Nutritional and Phytochemical Composition and Associated Health Benefits of Oat (Avena sativa) Grains and Oat-Based Fermented Food Products

Oats (Avena sativa L.) are a popular functional cereal grain due to their numerous health benefits. This review article summarized the information on the chemical composition and phytonutrients of oats grown in different countries. It also reviewed recently developed fermented oat products to highlight their potential for human health. Oats have an interesting nutritional profile that includes high-quality protein, unsaturated fats, soluble fiber, polyphenolic compounds, and micronutrients. Oat grain has a unique protein composition, with globulins serving as the primary storage protein, in contrast to other cereals, where prolamins are the main storage proteins. Oats have the highest fat content of any cereal, with low saturated fatty acids and high essential unsaturated fatty acid content, which can help reduce the risk of cardiovascular diseases. Oats are a good source of soluble dietary fiber, particularly β-glucan, which has outstanding functional properties and is extremely important in human nutrition. β-Glucan has been shown to lower blood cholesterol and glucose absorption in the intestine, thereby preventing diseases such as cardiovascular injury, dyslipidemia, hypertension, inflammatory state, and type 2 diabetes. Oats also contain high concentration of antioxidant compounds. Avenanthramides, which are unique to oats, are powerful antioxidants with high antioxidative activity in humans. Recognizing the nutritional benefits of oats, oat-based fermented food products are gaining popularity as functional foods with high probiotic potential.


Introduction
Oat (Avena sativa) is a cereal grain from the Poaceae family cultivated for cattle feed (70%) and human consumption (30%) [1]. Some studies suggest that oats can be traced back to around 2000 BC. [2]. However, the exact origins of the various Avena spp. are unknown. After many years of growing wheat and barley, oats became known and cultivated [3]. Te genus Avena contains a polyploid collection of wild, weedy, and cultivated species [4]. Avena sativa is a hexaploid species that is the world's most widely grown and popular oat cultivar today [5].
Oats' nutritional composition difers signifcantly from that of other cereals, with high protein content and an ample amount of essential amino acids [6,7]. Oats have a higher fat content (6-10%) than wheat and most other cereals (2-3%) [8]. It had the highest fat content of any cereal, with a high percentage of unsaturated fats [9]. Te high nutritional value of oats is also due to their high β-glucan content [10]. Betaglucan is a vital functional component in various food industries [11]. Furthermore, oats possess more than 20 unique polyphenolic compounds known as avenanthramides [12]. Te antioxidant activity of avenanthramides is 10 to 30 times higher than that of other cereals' polyphenolic compounds such as ferulic acid, gentisic acid, p-hydroxybenzoic acid, protocatechuic acid, syringic acid, vanillic acid, and vanillin [13].
Oats are used for food in various forms, including whole grains, rolled oats, crushed oatmeal, and oat four [14,15]. Oats are best known as a breakfast cereal food, whether eaten whole or in the form of rolled oats. Oatmeal is primarily used for porridge and to prepare several baked goods such as oatcakes, oatmeal cookies, and oat bread. Several novel products using oats have been developed [16][17][18].
Oat-based foods have recently gained popularity due to their health benefts. Consumption of oat products has been associated with a reduced serum cholesterol level, a lower risk of cardiovascular disease (CVD), and a lower risk of obesity, hypertension, cancer, diabetes, and gastrointestinal disorders [19]. Te European Food and Safety Authority and the US Food and Drug Administration (FDA) have accredited health claims for oat foods containing oat β-glucan to lower serum cholesterol and the risk of CVD [20]. Furthermore, the potential benefts of oats have been related to various other bioactive components in addition to β-glucan. Avenanthramides (AVAs), a unique oat antioxidant, help prevent free radicals from damaging low-density lipoprotein (LDL) cholesterol, and AVA-enriched oat extract combined with vitamin C inhibits LDL oxidation synergistically. AVAs have anti-infammatory, antiproliferative, vasodilation, anti-itch, cytoprotective, and anticancer properties [21][22][23]. Te purpose of this article is to review the nutritional potential of oat grains and oatbased fermented food products.

