What Is Grain Beta Glucan?

Cereals are an important part of the Eastern diet. Compared with refined grains, whole grains retain more bran and germ and are rich in nutrients, especially dietary fiber, micronutrients and phytochemicals such as polyphenols. There is much evidence to suggest that the consumption of whole grains can greatly improve a balanced diet and reduce the risk of chronic diseases such as type 2 diabetes, cardiovascular disease and colorectal cancer [1]. Therefore, encouraging the replacement of refined grains with whole grains is an important way to improve the nutrition of the population's diet. Dietary fiber is an important functional component of whole grain foods. Surveys have shown that dietary fiber from cereal sources has a greater effect on human health than dietary fiber from other sources, which is related to differences in structure [2]. β-glucan is an important dietary fiber component of cereals. It is found in the cell walls of endosperm and is a polysaccharide [3]. It is found in the highest amounts in barley (2.5%–11.3%) and oats (2.2%–7.8%), and also in smaller amounts in rye (1.2%–2.0%) and wheat (0.4%–1.4%) [4].

In recent years, with the public's increasing concern about nutrition and health, the consumption of whole grain foods has continued to increase. In particular, based on the health claims of the US FDA and the EU on the efficacy of β-glucan, the consumption of foods rich in β-glucan, such as oats and barley, has also increased year by year. The depth and breadth of research related to cereal β-glucan at home and abroad have been greatly expanded, and research has diversified from the extraction, isolation and purification methods of β-glucan; the impact of food processing and handling techniques on the structure and properties of β-glucan; the influence of food processing and handling techniques on the structure and properties of β-glucan; the interaction between β-glucan and substances such as proteins and lipids; the application of β-glucan in different types of food; and the research on the nutritional and health benefits of β-glucan. This thesis therefore provides a review of recent research progress in cereal β-glucan.

1 Extraction, preparation and purification of cereal β-glucan

Grain β-glucan is mainly found in the subaleurone layer and endosperm cell walls of grains. The properties and applications of grain β-glucan are mainly based on its molecular structure characteristics. Extraction conditions not only affect the β-glucan extraction rate, but also affect its molecular structure. Therefore, in recent years, a large number of literatures have reported methods for the extraction, separation and purification of grain β-glucan. A review of the literature [5-6] found that the current main methods for extracting cereal β-glucan are derived from the research of Wood et al. [7], and the basic steps are shown in Figure 1. Subsequently, on this basis, researchers have conducted in-depth studies on different cereal raw materials, preliminary extraction conditions, factors affecting yield and purity, etc. (see Table 1).

At present, the main raw materials used to extract cereal β-glucan include barley, barley, oats and oat bran. The extraction rate of β-glucan from barley and barley is relatively high, while the extraction rate is relatively low when oats and oat bran are used as raw materials, which is related to the distribution of β-glucan in cereals. Comparing different extraction methods, it was found that the β-glucan extraction rates of different methods ranged from about 50% to 87%, and the yields ranged from about 5% to 8.5%. The enzyme method had a relatively high extraction rate, while the microwave-assisted extraction method had a relatively high yield. In addition, Ahmada et al. [3] reported that the enzymatic extraction method yielded a β-glucan product with better stability and functional properties. However, extraction is a complex process that requires attention not only to yield, but also to functionality and product stability. Therefore, the extraction of β-glucan from cereals, especially its industrial preparation, requires a comprehensive consideration of technical and product quality indicators such as stability and energy consumption.

2 In-depth analysis of the functional properties of cereal β-glucan and its application in foods

In recent years, with the understanding of the health effects of cereal β-glucan and the deepening of research on its molecular properties, many researchers have paid more attention to the relationship between the structure and functional properties of β-glucan and its application prospects. Yang Chengjun et al. [20] reviewed the structure and physical properties of oat β-glucan, its nutritional properties and its application in the meat, bakery and beverage industries; Izydorczyk [21] reviewed the molecular structure, physicochemical properties and application of barley β-glucan in food.

