resource for including macroelements such as Na, K, Mg, and Ca [47, 67] and microelements as Se, Zn, Mn, and Cu into the human diet [47, 68]. It is a grain rich in vitamins B1 and B2 [69, 70]. The significant contents and potential antioxidant activities of rutin and other polyphenols are very important for the dietary value of buckwheat [47, 69, 71]. In its grains and hulls, 6 flavonoids have been identified, including rutin, quercetin, orientin, vitexin, isoorientin, and isovitexin. On the other hand, only rutin and isovitexin have been determined in buckwheat seeds [51, 59, 72].
The consumption of buckwheat and buckwheat products is associated with many healthy biological activities including antidiabetic, anticancer, hypocholesterolemic, anti-inflammatory, neuroprotective, and hypotensive effect. Buckwheat proteins and polyphenols are supposed to be responsible for these effects [73, 74]. It has been accepted that some of the effects mentioned can be related to these two compounds in buckwheat. However, recently identified action mechanisms may also be exerting the health-promoting benefits that have been observed in cases of buckwheat consumption [74–76].
3.2.3 Oat
Oat is a cereal that has multifunctional uses including human food, animal feed, and health effects [77, 78]. The production of oat is in 6th place among grains worldwide, respectively following maize, wheat, barley, sorghum, and millet [79]. The world’s oat cultivation area is significantly smaller than that of some of these other grains [77 ,78], related to lower oat yields in comparison to other cereal yields [78, 80]. Oat accounts for >2% of the total global cereal production [79]. Oats are resistant to damp weather conditions and acidic soil. They are also generally disease-resistant. Also, they require less agricultural chemicals and fertilizer input [78, 81]. The production costs of oats can be similar to barley or wheat [78]. Nowadays, the main oat producers are Russia, Canada, the United States, and Northern Europe [82]. Global oat production is slightly more than 25 million tons per year [79]. According to the oat taxonomy given by Youngs et al., oat species are categorized as diploid, tetraploid, and hexaploid oats [82, 83]. The majority of oats being grown these days are members of the hexaploid group of species. Avena sativa L., more popularly known as common white oat, is this group’s most widely utilized species. It is raised and harvested in the world’s temperate areas. Avena nuda (naked oat) provides groats, the hulls of which are removed in threshing. Due to their excellent grain quality, naked oats are desirable among producers and in various industries, and especially for specialty markets. However, this species has low yields and it is susceptible to both mechanical damage and mold [82, 84]. Avena byzantina, or the common red oat, is commonly harvested in the winter season in the south of the United States. Today, the majority of the varieties of oat that are being grown have come about as crosses of Avena sativa with Avena byzantina [82].
The oat grain comprises the groat, which in turn is enclosed within the external hull layer [79, 85]. Although structural differences between assorted varieties of oats have been highlighted by various researchers, generally speaking, an oat grain has a shell (25%); the pericarp, testa, and aleurone (9%); the endosperm (63%), which is a starchy substance; and the embryo (3%) [86]. Hulls must be separated from the groats before the beginning of the processing [85, 87]. After being dehulled, these grains or groats constitute approximately 75% of the whole grain’s total; this figure fluctuates between 65% and 80% due to both diversity and environmental differences [86]. Oat is generally consumed whole as oat flakes [85]. The metabolizable energy content of whole oats is 2.5 to 2.6 kcal/g, and it is quite low [88]. The highly nutritious nature of oat species means that they are beneficial components of human and animal diets [78]. The typical protein value of whole oats is 10%–12% [86, 89] and this value is similar to that of wheat [86]. Meanwhile, oats have amino acid contents that are better than those found in other grain species due to the main protein being globulin [78, 90]. Compared to prolamins, which are grain storage proteins, globulins contain higher amounts of lysine and other essential amino acids [78, 91]. Oats also contain high levels of lipids [78, 84, 92, 93]. The lipid concentration of oat groats is 2 to 5 times higher than that present in wheat. Lipid contents can reach amounts