Physical Properties of Oat β-Glucans

The mixed linkages that form oat β-glucans are important to their physical properties, such as viscosity and solubility. The presence of two types of linkages prevents compact folding of oat β-glucan chains, making them soluble in water. The (1→3)-linked residues result in kinks in the otherwise ribbon-like shape of the molecule, allowing water to penetrate and solubilize the fiber (1). The longer (1→4) sequences are believed to be responsible for the partial water insolubility properties of oat β-glucans, because they provide surfaces that are capable of forming junction zones (2). Oat β-glucans are asymmetric molecules that assume an extended conformation in aqueous solution, best described as worm-like chains (3). One theoretical model (4) describes β-glucans in solution as an extended random coil.

Oat β-glucans are large molecules. Available data on molecular weight distribution vary between 2.68 × 104 and 3 × 106 g/mol (5,6,7). These variations probably depend on differences in raw materials, processing, and methods of determination (8,9,10). Even at low concentrations in water solutions, oat β-glucan molecules interact, causing marked resistance in water flow and creating a viscous solution. If the concentration is high enough, a gel is formed through associations between molecules.

Increased viscosity is a fundamental characteristic of oat β-glucan solutions and has an important impact on their physiological behavior in the intestine and, thus, their physiological function. Viscosity plays an important role in cholesterol-lowering effects and glycemic response. Solubilization and the molecular weight of β-glucans influence viscosity and bioavailability (11,9,10,12). Viscosity is mainly determined by molecular weight but also by molecular structure, resulting from the distribution of (1→3) and (1→4) links. In conclusion:  To maintain functional attributes, the food containing oat β-glucan must be controlled during processing and in food matrices. The physiological effects of oat β-glucan depends at least in part on the viscosity and not only on the amount of β-glucans. Therefore it is critical not only  to know the ß-glucan concentration, but also the solubility, viscosity and molecular weight of β-glucans in the product (11,9).

References

1. Clarke, A. E., and Stone, B. A. Chemistry and biochemistry of b-1,3-glucans. Rev. Pure Appl. Chem. 13:134, 1963.

2. Wood, P. J. Physiochemical characteristics and physiological properties of oat (1→3)(1→4)-β-d-glucan. Page 91 in: Oat Bran. P. J. Wood, ed. American Association of Cereal Chemists, St. Paul, MN, 1993.

3. Gomez, C., Navarro, A., Manzanares, P., Horta, A., and Carbonell, J. V. Physical and structural properties of barley (1→3)(1→4)-β-d-glucan. Part II. Viscosity, chain stiffness and macromolecular dimensions. Carbohydr. Polym. 32:17, 1997.

4. Buliga, G. S., Brant, D. A., and Fincher, G. B. The sequence statistics and solution conformation of a barley (1→3)(1→4)-β-d-glucan. Carbohydr. Res. 157:139, 1986.

5. Rimsten, L., Stenberg, T., Andersson, R., Andersson, A., and Åman, P. Determination of b-glucan molecular weight using SEC with Calcofluor detection in cereal extracts. Cereal Chem. 80:485, 2003.

6. Roubroeks, J. P., Mastromauro, D. I., Andersson, R., Christensen, B. E., and Åman, P. Molecular weight, structure, and shape of oat (1→3)(1→4)-β-d-glucan fractions obtained by enzymatic degradation with lichenase. Biomacromolecules 1:584, 2000.

7. Suortti T. Size-exclusion chromatographic determination of β-glucan with postcolumn reaction detection. J. Chromotagr 632: 105 -110, 1993

8. Aman, P., Rimsten, L., and Andersson, R. Molecular weight distribution of β-glucan in oat-based foods. Cereal Chem. 81:356, 2004.

9. Anttila, H., Wood, P. J., Brummer, Y., Sontag-Strohm, T., Tosh, S., and Wang, Q. The effect of processing on the molecular weight and extractability of oat beta-glucan using a method to simulate physiological digestion. Abstr. 2004 AACC Annu. Meet. Published online at www.aaccnet.org/meetings/2004/abstracts/. The Association, St. Paul, MN, 2004

10. Beer, M. U., Wood, P. J., Weisz, J., and Fillion, N. Effect of cooking and storage on the amount and molecular weight of (1→3)(1→4)-β-d-glucan extracted from oat products by an in vitro digestion system. Cereal Chem. 74:705, 1997.

11. Anttila, H., Sontag-Strohm, T., and Salovaara, H. Viscosity determination of soluble dietary fiber in cereal products. Abstr. 2002 AACC Annu. Meet. Published online at www.aaccnet.org/meetings/2002/abstracts/. The Association, St. Paul,

12. Wood, P., Braaten, J., Scott, F. W., Riedel, K. D., Wolynetz, M. S., and Collins, M. W. Effect of dose and modification of viscous properties of oat gum on plasma glucose and insulin following an oral glucose load. Br. J. Nutr. 72:731, 1994.