Abstract: Oat β-glucan is an unbranched linear polysaccharide derived from the cell wall of oat aleurone, subaleurone and endosperm tissue. It belongs to water-soluble fiber and is an important active ingredient in oat. Oat β-glucan, as a natural food additive, is widely used in the food and pharmaceutical industries. This article summarizes the physiological functions of oat β-glucan, such as anti-diabetes, lowering cholesterol, enhancing immunity, anti-cancer, etc. Applications in the food industry such as food and beverages. The current research problems and solutions of oat β-glucan were prospected, in order to provide theoretical support for the further development and application of oat β-glucan.
Oats (Avena sativa L.) are widely cultivated around the world with a history of more than 2000 years. Oats are a crop rich in several bioactive substances, superior in nutritional value to many other cereals (barley, corn, millet, sorghum, etc.). Oats are usually eaten as grains, which can provide important nutrients such as protein, unsaturated fatty acids, soluble dietary fiber, vitamins and minerals. A large number of experimental and clinical studies have shown that eating oat products plays a positive role in reducing serum cholesterol levels, glucose uptake, and plasma insulin response. Oats will synthesize a variety of secondary metabolites during the ripening process, thus making oats contain a large number of biologically active substances. Oats have become an excellent source of active ingredients such as phenolic acids, flavonoids, carotenoids, vitamin E, and phytosterols, and contain two unique types of bioactive substances: avenantia alkaloids (AVA) and steroidal saponins. Plus, oats are a good source of soluble dietary fiber, especially beta-glucan. β-glucan has significant physiologically active functions and nutritional properties, such as unique cholesterol-lowering and anti-diabetic effects. In addition, the main active ingredient in oat soluble dietary fiber is considered to be β-glucan. Currently, the evidence on the beneficial physiological functions of oat beta-glucan has been reviewed and accepted by the U.S. Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA). According to existing research reports, oat β-glucan can also interact with the microbiota on cardiovascular disease, type I diabetes, cancer, Alzheimer's disease, hypertension, obesity, allergies, autism, positively impact major health or chronic conditions such as fibromyalgia and pancreatitis. Oat β-glucan has good water solubility, viscosity and gelling properties, and is widely used in food, such as breakfast cereals, beverages, bakery products and meat products. This article summarizes the current research on the functional properties of oat β-glucan and its application in food.
Oat β-glucan is mainly distributed in the cell wall of aleurone, sub-aleurone and endosperm tissue. It consists of glucopyranose units through β-(1→3) and β-(1→4) Linked by glycosidic bonds. The dry weight of β-glucan in oat bran is about 4%, composed of 70% β-(1→4) and 30% β-(1→3) glucosidic linkages, the basic units in the polymer chain are composed of trimers and tetramers, and their material ratio in oat is 1.5:2.3. According to NMR data and methylation analysis, the relative molecular mass of β-glucan in oats is between 0. 35×105 ~ 29.6×105. During the past 20–30 years, it has been recognized that the random distribution of β-(1→3) is the main reason for the functional properties of β-glucans. The molar mass of soluble oat β-glucan is about 5×105 g/mol, while that of insoluble β-glucan is less than 2×105 g/mol. The latest research uses computer technology to create a 3D model of β-glucan molecules. In form, β-glucan molecules are slender and winding chains with a distance of 41.35 Å. The rigidity of the molecular chain increases with the ratio of trisaccharides to tetrasaccharides. And oat β-glucan with higher ratio (tetrasaccharide/trisaccharide) has higher solution viscosity. The research trend shows that the fine structure of oat β-glucan will be one of the key researches in the future.