Carbohydrate, Dietary
Fiber, and β-Glucan. Oats contain fewer carbohydrates but more protein and lipids than other cereals [24]. However, starch remains the most abundant component like in other cereal grains, comprising approximately 60% of oat grains [25]. Amylose and amylopectin make up 98-99% of the carbohydrate constituents of oat starch granules. Oat starch has diferent characteristics such as short amylose, relatively high crystallinity, and a well-developed and small granule surface [26]. Tese special characteristics of oat starch make it unique from other cereal starches.
Oats have a well-balanced profle of soluble and insoluble dietary fbers [6]. Dietary fbers, also known as roughage, are edible plant parts that are essential components of human nutrition. Dietary fber enters the large intestine and is partially or completely fermented by gut bacteria [27]. Fermentation produces various types of byproducts, including gases and short-chain fatty acids. Te combined action of the fermentation process and products contribute to the benefcial efects of dietary fber on health [28]. Te proximate composition of oats is summarized in Table 1.
Oats were frst found to have cholesterol-lowering properties in 1997, and the active ingredient was identifed as β-glucans [59]. After reviewing 42 clinical trials, the FDA acknowledged the cholesterol-lowering features of oats [60]. Te recommended intake for a cholesterol-lowering efect is 3 g of oat β-glucan per day [19]. According to recent research, doses of 3-13 g/day resulted in total cholesterol reduction of 8.2-15.1 mg/dL and LDL reduction of 7.8-13.2 mg/dL [61]. Tese changes may appear insignifcant compared with those obtained through drug therapy. However, a 1% reduction in blood cholesterol can reduce the risk by 2-4% [62].
In general, scientifc studies have shown that eating oats, like other fber-rich cereal grains, helps lose weight, lowers blood cholesterol levels, and improves postprandial glycemic and insulinemic responses in noninsulin-dependent diabetes mellitus and healthy subjects. Oatmeal reduces the risk of colon cancer, regulates blood pressure, and prevents cardiovascular disease. Oat-based foods also strengthen the immune system's defenses against parasites, bacteria, fungi, and viruses [63] (Figure 1).

Protein and Amino Acid Profle of Oats.
Oat grain has high protein content and a distinctive protein composition [64,65]. Most cereals (including barley, wheat, and rye) rely heavily on prolamins as their main storage proteins, but oats are an exceptional case. Te main storage proteins in oats are globulins (which are salt-water-soluble and account for roughly 55% of the total Osborn protein solubility classifcation), with prolamins accounting for a minor percentage [66]. Avenins also serve as protein storage for oats, accounting for 10 to 13% of the total protein content [44]. Te oat protein consists of more limiting amino acids such as glutamine, lysine, and threonine and less proline compared to other cereal grains [44]. Te protein content of oat groats ranges from 12.4 to 24.5% [66]. Te embryonic axis and the scutellum contain greater quantities of amino acids than other parts of the kernel.
Enzymes are the most essential metabolically active proteins in the oats. Oats, like other cereal grains, contain a lot of enzymes. Previous research identifed maltase, proteases, phenoxyacetylase, hydroxylase, α-amylase, lichenase, tyrosinase, phosphatase, and lipase as oat enzymes [67]. Table 3 shows the amino acid composition of oats from various sources.

Crude Fat and Fatty Acid Composition of Oats.
Oats have the highest fat content of any cereal [44]. Tey are high in linoleic acid and low in saturated fat, which can help reduce the risk of heart and vascular diseases [70]. Monounsaturated fatty acids (MUFA, C18 : 1) and polyunsaturated fatty acids (PUFA, C18 : 2) are the most abundant fatty acids in oats, followed by saturated fatty acids (C16 : 0) [44]. Triglycerides also constitute the main component of lipids and phospholipids; glycolipids and sterols are also present in considerable amounts [67]. Te high lipid content makes them a valuable functional food ingredient in a wide range of industries [9]. According to Van Den Broeck et al. [44], the fatty acid content in mg/100 g of the oat four sample was 193.5-292.9 (C16 : 0), 11.5-33.3 (C18 : 0), 385.0-718.0 (C18 : 1), 532.3-748.9 (C18 : 2), and 12.3-16.1 (C18 : 3). Te relative proportion of fatty acids of oat grains is summarized in Table 4.

Micronutrients of Oats.
Micronutrients are minerals and vitamins that the body needs in minute amounts. On the other hand, they cannot be compromised, and defciencies in any of them can result in life-threatening conditions [74]. Vitamins and minerals are essential for proper metabolism and tissue maintenance Micronutrients must be obtained through the diet because the body does not produce them. To achieve the best results from the diet, a proper balance of micronutrients and macronutrients is required [75].
Minerals are classifed into two types: major and minor. Sodium, potassium, magnesium, calcium, phosphorus, chlorine, and sulfur are examples of major minerals with daily requirements greater than 100 mg for adults [76]. On the other hand, minor or trace elements are minerals with daily requirements of less than 100 mg, such as iron, zinc, copper, chromium, cobalt, molybdenum, selenium, nickel, manganese, fuorine, iodine, silicon, tin, and vanadium [76]. As with other cereal grains, the mineral content of oats ranged from 2 to 3%. According to Bhardwaj et al. [56], the iron and zinc contents of 43 oats ranged from 1.8 to 6.8 mg/ 100 g and 6.5 to 10.2 mg/100 g, respectively. Butt et al. [6] reported that the mineral content of oats per 100 g of four sample was 60 mg of calcium, 372 mg of phosphorus, 3.8 mg of iron, and 3.9 mg of zinc. Table 5 updates the mineral content of the oat grains.
Oats' micronutrients include not only trace minerals but also vitamins, which are organic compounds required in microgram or milligram amounts. Oat grains contain signifcant amounts (mg/100 g) of vitamins such as thiamine

