35].
Due to their beneficial activities in both treating and preventing many different diseases, and considering the increasing healthcare costs of modern times, the desire to better understand the potential of functional foods has increased among not only food scientists but among regulatory agencies, consumers, and food producers [37]. This chapter explores the structure, chemical compositions, and health-promoting effects of major cereals such as wheat, buckwheat, oat, barley, flaxseed, psyllium, brown rice, millet, sorghum, maize, and rye, and functional foods produced from these cereals.
3.2 Structure and Chemical Composition of Cereal Grains
3.2.1 Wheat
Among the most important grains in the world is wheat, produced annually at quantities reaching approximately 600 million tons. About 70% of this wheat is utilized for the production of foodstuffs [38, 39].
Triticum aestivum L., commonly known as wheat, is a grain comprising the 3 basic sections of an embryo (also known as the germ), endosperm, and pericarp (also known as the bran). A whole wheat kernel consists of 3% germ, 12% bran, and 82% endosperm [37, 40]. A wheat grain is about 8 mm long and weighs approximately 35 mg, while kernel size changes based on its location within the spike or the head of the plant, and also based on the cultivar. Wheat kernels have longitudinal folds on the ventral side, which run the length of the kernel. This fold makes it difficult for the bran to separate from the endosperm and keeps out both dust particles and various microorganisms. In the seed coat, there is pigmentation that gives the wheat kernel its coloring. Color is determined by genetics but is usually white or red. The grower may choose a preferred kernel color by genetic manipulation. The wheat kernel has an aleurone layer surrounding the endosperm, which is starchy in nature, and the germ. The bran and germ will be detached from the endosperm in the milling process. Most of the endosperm fraction (71%) is found in white flour. Amounts of the remaining parts of the endosperm, like the outermost layer of aleurone, are low [37].
Wheat bran has high mineral contents, as well as containing high levels of B vitamins. It also contains water and insoluble fiber, the presence of which protects the grain and the endosperm. Over half of the bran, or 53%, is made up of fiber components. Wheat bran fiber possesses a very complicated chemical composition, but it primarily comprises cellulose, pentosanes, and polymers based on arabinose and xylose that are bound closely to proteins. Such standard polymers are located within the wheat’s cell walls and cell layers like the aleurone layer. Both wheat bran carbohydrates and proteins account for 16% of wheat’s total dry weight. At the same time, wheat bran’s mineral contents are quite high (7.2%). In contrast, the pericarp and seed coat, which are the grain’s outermost layers, are composed of empty cells that are not alive. In the aleurone layer (i.e., the inner bran layer), cells are full of living protoplasts, which is why there is a large amount of carbohydrates and proteins in wheat bran. The amounts of amino acids in flour and in the aleurone layer vary dramatically. The levels of proline and glutamine in the aleurone layer are about half, the amount of arginine is three times, and the contents of asparagine, alanine, glycine, lysine, and histidine are twice that of wheat flour [41, 42]. Glutathione, alkylresorcinols, sulfur compounds, carotenoids, and α-linoleic acid are other key components of the bran. In wheat bran oil, steryl ferulates, tocopherols, alkylresorcinols, and other polyphenols are found [43, 44].
The endosperm layer of wheat is rich in energy-yielding starch. It contains proteins (13%) and fats (1.5%) apart from carbohydrates. Proteins in the endosperm of wheat are globulins, albumins, gliadins, and glutenins. Gliadins and glutenins are important proteins that make up gluten in dough-making. The mineral and dietary fiber contents of wheat endosperm are quite low. The mineral (ash) content is 0.5% and the dietary fiber content is 1.5% [42, 45].
Wheat germ has high levels of lipids (8%–13%), proteins (25%), and mineral substances (4.5%). At the same time, it represents a valuable source of vitamin E. While wheat germ contains 50% of the proline and glutamine content of the flour, its amounts of asparagine, arginine, alanine, glycine, threonine, and lysine have been measured to be twice as high [41, 42].
