Rheological behavior of binary and ternary mixtures of polysaccharides in aqueous medium

Abstract

The rheological behavior of mixtures of xanthan with different galactomannans is examined to evaluate the influence of the structure of galactomannan and that of the mixture composition on the physical properties; the larger synergy is observed for locust bean gum in the presence of xanthan. It is also shown that pH has only a slight influence on the rheology down to pH = 3.59; at lower pH, the G′ modulus decreases significantly. Then, the behavior of xanthan–methylcellulose mixtures is studied, paying particular attention to the storage modulus (G′) of the system, often equated to “gel strength”. The modulus values for direct dissolution of the two polysaccharides in 0.1 M NaCl show that xanthan and methylcellulose are incompatible. The rheological behavior observed is in agreement with DSC results which indicate that no specific interaction between the two polymers exists. However, upon increasing temperature, the modulus of methylcellulose increases substantially between 65 and 70 °C because of physical gelation and dominates the rheology of the mixtures. When temperature is decreased to 37 °C, the clear methylcellulose gel formed remains stable, still giving the main contribution to the overall rheology of the system. Finally, ternary systems are studied when xanthan is mixed with galactomannan and methylcellulose. In this case H-bonds involved in galactomannan–xanthan interaction break when temperature increases causing a decrease in rheological moduli which is then compensated by the gelation of methylcellulose giving an original large increase in moduli for the ternary systems. This can be interpreted as the presence of two independent but interpenetrating networks. The role of pH for these ternary systems is interesting: it is still dominated by methylcellulose and nearly independent of acidic pH (down to pH = 1.8).