Studies have shown that the physiological function of β-glucan mainly depends on its water solubility and viscoelastic properties. Under the condition of low concentration of β-glucan, its viscosity is relatively high. When its mass concentration is higher than 2g/L, its viscosity decreases with the increase of shear rate, showing pseudoplasticity. β-glucan also has high water retention capacity and gel performance. When β-glucan dissolves in water, it can form a viscous solution, and its special rheological properties are closely related to its physiological properties. Shen Ruiling et al. compared the rheological properties of oat β-glucan with two molecular weights. The results showed that oat β-glucan solution is a non-Newtonian fluid. When the solution mass fraction is in the range of 0.2%~3%, its viscosity increases with shear decrease with increasing speed. Wang et al. reported that the viscosity of oat β-glucan solution decreased with the increase of shear rate, which is a typical non-Newtonian fluid. Studies by Johansson et al. have shown that oat β-glucan has a higher viscosity in the same concentration of oat β-glucan and barley β-glucan, indicating that the fine structure of the two is different. Studies have shown that when the ratio of three and four oxygen groups is the same, the viscosity of oat β-glucan at the same concentration is 100 times that of barley β-glucan, which may be due to the difference in structural properties (molar mass) and fine structure. In summary, oat β-glucan solution is a non-Newtonian fluid, and its concentration, molecular mass and temperature will all affect it. Due to differences in structural properties, oat β-glucans are more viscous than other cereal β-glucans, but the reasons for their viscosity are still debated. Therefore, the influence of the fine structure of oat β-glucan on its viscoelasticity and functional properties will be one of the focuses of future research.
Oat β-glucan plays an important role in promoting health and preventing diseases, normalizing intestinal flora, preventing diabetes, lowering cholesterol and blood pressure, controlling postprandial blood sugar and weakening insulin response, reducing the risk of cardiovascular disease and regulating appetite has a positive effect and alleviates complications of diabetes. Oat β-glucan improves postprandial satiety, possibly due to the non-digestible properties of β-glucan in the gastrointestinal tract. In addition, studies have shown that oat beta-glucan plays an important role in reducing belly fat and obesity, mainly in reducing body weight, body mass index, body fat and waist-to-hip ratio. The latest research shows that consumption of oat beta-glucan products can increase endurance and improve recovery from fatigue of rats.
It is beneficial to the health of the stomach and intestines. In the past 10 years, the research on intestinal and gastrointestinal microbiota has reached a new height, which has attracted widespread attention of food science researchers. The microbial flora in the body can help the host resist various external adverse factors and provide important protection. Clinical trials have confirmed that the bacterial flora in the stomach and intestinal tract can cure a variety of pathological conditions. Changes in the microbial community have a significant impact on the physiology and function of the host, and β-glucan, as an important component of prebiotics, can have a positive impact on the stomach and intestinal microbiota. Brennan et al. reported that oat β-glucan can form a gel-like network in the human body and change the viscosity of gastric and intestinal juices. In addition, oat β-glucan also has a positive effect on the intestines and stomach. If it cannot be digested by digestive enzymes (saliva amylase, etc.), it can well inhibit the growth of mucosal food-loving bacteria in the intestines and stomach. Oat β-glucan can also increase the number of microorganisms and protect the liver by improving the breeding conditions of beneficial bacteria (such as lactic acid bacteria and bifidobacteria); In the large intestine, oat β-glucan is fermented by microorganisms, especially by lactic acid bacteria and bifidobacteria in the cecum, resulting in the production of bifidobacteria that are beneficial to human health. Studies by Shimotoyodome et al. and Hedemann et al. have shown that oat β-glucan can promote the growth of rat colonic mucosa and play an important role in the intestinal tract. The weaned pig experiment of Metzler-Zebeli et al. showed that oat β-glucan can change the expression of some genes by increasing the production of short-chain fatty acids. Therefore, oat β-glucan can be used as a prebiotic to promote human health. O'SHEA et al. found that the mixed-linked β-(1→3)/β-(1→4) β-glucan in oat β-glucan may be the fermentation substrate of symbiotic bacteria, oat β-glucan The consumption of stimuli stimulates the growth of more commensal gastric and intestinal microbiota. Oat β-glucan also increases insulin sensitivity index, consumption of whole oat flour or oat β-glucan increases intestinal Na+K+-ATP, Ca2+Mg2+-ATP enzyme activity and energy charge, especially in ileum. However, the mechanism by which oat β-glucan affects the microbial populations in the stomach and intestine is not yet clear, which deserves further study.
