# Insight on Extraction and Preservation of Biological Activity of Cereal \(\beta\)-D-Glucans
## 2 The Molecular Structure of Cereal \(\beta\)-D-Glucans  
Cereal \(\beta\)-D-glucans are linear homopolymers formed from D-glucose residues bound primarily through two or three consecutive 1,4-\(\beta\)-glycosidic linkages (oligomeric cellulose segments) separated by a single 1,3-\(\beta\)-glycosidic linkage. Longer segments with glucose residues bound consecutively by 1,4-\(\beta\)-glycosidic linkages and with a DP between 5 and 28 are less frequent. There is no evidence that two or more adjacent 1,4-\(\beta\)-glycosidic linkages are present in the \(\beta\)-D-glucan chains of cereals [17]. The molecular characteristics of \(\beta\)-Dglucans generally derive from the analysis of oligomers obtained from the depolymerization of polymers with enzyme lichenae, a (1\(\rightarrow\)3; 1\(\rightarrow\)4)-\(\beta\)-D-glucan-4-glucuronohydrolase (EC 3.2.1.73) specifically cleaves the 1,4-\(\beta\)-glycosidic linkages of the three-substituted glucose residues in \(\beta\)-D-glucans, yielding oligomers with different DP. The major hydrolysis products for the cereal \(\beta\)-D-glucans are 3-O-\(\beta\)-cellobiosyl-\(\beta\)-D-glucose (DP3) and 3-O-\(\beta\)-cellotriosyl-\(\beta\)-D-glucose (DP 4), which account for 90-95% of total oligosaccharides, while longer oligosaccharides (DP \(\geq\) 5) account for only 5-10% of the total [4, 11]. The molar ratio is considered as the ratio of cellotriosyl to celloteraosyl units (DP3/DP4) after depolymerization of the \(\beta\)-D-glucan polymer chain [7]. In \(\beta\)-D-glucans, molar ratio is a unique feature of each cereal and is strongly influenced by cultivars and environmental conditions. In major cereals, the molar ratios are approximately as follows: wheat (3.0-4.5), barley (2.3-3.4), rice (2.4-2.7), and oats (1.5-2.3) [7]. The generalized structure of cereal \(\beta\)-D-glucans and their cleavage with lichen is shown in Figure 1.  
Despite being classified as soluble fibers, the molecular irregularity of \(\beta\)-D-glucans is reflected in their solubility in water. Although the molecular structure of \(\beta\)-D-glucan allows numerous interactions and connections between its chains and water molecules, a high DP can reduce its solubility. Cellulose-like segments in cereal \(\beta\)-D-glucans could contribute to the rigidity of molecules in a solution; \(\beta\)-D-glucan chains containing adjacent 1,4-\(\beta\)-glycosidic linkages segments may show a tendency for intermolecular aggregation and consequently to lower solubility through hydrogen bonds along these portions. The insertion of 1,3-\(\beta\)-glycosidic linkages leads to the breakage of internal hydrogen bonds in the cellulose-like sequence, resulting in flexible conformation and better solubility in water due to easy solvation with water molecules [4, 5, 18]. Solubility appears to decrease as the DP3/DP4 ratio along the \(\beta\)-D-glucan chains increases. Moreover, the coexistence of different biopolymers in the cell wall, their spatial organization, and interactions between wall components will likely affect mechanical strength and permeability, thus altering the solubility of the compounds [17]. The highest \(\beta\)-D-glucan content of cereals is found in the outermost layers. Depending on the tissue considered (pericarp, aleuronic layer, and endosperm), their molecular structure changes. Thus, \(\beta\)-D-glucans in the outer layers have a lower solubility and a higher molar ratio in DP3/DP4 than \(\beta\)-glucans located in the endosperm. The \(\beta\)-D-glucans present in pearl byproducts show a higher molar ratio in  
Figure 1: Generalized structure of cereal \(\beta\)-D-glucans and their cleavage with lichenae; scissors indicate the hydrolysis sites of lichen on the polysaccharide chain. Hexagons: glucose monomers; DP3: 3-O-\(\beta\)-cellobiosyl-D-glucose; DP4: 3-O-\(\beta\)-cellotriosyl-d-glucose; DP \(\geq\) 5: cellodextrin-like oligosaccharides containing more than three consecutive glucose residues bound to 4-O. DP3/DP4 (~3.3-3.6) than those present in flours or fiber-enriched fractions (~2.8-3) [19].