Effect of pH on viscosity of oat beta-glucan

The aim of our study was to analyze aggregation behaviour of oat beta-glucan molecules by measuring pH effects on viscosity of beta-glucan solutions. Commercial oat bran concentrate (Natureal OBC native, Suomen Viljava Oy, Helsinki, Finland) was used as raw material to isolate high molecular weight beta-glucan by acid extraction (Suortti et al. 2003). HPLC-SEC was used for analysis of the molecular weight of the isolated beta-glucan (Suortti 1993). The viscosities of beta-glucan solutions at different pH were measured at shear rates 16.4 - 157 1/s (Autio et al. 1992). The extraction of oat bran at low pH gave beta-glucan solutions where no other polymeric compounds could be detected. The molecular weight of beta-glucan was similar to highest molecular weights reported thus indicating that no molecular weight reduction had occurred during the isolation procedure. The molecular weight of the peak representing beta-glucan was 1.8-1.4*106 with polydispersity ranging from 1.3-1.5 in all the samples. Lichenase hydrolysis of the acid extracted beta-glucan showed that the preparation was pure from other polysaccharides. Viscosity of beta-glucan at pH 2 was high. When the pH was adjusted to pH 6 viscosity was decreased by about 20%. When the pH was adjusted to pH 12 the viscosity of the solution was highly decreased, but adjustment of pH back to pH 6 partly restored the viscosity. However, the level was much lower than in the sample where pH was adjusted directly to pH 6 without going by pH 12. The results show that only the change of pH to highly alkaline region reduces viscosity of beta-glucan. As this phenomenon is partly reversible the conclusion is that beta-glucan is extracted as a network where beta-glucan molecules have a strong interaction with each other. Under alkaline conditions this network is dispersed because of the ionization of the hydroxyl groups. The molecular weight of beta-glucan is not affected during this change. Autio et al. 1992. Food Hydrocolloids 5: 513-522; Suortti, T. 1993. Journal of Chromatography 632: 105-110. Suortti et al. 2003. AACC Annual Meeting. Portland, OR, 28 Sept. - 1 Oct. 2003, 109.