Anti-diabetes, diabetes is generally characterized by high blood sugar, polydipsia, polyphagia, polyuria, weight loss and other typical symptoms. Current studies have shown that oat β-glucan can lower cholesterol and triglycerides and maintain blood sugar levels stable. Therefore, oat β-glucan has been extensively studied in controlling diabetes. Tappy et al showed through clinical trials that the hypoglycemic effect of oats and oat bran is mainly attributed to oat β-glucan. Abbasi et al. confirmed that oat β-glucan can reduce postprandial blood glucose concentration, and can regulate the activity of intestinal glucose transporter, which provides an effective way to reduce blood glucose level in diabetic patients. Biörklund et al. found that beverages rich in β-glucan had a positive effect on glucose and insulin levels, and intake of 5 g oat β-glucan improved insulin levels and kept glucose levels stable. Hooda et al. found that adding 6% oat β-glucan concentrate to the diet can significantly reduce the glucose level and increase the short-chain fatty acid and insulin levels in pigs. These changes are related to gastric inhibitory peptide and GLP-1. Alminger et al. studied the changes of microbial flora and found that the ratio of Firmicutes/Bacteroidetes increased. In vitro studies showed that oat supplementation can increase the number of Bacteroidetes, and after β-glucan is digested by the microbiota, it Increased production of propionate and butyrate，oat beta-glucan-enriched products are more effective in reducing glucose and insulin responses than low-fibre products. Shen et al. fed diabetic mice with oat β-glucan for 6 weeks and found that oat β-glucan significantly reduced fasting blood glucose and glycated serum protein levels, increased glycogen levels, decreased free fatty acids and inhibited pancreatic apoptosis. Rumberger et al. found that oat beta-glucan produced more butyrate than other fibers, suggesting oat beta-glucan as a potential drug for treating diabetes. Wang et al. found that β-glucan solutions with higher viscosity had better hypoglycemic ability. Studies have also shown that the intake of foods rich in oat β-glucan can reduce the blood sugar level of the human body, especially in patients with diabetes, which may be due to the fact that β-glucan can increase the viscosity of chyme, thereby delaying the intestinal absorption of glucose. Turnbaugh et al found that oat β-glucan positively affects obese mice by increasing microbiota genes encoding dietary polysaccharide-decomposing enzymes in obese mice. In summary, the mechanism of action of oat β-glucan on the enzymes encoding dietary fiber polysaccharides is one of the focuses of future research on the anti-diabetic mechanism of oat β-glucan.
Lowers cholesterol, oat beta-glucan is known for its cholesterol-lowering effects, the US Food and Drug Administration (FDA) and the European Food Safety Authority (EFSA) have shown that 3 g of oat or barley beta-glucan per day is beneficial to lower blood cholesterol levels and reduce the risk of coronary heart disease. Studies by Whitehead and others have shown that daily intake of at least 3 g of oat β-glucan can reduce plasma total cholesterol and low-density lipoprotein cholesterol levels in patients with normal or high cholesterol by 5% to 10%, and a dose greater than 3 g/d can reduce total blood cholesterol levels. and LDL cholesterol levels were reduced by 0.25 mmol/L and 0.30 mmol/L, respectively, without changing HDL cholesterol or triglycerides. Analysis by Ho et al. showed that oat β-glucan has a lowering effect on LDL-cholesterol and non-HDL-cholesterol, therefore, foods rich in β-glucan may have the potential to reduce the risk of cardiovascular disease. Wolever et al. observed that the physical and chemical properties of oat β-glucan at the molecular scale have a positive impact on blood cholesterol levels, especially in relation to the weight-average molecular mass, solubility, and viscosity of β-glucan. At present, the mechanism of oat β-glucan lowering blood cholesterol is still unclear, but the ability of oat β-glucan to interfere with the recycling of bile salts and the mechanism of influence on cholesterol metabolism are relatively authoritative results. Theuwissen et al. showed that the main mechanism of oat β-glucan to produce short-chain fatty acids (propionate) through the gut microbiota is to lower cholesterol, and the gut microbiota metabolizes fiber and produces short-chain fatty acids to the host, an increase in the ratio of propionate to acetate (the main substrate for cholesterol biosynthesis) resulted in a decrease in cholesterol biosynthesis. There are many studies on oat β-glucan lowering cholesterol. However, the mechanism of its influence on the ability to interfere with bile salt circulation is still unclear, and whether the physical and chemical properties of oat β-glucan, such as particle size, solubility, conformation, etc., can also affect cholesterol metabolism is also one of the important research directions in the future.
