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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 living cells is the aleurone layer [123, 125].

Barley genotypes are classified as hulled and hull-less. The hull-less variety has better nutritional value compared to the hulled one [120, 128]. Hull-less barley generally has higher amino acid, total protein, and digestible energy contents compared to hulled barley [129–131]. Barley can be described as a perfect source for obtaining β-glucans and complex carbohydrates, two critical constituents of dietary fiber [132]. Barley grain contains 60%–80% carbohydrates, 9%–13% protein contents, 1%–2% fat, and 10%–15% water [132, 133]. The most limiting essential amino acid of cereal species, namely lysine, is found in higher levels in some cultivars [129–131]. Starch is the carbohydrate to be found most plentifully in barley grain, its levels varying between 62% and 77% in terms of the dry grain weight [132, 134]. The dietary fiber content of barley is 14%–25%. Arabinoxylan content is 4%–11%, while the β-glucans, cellulose, and lignin contents are 3%–7% [135–138]. Barley is rich in vitamins, particularly B vitamins, and it also has valuable mineral contents, especially trace minerals [139, 140]. Apart from β-glucans, the compounds in barley such as tocotrienol, phytoestrogens, lignans, polyphenols and phytic acid have been associated with health benefits [140, 141].

Epidemiological studies show that regular barley consumption has the possible ability of lowering the risks of various chronic diseases including colon cancer [111, 142, 143], chronic heart disease [111, 118, 144, 145], gallstones [111, 146, 147], and high blood pressure [111, 148]. There are reports of the role of barley in the maintenance of colon health [111, 149], the induction of stimulation of the immune system [111, 150], and immuno-enhancement [111]. In addition, barley consumption provides a protective effect against toxins, diabetes mellitus, and neurological problems such as Alzheimer disease [111, 151, 152].