β-glucan in the food industry mainly includes products such as bakery products, dairy products, beverages, meat products and snack foods (see Table 2). In recent years, research on the application of cereal β-glucan has been increasing. On the one hand, β-glucan is added to different foods to study the effect on the properties of food system components and food quality; on the other hand, based on the interaction between β-glucan and different molecules in the food system, the functional properties and application of β-glucan complex's functional properties and applications. To this end, this thesis takes the application of β-glucan in bakery products as an example to outline the effects of β-glucan addition on dough properties and food quality. At the same time, it also provides a review of the research and applications of β-glucan complexes.

2.1 Application of β-glucan in bakery products

Adding β-glucan to bakery products can increase the water-soluble dietary fibre content on the one hand, and affect the rheological properties, hydration characteristics and product texture of the dough on the other. Studies have shown that adding the right amount of oat β-glucan (OG) can improve the rheological properties of the dough. Adding 0.5% to 5.0% OG to low-gluten, medium-gluten and high-gluten flour and steamed bread flour, as the amount added increases, the water absorption rate, formation time and stability time of the dough all increase. Adding 0.5% to 1.0% OG can make the extensibility of low-gluten flour similar to that of steamed bread flour . OG can slightly increase the gelatinization temperature of medium-gluten flour, but it can also reduce the gelatinization temperature of steamed bread flour and the final viscosity, attenuation value and recovery value of the four types of flour [36]. Some studies have also shown that the addition of β-glucan has a deteriorating effect on the dough. When adding barley β-glucan (BG) ≥0.5%, the resistance of wheat dough to extension increases, and the dough formation time, stability time, weakening degree (value) and extensibility are all significantly reduced. When the amount of β-glucan added is ≥1.5%, the specific volume of wheat flour bread is significantly reduced, the hardness is increased, and the elasticity is reduced [37].

Beta-glucan also affects product quality by influencing the hydration characteristics of the dough. Studies have shown that the addition of OG to noodles and steamed buns can inhibit moisture migration and starch aging, reduce water loss and cooking loss [38-39]. Water-soluble dietary fiber containing 70% OG was added to wheat flour, and by optimizing the water content, bread with a texture similar to white bread and rich in soluble dietary fiber (SDF) can be obtained [40]. The effect of β-glucan on the hydration properties of dough is related to its fine structure, such as molecular size [41]. Skendi et al. [42] studied the effects of two different relative molecular masses (1.00×105 and 2.03×105) of BG on the rheology, viscoelasticity and bread quality of two wheat flour doughs.

The results showed that both molecular weight BGs increased the elasticity, deformation resistance and fluidity. Among them, low molecular weight BG added to low gluten wheat flour can obtain a similar quality of flour as high gluten wheat flour. Rieder et al. [43] pointed out that high molecular weight β-glucan can increase the viscosity of the dough aqueous phase and stabilize the pores; however, Gill et al. [44] pointed out that high molecular weight β-glucan will have a more adverse effect on the dough, making the dough more resistant to extension and less extensibility. This is because high molecular weight β-glucan produces a highly viscous gel when it comes into contact with water, which adheres to the surface of gluten proteins, competing with gluten proteins for moisture and affecting the formation and stability of the gluten network structure [45].

2.2 The physical and chemical properties of β-glucan complexes and their application in foods

In recent years, research on cereal β-glucan has expanded to include the study and application of its physical and chemical properties in combination with other macromolecules.

2.2.1 β-glucan polysaccharide complexes

Beta-glucan has a certain degree of gelling properties, and combining it with other polysaccharides can enhance its gelling properties. The interaction between konjac glucomannan and beta-glucan can significantly enhance the fluidity, water retention, viscoelasticity, cohesion and storage stability of the composite gel through hydrogen bond adsorption and embedding of beta-glucan molecules, but it has a significant effect on reducing hardness [43]. Therefore, adding the right amount of konjac glucomannan can increase the application potential of β-glucan in spreadable foods. Adding β-glucan to oat starch can also form a uniform and dense network structure through hydrogen bonding. β-glucan has a certain protective effect on the starch crystallization zone, and can form nuclei under ultra-high pressure processing conditions to inhibit starch aging [44]. When barley β-glucan is mixed with wheat starch, it also binds to the surface of the starch granules through hydrogen bonds, promoting water absorption and swelling, and the orderly arrangement of amylose, and increasing the weight-average relative molecular mass of amylose [46].