Bioactive Components and Health Benefits of Oats
Oat has gained popularity as a healthy food in recent years due to its high content of bioactive compounds that can beneft human health, such as β-glucan, avenanthramides, tocols, sterols, and avenacosides [20]. Tese compounds help   [73] "-" represents the values that the authors did not report. "-" represents the values that the authors did not report. prevent gastrointestinal disorders, type 2 diabetes (T2DM), CVD, and cancer [24]. In human cells, radicals are formed from normal metabolism and environmental radiation. Tese radicals can cause deoxyribonucleic acid (DNA) changes, which may produce cancerous cells or diseases like atherosclerosis [79,80]. Free radicals are also known to oxidize LDL cholesterol, contributing to heart disease and stroke [81]. Tus, free radicals are key causal factors in many chronic diseases. Te human body has a natural defense system against these reactions, but dietary antioxidants also contribute to body defense [82]. Some phenolic compounds found in oats have free radical scavenging activities with potential health-benefcial properties [83].
Anthranilic acid amides aid in the prevention of free radical damage to LDL cholesterol [84]. Anthranilic acid amide-enriched oat extract combined with vitamin C inhibits LDL oxidation synergistically in vitro [84,85]. Oats contain a unique group of approximately 40 diferent anthranilic acid amides consisting of anthranilic acid derivatives and hydroxycinnamic acid derivatives [84,86]. Oat antioxidants have been shown in animal and human studies to reduce CVD risk by inhibiting LDL cholesterol oxidation and peroxidation and reducing serum cholesterol [84]. As a result, eating oats and other foods is recommended as part of a healthy diet.

Phenolic Compounds of Oats.
Te importance of phenolics stems primarily from their high antioxidant capacity and health benefts. Oat products have recently gained popularity as bioactive ingredients for industries such as pharmaceuticals, food, and cosmetics [87]. Oats' primary antioxidants are polyphenolic compounds, favonoids, and sterols [24]. Te total phenolic content (TPC) and total favonoid content (TFC) of oats are summarized in Table 6.
Phenolics serve as potent antioxidants by scavenging reactive oxygen and nitrogen species and chelating transition minerals [84]. Oats contain 2.3 mg/100 g of tocopherols [94] and 12.4 to 586.6 mg/kg of total avenanthramides [95]. Te total avenanthramide content varies depending on the milling fractions. Hitayezu et al. [96] reported 323.7 to 775.5 μg/g of avenanthramides in diferent oat milling fractions such as medium bran, fne bran, low bran, and whole oat groats four. Avenanthramides are unique to oat grains and are not found in other cereal grains [85,97]. Pioneering work in the identifcation of avenanthramide structures has been performed by Collins [98], Collins [86], and Collins et al. [99]. Avenanthramides have been reported to improve health parameters in animal and human studies. Avenanthramides have antioxidant, antiproliferative, antiatherogenic, and anti-infammatory properties [100,101]. Table 7 presents the total avenanthramides content in dehulled oat from diferent geographies.
Avenanthramides are heat-stable under commercial processing conditions [109,110]. Magee et al. [111] developed an anti-infammatory oat-based product that contains a sufcient amount of avenanthramide (0.05-100.00 ppm) combined with hydrocortisone (0.1-1.0% w/w, based on the total weight of the composition). Tis patent was received under the category "Compositions for inhibiting or reducing skin infammation." Yang et al. [112] conducted an extensive study on the antioxidant capacity of avenanthramides (AVAs). Te report showed that the antioxidant activity of AVA was 10-30 times higher than that of antioxidants of typical cereal grain components such as ferulic acid, gentisic acid, p-hydroxybenzoic acid, protocatechuic acid, syringic acid, vanillic acid, and vanillin. Flavonoids are a type of polyphenolic unit that has a C6-C3-C6 skeleton and contains over 4,000 phenolic complexes. Flavonols, favones, favanols, favanones, favans, and anthocyanins are the most common favonoids [113]. Collins [114] identifed the major favones present in oat four as apigenin, luteolin, and tricin. Table 6 summarizes the TFC of oats. Flavonoids have generated interest because of their broad human health-promoting efects, which are related to their antioxidant properties and synergistic efects with other antioxidants and metals [115]. Te synergistic efects of the antioxidant properties increased after interaction with iron and copper [116].
Natural antioxidants obtained from oat extracts, particularly avenanthramides, are currently the focus of intense research and interest among food scientists and health professionals. Several studies reveal a positive correlation between antioxidant-rich oat-based foods and a reduced risk of diseases associated with oxidative stress, such as cancer, cardiovascular, and neurodegenerative diseases [12,23]. Saltzman et al. [117] compared the efects of an oat diet (oatmeal, oat mufn, spice cookies with oats, chocolate cookies with oats, custard with oats, and berry drink with oatmeal) with that of a control diet (cream of wheat, wheat mufn, spice cookies, chocolate cookies, custard, and berry drink with wheat bran) on cardiovascular risk and weight loss in 43 adults (men and women) for two weeks. Te fndings revealed that an oat-containing hypocaloric diet signifcantly lowers systolic blood pressure and promotes weight loss in both men and women. Ji et al. [118] reported that supplementing rats' diet with avenanthramide-enriched oat extract at 100 mg/kg diet (providing about 20 mg avenanthramides (Avns)/kg) increases superoxide dismutase (SOD) activity in skeletal muscle, liver, and kidneys, as well as glutathione peroxidase activity in heart and skeletal muscles. Table 8 reviews in vivo and in vitro studies on the positive health outcomes of oat extract and avenanthramides and how these properties were demonstrated. Many authors have reported multiple antioxidative and bioactive molecules of oats with a signifcant positive health outcome [22,85,92,96,105,124].