Immunological effects, studies have shown that polysaccharides can enhance immune ability, which can provide a broad field for the research of new safe and tolerable immune adjuvants. Among the studied polysaccharides, β-glucan has the most application potential. Oat β-glucan is considered to be a potent immunostimulant capable of binding to the surface receptors of immune cells (monocytes, granulocytes and NK cells), activating and modulating humoral and cellular immunity, thereby stimulating immunity reaction. Oat β-glucan may function through 3 immunostimulatory mechanisms, modulating the activity of macrophages, T lymphocytes and the complement system, due to these properties, oat beta glucan can regulate natural immune responses and enhance fitness by stimulating the activity of T lymphocytes. Studies in animal models have shown that dietary supplementation with oat beta-glucan can activate T cells expressing CD8 and TCR1 surface molecules, thereby enhancing the immune system. Jin et al. found that oat β-glucan can regulate the immune response when applied alone, and can connect innate immunity and adaptive immunity to improve the immunogenicity of the vaccine. When oat β-glucan is used as an immune stimulant or immune adjuvant, depeptidase, CR3, CD5, lactyl ceramide, etc. can recognize receptors of oat β-glucan. Yun et al found that β-glucan can effectively change the number of cells (Thy1.2, CD4 and CD8 cells) in mouse mesenteric lymph nodes and Peyer's patches. Through oral or parenteral oat β-glucan injection It can enhance the resistance of mice to Staphylococcus aureus or pest infection, and enhance the resistance of mice to Staphylococcus aureus or Escherichia coli infection by oral or parenteral treatment of oat β-glucan. Udayangani et al. also found that β-glucan has immunomodulatory properties, and the survival rate of zebrafish (zebrafish attacked by pathogenic bacteria) administered with nano-β-glucan was significantly increased. Sahasrabudhe et al. showed that enzymatic pre-digestion of oat β-glucan enhanced its effects on specific immune receptors, effective functional feed and food additives can be designed by controlling the particle size, molecular mass, and receptor-specific binding sites of oat β-glucan. Studies by Rösch et al. have shown that oat β-glucan molecular specificity plays an important role in immune regulation, such as characteristics such as oat β-glucan insolubility, particle size, particle conformation, and particle uniformity. It can be seen that the specific binding site of oat β-glucan receptor will be one of the key researches in the direction of immune function of oat β-glucan in the future.
Anti-cancer, recent studies have found that many herbal medicines rich in complex polysaccharides have good anti-cancer effects. These complex polysaccharides such as β-glucan, a non-cellulose polymer, the glycoside position is at β-(1-3), β-glucan on β-(1-4) or β-(1-6). The anticancer effect of β-glucan has been proved to be related to the complexity of its structure, and some results show that the immunomodulatory and anticancer functions of β-glucan are closely related to its structure, molecular mass, branching degree and conformation. At present, there is no clear evidence that β-glucan can be effectively used as an anticancer agent, however, many studies have elucidated its effect on cancer cells in vitro and in vivo. Choromanska et al. tested 3 cell lines (human lung adenocarcinoma, human multidrug-resistant small cell lung cancer and normal human keratinocytes) and found that oat β-glucan has strong anti-tumor properties, meanwhile, it is non-toxic to normal cells. Cheung et al. and Demir et al. found that oat β-glucan plays an important role in immune activation in killing cancer cells. In addition, studies have shown that oat β-glucan has a positive effect on the health status of rats with LPS injection-induced enteritis. Blaszczyk et al. concluded that oat β-glucan played a protective role in LPS-induced inflammatory rats through changes in gene expression, and pointed out that the mode of action of high-molecular-weight and low-molecular-weight β-glucan in vivo is different. The molecular weight of oat beta glucan determines how it works in the body. Studies have shown that high-molecular-weight oat β-glucan is more effective in reducing LPS-induced oxidative stress in colonic tissues and enteritis in stomach, liver, or spleen, and dietary supplementation of low-molecular-weight oat β-glucan can improve the morphology of colon tissue in healthy control rats and LPS-induced enteritis animals. The influence of the physical properties of oat β-glucan on the immune activation of cancer cells will be one of the focuses of future research on the functional properties of oat β-glucan.
β-glucan has multiple functions such as thickening, stabilizing, emulsifying, and gelling. In recent years, studies have found that obese people have a higher risk of coronary heart disease, hypertension, stroke, diabetes and cancer due to excessive intake of fat, and functional foods are a safe way to treat various diseases. Therefore, in Oat β-glucan has received extensive attention in the field of food research. In the preparation of meat, baked goods, sauce soup, beverages and other foods, it mainly uses the emulsification, thickening, stability and gelling properties of oat β-glucan to prepare functional foods. The application of oat β-glucan in several food industries is introduced in detail below.
