Effects of polyols on cryostructurization of barley β-glucans

The effects of polyols (fructose, glucose, sorbitol, sucrose, and xylose) on cryostructurization of three barley mixed-linkage (1→3), (1→4) β-d-glucan isolates (β-glucans) differing in molecular size (apparent molecular weights of 210, 140, and 70×103 Da) were investigated. Polyols were incorporated at concentrations ranging from 5% to 30% (w/w) into aqueous β-glucan solutions (3% w/w) and the dispersions were subjected to 14 or 22 repeated freezing (−15 °C for 24 h) and thawing (5 °C for 24 h) cycles. Such a treatment yielded compact, porous cryostructurates that were smoother and less porous with the addition of polyols. Optical microscopy confirmed these observations showing a fibrillar network for the cryostructurates that became less pronounced with increasing polyol concentration. For the high molecular weight (210×103) preparation, with exception of sorbitol, the addition of polyols to β-glucan solutions retarded the cryostructurization, as shown by phenomenological observations after each freezing–thawing cycle, and resulted in weaker and less thermostable cryogels compared to those of control, as determined by small deformation mechanical measurements. Xylose and fructose showed a stronger inhibitory effect on structure formation, compared to sucrose and glucose, whereas sorbitol was a promoter when added up to 20% w/w level. However, the above effects of polyols were largely diminished with decreasing molecular weight of the β-glucan and seemed to be independent of the type of polyol for the 140 and 70×103β-glucan samples. With increasing sugar concentration there was an increase in the number of cycles, at which cryostructurates were formed, and the resultant gels exhibited lower elastic modulus (G′) and melting temperature (Tm), and higher tanδ values, compared to cryogels without polyol added. On the other hand, with increasing sorbitol level the elasticity of the cryogels decreased, whereas their stability upon heating increased. Moreover, differential scanning calorimetry exhibited a more cooperative gel↔sol transition and significantly higher apparent melting enthalpy values (ΔH), calculated from the endothermic peaks, for β-glucan cryostructurates fortified with polyols compared to control preparations. Apparent yield stress values, as determined by stress sweep and creep tests, showed that addition of polyols resulted in more “brittle” gels, following the order: control>sorbitol>sucrose>glucose>xylose>fructose. On the other hand, large deformation mechanical tests (compression mode) revealed an increase in firmness and strength of β-glucan cryogels with inclusion of polyols in the order: sucrose, fructose