Oat-Based Processed Food Products
Te demand for oat-based products has increased in recent years as people have become more aware of the numerous nutritional and health benefts of oats [67]. In 1997, the Food and Drug Administration (FDA) ofcially recognized the importance of oat fber with at least 0.75 g of beta-glucan per serving size as a functional food [16]. Furthermore, subsequent studies on the health implications of oat constituents like avenanthramides have raised hopes that the nutritional benefts of oats in human diets may extend far beyond those currently recognized [67]. Tere are many nonfermented and fermented oat-based products available in the market. Oat is used to make oat-based breakfast cereals, oatmeal, fakes, porridge, granola bars, muesli, oat bread, cookies, biscuits, oatrim, oat milk, infant food, and an oat-based fermented probiotic drink [134]. Table 10 presents some recent fermented oat-based products.
Te incorporation of oats has been shown to improve the overall quality of food [134]. Sanchez-Pardo et al. [149] found that pound cake made with 25% (w/w) oat fber had better textural characteristics than the conventional product. Bread is an integral part of the daily diet of a large part of the world's population. According to Flander et al. [150], oatbased bread has a mild nutty and pleasant favor. Because oats retain moisture well, bread stays fresher for longer [151]. Adding oat starch or oat lecithin to wheat bread was found to slow the bread's staling rate [152]. Oat starches and their modifed products were organoleptically comparable to conventional ones used in pasta products [153]. Boukid [154] identifed oat proteins as an emerging ingredient for food formulation.
Khanna and Mohan [155] discussed the suitability of the incorporation of oats into Indian diets such as bread (chapattis and missi rotis), breakfast items (upma and poha), snacks (biscuits), and beverages (smoothies and shakes). With a partial substitution of up to 50% of oats, the modifed traditional products had acceptable sensory and textural characteristics. Chauhan et al. [151] also demonstrated that the incorporation of oat four in the preparation of valueadded functional foods such as bread and noodles was successful. Oats and their resistant starches make lowcalorie, low-fat, high-fber granola bars and cereals [26]. Breakfast cereals made with oats have received a lot of attention recently. Tese are high in functional ingredients such as β-glucan and bioactive components (high polyphenol and antioxidant content), which are known to lower serum and plasma cholesterol levels and postprandial glycemic response [156].
Te premix was made by mixing four of 2.8% w/w toasted black cardamom, 2.8% w/w malted wheat, 2.6% w/w pumpkin pulp, 1.1% w/w spiced chili pepper, and 0. 7% table   salt Spontaneous fermentation [148] 10 Te Scientifc World Journal which provides the elasticity and structure required for bread dough. As a result, most oat bread still contains wheat four to make the dough rise and have a pleasing appearance [159].

Conclusion
Tis review shows that oats are signifcant sources of valuable nutrients, particularly protein and fat, with a signifcant concentration of healthy mono-and polyunsaturated fatty acids and a balanced amino acid composition. Tey are a good source of minerals that our bodies require. Oat grains are also an important source of natural antioxidants, which beneft human health by reducing the risk of various diseases. Oats are now used in many functional food formulations around the world, and they are important component of a healthy diet. Terefore, with respect to their nutritional, medicinal, and therapeutic properties, oats are credited as a good plant food for the future and are recommended for a healthier world.

Data Availability
Te data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest.