the expansion of its production worldwide^[9].

2.1. Protein and Amino Acid Content

The protein content of quinoa seeds ranges between 11% and 19% (Table 1). Moreover, quinoa seeds contain all nine essential amino acids (EAA) for proper human health as noted in Table 2^[11].

Table 2. Amino acid composition of quinoa seeds (g 100 g⁻¹ crude protein).

Quinoa has garnered attention as a protein source due to the high quality and balanced composition of amino acids content of its protein—superior to wheat, barley, and soybean. Quinoa essential amino acid scoring patterns (Scoring patterns, as defined by FAO, are based upon on the amino acid requirement values divided by the mean protein requirement^[12]) can be seen in Table 3, which shows quinoa exceeds the scoring patterns for 8 essential amino acids^[12][13][14].

Table 3. Essential amino acid profile of quinoa and other grains, compared to the FAO recommended amino acid scoring pattern for older children (3 to 10 years old), adolescents, and adults^[12][13][14].

The appreciation of quinoa as a food by Andean populations relies on its high nutritional value, as it is the principal protein source for rural populations, substituting the lack of animal protein^[15]. Moreover, due to its high protein content and amino acid profile, quinoa is suggested to be an alternative to dairy products^[15][16].

The protein and respective amino acid profile of quinoa can vary significantly from cultivar and location (Tables 1 and 2). Quinoa can be grown on various types of soils; nevertheless, the plant responds well to nitrogen fertilization, increasing yields, and protein content of seeds. The application of organic matter is important for topping nutrients and promoting water use efficiency in arid regions and sandy soils, thus enhancing the seed yield^[17].

2.2. Carbohydrates

The carbohydrate content of quinoa seeds ranges between 49% and 68% (dry matter weight) (Table 1). Starch is the main biopolymer constituent of plant organs, and is the most abundant carbohydrate present in the seeds. Native quinoa starch consists of uniform small granules less than 3 µm in diameter^[16][18]. Quinoa starch also presents interesting functional applications, due to its low temperature of gelatinization (range of 54–71 °C) and enthalpy (11 J g⁻¹ starch)^[4]. Compared to the starch of wheat and barley, quinoa presents a higher maximum viscosity, water absorption capacity, and greater swelling power^[18]. Its excellent freeze-thaw stability makes it an ideal thickener for food products where resistance to retro degradation is desired^[4][18]. Additionally, due to the small-sized granules and high viscosity, quinoa starch has the potential to be used in specialized industrial applications, such as dusting starches in cosmetics and rubber type mold release agents^[16].

Another carbohydrate group present in quinoa seeds is dietary fiber. The total dietary fibers content of quinoa seeds is close to what is found in other cereals ranging from 7.0% to 9.7 % (DM)^[18]. Pulvento et al. reported an average of 17.2% of dietary fiber in quinoa harvested in the south of Italy. Although representing a high content, dietary fiber can decrease significantly after post-harvest processes to eliminate anti-nutritional micro components present in seed coats^[5]. Table 1 notes the fiber content of quinoa found in the literature.

2.3. Fat

The fat content of quinoa seeds varies between 2 and 9.5%, which is higher than maize and other cereals but less than soybean (Table 1). Quinoa oil is rich in essential fatty acids such as oleic [C18:1] (19.7%–29.5%), linoleic [C18:2] (49.0%–56.4%), and linolenic [C18:3] (8.7%–11.7%). The portion of (poly-) unsaturated fatty acid accounts to 87%–88% of total fatty acids of the seed^[4][18]. These compounds have gained importance since they promote health benefits such as positive effects on the immune system, cardiovascular diseases, cell membrane function, and increased insulin sensitivity^[10][18]. Table 4 shows the reported results of determining the fatty acid content and profile of quinoa seeds of different varieties cultivated in different locations. Quinoa may also be considered an alternative oilseed. The oil contains a high concentration of antioxidants such as α- and γ-tocopherol, which ensures quinoa oil a long shelf life due to its natural antioxidant potential at the level of cell membrane, protecting fatty acids against damage by free radicals^[18].

Table 4. Most relevant fatty acids content of quinoa seeds.

* Reported values are average for 21 accessions (from Northwest Argentina) in g 100 g⁻¹ of total fatty acids. ^† Reported values in g 100 g⁻¹ fat. ^‡ Reported values in g kg⁻¹ of total fatty acids.

3. Micro Components

Distributed across the macro components of quinoa seeds are micro constituents such as minerals and bioactive compounds are present in minor scales. Such micro constituents contribute to not only the nutritional composition of quinoa but also may be used due to their functionality. Moreover, the exceptional nutrient profile from quinoa can provide valuable therapeutic properties such as enhancing immune function, assisting in cell repair, calcium absorption and transport, participation in the metabolism of fatty acids for human health, and even preventing cancer metastasis^[11][18].

3.1. Minerals

As it can be seen in Table 1, the ash content of quinoa seeds ranges from 2.4% to 4.8%. The ash contains a diversified profile of minerals including a high content of calcium, magnesium, iron, copper and zinc. The mineral content of quinoa seeds is found to be at concentrations greater than most grain crops^[17]. Table 5 shows the mineral content of quinoa and its comparison with other grains^[13][14].

Table 5. Mineral content of quinoa and other grains^[13][14].

Vega-Gálvez et al. reported that mineral concentrations seem to change drastically when quinoa is cultivated in different soil types—thus with particular mineral compositions—and fertilizer application^[17]. Table 6 summarizes the reported values for mineral content of quinoa seeds reported in the literature.

Table 6. The mineral content of quinoa seeds of different varieties.

3.2. Bioactive Compounds

Quinoa seeds are the main edible part of the quinoa plant, nevertheless quinoa leaves are rich in phenolic compounds that present antioxidant and anticancer properties. Plant polyphenols and phenolic content are beneficial to human health, due to their antioxidative potential. It has been suggested that such compounds can aid the risk reduction of cardiovascular diseases, neurodegenerative disorders, and diabetes^[21][22].

Considerable amounts of ferulic, sinapinic, and gallic acids, kaempferol, isorhamnetin, and rutin were obtained in quinoa extracts. These named compounds were linked to an inhibitory effect on prostate cancer cell proliferation and motility^[22].

3.3. Saponins

Saponins are grouped among the minor components, secondary metabolites, broadly studied due to their biological properties. They are considered to be the most anti-nutritional factor in quinoa seeds, acting as a natural protection against pathogens and herbivorous. Over 30 types of saponins can be found distributed in quinoa plant parts^[23]. The quantification of saponin content is important in order to differentiate between ‘sweet’ (having saponin content of 20–40 mg g⁻¹ dry weight) and ‘bitter’ genotypes (>470 mg⁻¹ dry weight)^[5]. The saponin content found in quinoa seeds reported in the literature is presented in Table 7. Saponins confer the bitter taste and are mostly found in the outer seed coat. The compound is removed by post-harvest processing techniques like cold water washing, abrasion, and dehulling^[11]. In addition, saponins extracted from quinoa seeds can be used in other industries such as cosmetics and pharmaceuticals.

Table 7. Saponin content of quinoa seeds of different varieties.

* mean value of two years in mg g^−1.

3.4. Vitamins

Quinoa is also a source of vitamins, namely riboflavin and folic acid, offering similar values of thiamine, but is a lesser source of niacin. It has been noted that the removal of the saponins (to reduce the bitter taste) does not seem to affect the vitamin content^[13][14]. Vitamin content of quinoa and compared to the other grains can be seen in Table 8.

Table 8. The vitamin content of quinoa seeds compared to other grains (mg 100 g⁻¹ DM)^[13][14].