In meat food, the functional food market has grown since it was first launched in the mid-1980s as people become more aware of healthy eating, meat has benefited from this and one way to develop functional foods is by using prebiotics etc. Prebiotic substances enhance the activity of probiotics in the gut, and accumulating scientific evidence shows that oat beta-glucan and microbial exopolysaccharides have positive effects on human health. Studies such as Wollowski have shown that a larger meat diet can increase the risk of colon cancer, while prebiotics and probiotics have anti-cancer effects, which can reduce DNA damage in colon cells, reduce the activity of cancer-promoting enzymes, protects by blocking mutagen binding and increasing immune stimulation. Amini et al. showed that oat β-glucan had a significant effect on the physical and sensory properties of sausages, and a combination of oat β-glucan and resistant starch could be used to produce probiotic sausages. Afshari et al. added β-glucan to hamburger patties, which improved their cooking rate, moisture retention rate and acceptability, moldability, etc. Thereafter, oat β-glucan can be added to various meat products as a prebiotic ingredient to improve their product quality and enrich meat functional foods.
In baked foods, adding oat β-glucan in the preparation of bread and cakes can improve the physical and chemical properties of bread, and adding oat β-glucan in pasta foods can reduce the glycemic index and effectively fight against metabolic diseases. Adding oat β-glucan to bread and cakes can delay the release of glucose and prevent the occurrence of hyperglycemia. Ekström et al. found oat β-glucan to be suitable for baking because of its high molecular mass and its ability to regulate the glycemic profile of bread products. Compared with barley β-glucan, oat β-glucan has better rheological properties and can produce higher quality bread, which may be due to the higher molecular weight of oat β-glucan Viscous, therefore, oat β-glucan can be added to baked products to improve product quality and enrich functional baked products.
In beverages, oat β-glucan can be used not only in cereal foods, but also in low-fat ice cream, yogurt, beverages and other foods. Studies have shown that the addition of oat β-glucan can promote the growth and vitality of Lactobacillus in yogurt. Rezaei et al. showed that adding oat β-glucan to frozen yogurt would increase the viscosity, expansion, firmness and stability of yogurt, and prolong the aging time at low temperature, which could adjust the texture properties of frozen soybean yogurt, thereby improving the quality of this frozen desserts. Ladjevardi et al. increased the activity of probiotics by adding oat β-glucan, reduced the fat content of yogurt and improved the quality of yogurt. Mahrous et al. showed that the addition of oat β-glucan had no significant effect on the chemical composition of stirred and concentrated yogurt. Sharafbafi et al. added high molecular weight oat β-glucan to milk to prepare dairy products with low calorie and low cholesterol content. Rinaldi et al. found that yogurt containing β-glucan and pectin had faster protein breakdown rate, faster release of peptides and higher ratio of free amino acids than yogurt containing starch and β-glucan. Lyly et al. and Mielby et al. found that the addition of oat β-glucan reduced the saltiness and sharpness of tomato soup and the acidity of fruit drinks, but had no significant effect on the aftertaste perception of drinks. Brennan et al. believed that the addition of oat β-glucan can also be used to control the blood sugar response of puffed snack products. Therefore, oat β-glucan has broad application prospects in the beverage industry.
As a soluble dietary fiber, oat β-glucan has rich functional properties and can be widely used in the fields of functional food and biomedicine. This paper reviews the development and utilization of oat β-glucan resources, in order to further expand the application value of oat β-glucan. The current focus is on oat beta-glucans improving the digestive properties of the gastrointestinal tract, although it is well established that beta-glucans have a significant impact on the microbiota and have a positive effect on gut health. However, its exact mechanism of action and the target of oat β-glucan on the microbiota are still unclear, which deserves further study. In addition, the current research mainly focuses on the single raw material of oat β-glucan, and whether the raw material rich in oat β-glucan and oat β-glucan-ligand complex also have similar properties to oat β-glucan, and its mechanism of action is also an important scientific issue worthy of study. Finally, the relationship between the molecular structure of oat β-glucan (such as molecular weight, unit composition and particle size, etc.) and the physical and chemical properties and functional properties should be systematically studied, especially the molecular mechanism of enhancing the immune system and anti-cancer, so that oat β-glucan can better serve human beings.
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