For dietary antioxidants, whole wheat and wheat bran both constitute valuable sources. The phenolic acids found in wheat, both free and esteri-fied, exert the largest influences among the health effects of wheat [22, 46, 47]. Phenolic acids, which are taken into the body with the consumption of a normal portion of whole wheat, exhibit marked antioxidant activities under in vitro conditions. The solubility and activity of wheat polyphenols are increased by acidic conditions and enzymatic hydrolysis [22, 46].
Whole wheat exerts many valuable health-promoting activities and effects. These are understood to be the result of the nutraceuticals and bioactive phytochemicals of whole wheat. The consumption of whole wheat and foods produced from it can reduce the risks of some chronic diseases, including type-2 diabetes mellitus, obesity, cardiovascular diseases, and cancers, especially colon cancer. This is achieved through both antioxidant and anti-inflammatory properties and gut microbiota modulation, as well as immunity improvement [48, 49].
3.2.2 Buckwheat
Fagopyrum esculentum Möench, commonly called buckwheat, is a traditional agricultural product historically grown in Asia and Central and East Europe [50]. It belongs to the family Polygonaceae, unlike most other cereals [51, 52]. However, buckwheat seeds can be classified as a cereal due to their chemical and usage properties, similar to cereal grains [53]. It is also known as a pseudocereal because of its differences from cereals [50, 53, 54]. The fundamental difference in terms of its structure is that buckwheat is dicotyledonous, as opposed to monocotyledonous species of cereal. The buckwheat seed’s embryo is found within the endosperm’s center, having two cotyledons. The embryo, endosperm, and seed coat are tightly surrounded by the hull (also known as the pericarp), which is hard and fibrous. Meanwhile, the walls of the cells that form the endosperm are quite thin [50, 55, 56].
Although numerous species of buckwheat are harvested globally, only 9 of them possess value for agricultural purposes [57–59]. There are varieties of buckwheat that can be harvested in summer, in autumn, and in the middle of the two seasons [53]. In general, two species are utilized globally: that known as common buckwheat (F. esculentum) and that known as Tartary buckwheat (F. tataricum). While the former is grown in more widely spread and diverse environments, the latter is farmed in mountainous areas [57–59]. Buckwheat seeds are triangular and have blackcolored hulls that cover the kernels, with the kernels ranging in color from white to light shades of green. Color saturation opens towards the kernel’s innermost layers. The density of the hull is lower than that of water; therefore, hulls can be removed more easily from kernels [57, 59]. Hull hardness varies among different species. F. esculentum is generally less hard than F. tataricum. F. esculentum has a harsher flavor, while F. tataricum is slightly bitter. The bitter compounds in F. tataricum seeds can be extracted by assorted techniques, such as various chemical methods and isoelectric precipitation [57, 60].
Buckwheat contains relatively high amounts of dietary fiber, protein, some types of vitamins, and some types of minerals in comparison to other types of wheat or rice [53]. Protein levels of buckwheat range from 12% to 18.9% [61, 62]. The proteins found in buckwheat possess well-balanced amino acids and generally have high biological value [47, 61]. Buckwheat flour has lower ratios of glutelin and prolamin but higher ratios of globulin and albumin [53, 63, 64]. Gluten content is either very little or nonexistent [53, 65]. Thus, buckwheat plays an important dietary role as a gluten-free option that is suitable for people afflicted with celiac disease [53]. Starch levels are in the range of 59%–70% while lipid levels range from 1.5% to 4%. More than 40% of the 80% unsaturated fatty acids in its structure are polyunsaturated fatty acids (PUFA) [47, 59]. Multiple studies have reported that buckwheat’s total polyphenol content is 6.8–20.7 mg GA/g, while free polyphenol content is 4.5–17.1 mg GA/g, insoluble dietary fiber content is 2.3%–8.6%, soluble dietary fiber content is 1.4%–3.4%, and finally, total dietary fiber content is 3.6%–10.6%