Beta-glucan has a certain degree of gelation, and combining it with polysaccharides can enhance its gelation, further affecting the processing quality of foods. Studies have shown that the interaction between konjac glucomannan and beta-glucan can significantly enhance the fluidity, water retention, viscoelasticity, cohesion and storage stability of the composite gel through hydrogen bond adsorption and embedding of beta-glucan molecules, but it has a significant effect on reducing hardness.

Konjac Mannan and β-glucan can be combined to increase the application potential of β-glucan in spreadable foods [47]. When added to oat starch, β-glucan can form a uniform and dense network structure through hydrogen bonding. β-glucan has a certain protective effect on the starch crystallization zone, and can form nuclei under ultra-high pressure processing conditions to inhibit starch aging [48]. Barley β-glucan can promote the swelling and gelatinization of wheat starch. BBG is bound to the surface of starch granules through hydrogen bonds, promoting water absorption and swelling, the orderly arrangement of amylose, and an increase in the weight-average relative molecular mass of amylose. A composite gel is formed to reduce hardness and enthalpy during refrigeration, and to delay the long-term recrystallization of wheat starch [49]. Using spray drying, barley β-glucan-modified corn starch microcapsules can encapsulate fish oil (EPA) and prevent it from oxidizing [50].

2.2.2 β-glucan lipid complexes

In food systems, cereal β-glucans may form complexes with different lipids in them, which have a certain loading effect on lipophilic small molecules and can promote their targeted release and improve bioavailability. Oat β-glucan stearate can be obtained by hydrophobic modification of oat β-glucan with stearic acid, a saturated fatty acid, and is used to load myricetin. At a concentration of 1.5 mg/mL of oat β-glucan stearate and a ratio of 1:1 of oat β-glucan stearate to myricetin, the complex can achieve a loading capacity of 55.86 µg/mg of myricetin and has a certain sustained-release effect on myricetin [46]. min at a homogenization speed of 12 kr/min. The loading capacity of myricitrin in the complex can reach 55.86 µg/mg, and it has a certain slow-release effect on myricitrin [46].

Oat β-glucan and octenyl succinic anhydride (OS) can be obtained by esterification to form OS-oat β-glucan ester (OSβG). OSβG with different degrees of substitution and weight-average molecular weights can self-assemble into negatively charged spherical micelles with a particle size of 175–600 nm. It also has the effect of loading curcumin. OSβG with a degree of substitution of 0.01 9 9 and the OSβG with a weight-average molecular weight of 1.68×105 g/mol can load curcumin (4.21±0.16) µg/mg [51]; however, the amino acids in food have a certain effect on the stability of OSβG loaded with curcumin [52]. A complex ester formed by octenyl succinic anhydride and barley β-glucan can be used as a wall material, and blackberry wolfberry anthocyanins can be used as the core material. In an aqueous system, 46% of the anthocyanins can be encapsulated. Anthocyanin microcapsules are stable at low temperatures and low pH, and provide some protection against oxidative degradation [53].

2.2.3 β-glucan protein complexes

The interaction of cereal β-glucans with proteins can enhance their functional properties, broaden the scope of β-glucan applications, and provide new ideas for the precise processing and precise nutrition of foods rich in β-glucans. Barley β-glucan (BG) and gluten protein can directly interact in an aqueous dispersion system. When there is an excess of water, BG increases the water holding capacity and the freeze-drying water content of gluten protein by increasing the binding capacity of gluten protein for weakly bound water in the aqueous phase, weakening the cross-linking of gluten protein. The use of BG to glycosylate wheat gluten protein can significantly improve the solubility, emulsifying properties and foaming properties of wheat protein . These results provide new ideas for the preparation and application of barley β-glucan complexed wheat protein as a fat analogue [12,54].

Oat β-glucan (OG) powder and lactoferrin can change the secondary structure of lactoferrin to form self-assembled bodies and thermally aggregated bodies at 25 °C and 90 °C. After heat treatment, spherical particles are formed, which can be further spray dried and used to deliver curcumin [55]. Oat β-glucan and soy protein isolate can enhance the emulsifying and gelling properties of the mixed gel through hydrogen bond interactions, and improve the glass transition temperature (Tg) and thermal stability of the mixed gel [56]. Adding different concentrations (0.25% to 1%) of oat β-glucan to a 4% myofibrillar protein solution and heating at 80 °C for 20 minutes to form a composite gel can significantly improve the water retention, gel hardness and viscoelasticity of the myofibrillar protein gel [57]. Adding barley β-glucan to sausages can cause the muscle protein to form a tighter network structure, thereby improving the water retention and protein denaturation temperature of the sausage [58]. These studies provide a theoretical basis for the development of meat products rich in β-glucan.

In recent years, there has been a growing trend in the consumption of plant-based drinks or dairy products. Adding high-molecular-weight oat β-glucan to milk can reduce the energy of milk and has a cholesterol-lowering effect. Therefore, there have been many studies on the interaction between β-glucan and milk proteins. The addition of β-glucan has a certain effect on the viscosity, flowability and stability of the milk system. Acid coagulation: The sodium caseinate and BG mixed gel has phase separation at the microscopic level. At low concentrations of β-glucan (3% w/w), the properties of the mixed system are controlled by the composition of the protein. However, as the concentration of the polysaccharide increases, the gel strength and thermal stability of the mixed system are affected by the structure of the polysaccharide, that is, the gel of acidified skim milk containing BG can weaken the protein network structure [58].

Changes in the molecular weight of polysaccharides can also cause phase separation in protein/polysaccharide mixtures. The OG content required for phase separation in the OG and sodium caseinate mixture depends on its molecular weight. When the relative molecular weight (Mr) of OG increases from 3.5×104 to 6.5×104 , the required content decreases from 2% to 2.5% (w/w) to 1% to 1.5% (w /w] can indicate thermodynamic incompatibility [59]. In a thermodynamically stable state, the viscosity of the low-molecular-weight β-glucan in the mixed system is a factor affecting the equilibrium state of the system, and the high-molecular-weight β-glucan can quickly aggregate when the protein concentration changes [60]. The driving force for the phase separation of skim milk by BG (product name GLucagel) is the flocculation loss of casein particles in the polysaccharide molecules. With the casein particle volume fraction and the barley GLucagel concentration , the two-phase system separates either due to transient gelation or the formation of a precipitate. Higher concentrations of β-glucan can increase the volume fraction of casein micelles [61]. Therefore, the thermodynamic incompatibility of milk proteins and β-glucan and phase separation pose a significant challenge for the product.

3 Nutritional research on cereal β-glucan

Cereal β-glucan is an important type of water-soluble dietary fiber. In recent years, research on the digestion, absorption, transport and metabolism of β-glucan and its health benefits has been continuously deepening, especially in terms of the correlation between the molecular properties of β-glucan and precision nutrition. The main research content is shown in Table 3. The nutritional functions of β-glucan mainly include the effects on gastrointestinal health, lowering blood sugar, reducing fat and weight loss, improving intestinal flora, anti-oxidation and anti-inflammatory, immune promotion and some anti-cancer functions. These studies have characterized the nutritional effects from various aspects such as the source of β-glucan raw materials, processing methods, molecular sizes or viscosities, etc., using in vitro and in vivo studies and other different subjects, from biochemical indicators, metabolic regulation and metabolomics, genomics and transcriptomics, etc. These studies not only theoretically explain the nutritional effects of β-glucan, but also provide a scientific basis for the future development of new health foods.

4 Conclusion

As a dietary fibre component with obvious health benefits in whole grain foods, cereal β-glucan has been isolated and purified from a variety of cereals and their by-products (such as bran), and is used in the production of various types of food. Adding cereal β-glucan to foods not only increases the dietary fibre content of the food and improves its health benefits, but also improves the quality of the food by taking advantage of the functional properties of β-glucan, such as its viscosity, gelling properties and flow characteristics. Therefore, cereal β-glucan has become one of the most popular raw materials or food ingredients in the health food industry.

However, although many studies have focused on improving the extraction rate and purity of β-glucan, the process conditions are still limited to the laboratory scale, and there is a lack of an extraction and purification process suitable for industrial production. This is still the main factor restricting the further industrial development of cereal β-glucan. In addition, the functional properties of complexes formed by cereal β-glucan and other macromolecules such as starch, proteins, and lipids, and their application in foods have become new research hotspots in this field. However, the health benefits and mechanism of action of the complexed cereal β-glucan compared to that of the simple cereal β-glucan are scientific issues that warrant further research.

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