of up to 10% of the total oat mass [93,94]. The major fatty acids in oatmeal can be listed as linoleic acid (38%), oleic acid (36%), and palmitic acid (19%), followed by linolenic acid (2%) and stearic acid (2%). The water content of oatmeal is 8.5% and the ash content is 1.8%. About 57.8% of oatmeal consists of carbohydrates, so it can be said to be a valuable source for obtaining dietary fiber [79]. Total sugar contents of oats are about 1%, while reducing sugar contents are less than 0.1% [86, 95]. Meanwhile, there are rising levels of interest in the consumption of oat-derived food items due to the requirement for soluble fibers in the human diet, and especially β-glucans, having valuable health-promoting effects [78, 96–98]. 1,3- and 1,4-β-Glucans are present as 2% to 6% of the total mass of groats and 7% of the starchy endosperm [86, 99, 100]. β-Glucans create viscous gums with water [86, 101] and contribute to water retention capacity, processing behavior, and viscosity [86, 102]. As the most highly abundant component of oats, starch accounts for 60% of whole oat grain’s dry matter. Oat starch’s iodine affinity is about 19.5%. This is reasonably close to the values of wheat, rye, and barley [86, 103]. Also, the α-tocopherol content of oat grain is 4.5–12.3 mg/kg and can be considered as high [104].
In the daily diet, the consumption of oats, which contain β-glucan polysaccharides, various dietary fiber components, and antioxidant compounds like tocopherol, wields a positive influence on consumer health and decreases the risks of various illnesses and malignancies [104]. In many studies, the valuable fiber components in oats were found to exert both therapeutic and protective activities against cardiovascular diseases, type-2 diabetes mellitus, and various types of cancer, like colon cancer [105–109]. The β-glucans found in oats are reported to have an association with oat’s ability to lower blood cholesterol levels [109]. New data also show that oats positively affect body weight and blood pressure [79].
3.2.4 Barley
Barley is known as one of the most ancient cereals to have been produced throughout the world. Archaeological proof demonstrates barley’s presence along Egypt’s Nile River about 17,000 years ago [110, 111]. Production of barley, scientifically known as Hordeum vulgare L., ranks 4th globally after respective maize, rice, and wheat production [112, 113]. Approximately 12% of the grain cultivated in the world is barley [111, 114]. Barley is utilized as one of the necessary raw materials for producing malt and beer, also being widely consumed in the production processes for animal feed [113, 115]. It has been generally used in the feeding of poultry, pigs, and adult ruminants and monogastrics [116, 117]. Of the barley being grown today, approximately 65% is utilized for animal feed, while 33% is used for malt products and 2% is reserved for human consumption [111, 118]. Barley can be farmed on diverse types of soil and in various climates, and it is used by many different cultures and nations [119, 120]. Compared to other types of cereals, it performs better under varied environmental pressures because it is drought-resistant, survives well in winter, and matures early; therefore, it is commonly more economically cultivated compared to other possible crops [111, 121]. The United States, China, India, Russia, and other countries from the former USSR are the major barley-producing countries [122].
Barley grains primarily comprise the husk, the embryo, and the endosperm, consisting of the aleurone and of endosperm cells, which are starchy [123]. The husk is the outer layer of the barley and forms 10%–13% of the grain weight [123, 124]. The embryo is made up of an acrospire, which is a nodal region located in between the shoot and root, as well as a primary root, which is enclosed by a root sheath known as the coleorhiza. Embryos are disconnected from the endosperm during germination by the modified cotyledon (the scutellum) [123, 125]. The endosperm comprises a starchy part and a layer of aleurone around it. The endosperm constitutes the major unit of a barley grain, representing 75% of a grain’s weight [123, 124]. The endosperm’s task is to function like a starchy storage center for nutrients that embryos can utilize in the process of germination [99, 123]. The cells in the aleurone layer include lipids, protein, minerals, and vitamins [123, 124, 126, 127]. Aleurone cells are isodiametric, which is different from the endosperm cells. Apart from the embryo, the sole unit of the grain that contains
