Gelation and rheology of xanthan/enzyme-modified guar blends

The rheological behavior and synergistic character of mixed polysaccharide systems are examined for blends of xanthan with enzymatically-modified guar. In particular, the enzyme α-galactosidase is used to selectively cleave off the galactose side chains of guar in order to obtain galactomannans with tailored molecular architecture: EMG1 with a relatively high galactose content of 33.6% and a mannose (M) to galactose (G) ratio of 1.85, and EMG2 with a lower galactose content (25.2%) and an M/G ratio of 2.86. Blends of xanthan with enzymatically-modified guar gum samples are examined in terms of their dynamic rheological properties and compared to those of xanthan — locust bean gum blends. The extent of synergism, illustrated by the gel elastic modulus G′ and yield stress τc, is found to increase with increasing extent of enzymatic modification. At constant ionic strength, the EMG2 and locust bean blends behave similarly, with increasing extent of synergy as the temperature of mixing is increased. Additionally, at a fixed mixing temperature, the blends made in water have a higher elastic modulus than those made in salt. In contrast, the EMG1 blends are weaker and the dynamic moduli are unaffected by changes in the mixing temperature or ionic strength. These results are consistent with those of other researchers and are directly related to both the level of disorder in the xanthan molecule as well as the galactose content and fine structure of the galactomannan.