Tailoring biopolymers using enzymes: Interactions in xanthan-galactomannan blends

Biopolymers and their derivatives find use in a variety of commercial applications as rheology modifiers and stabilizers. In particular, the synergistic effects exhibited by mixtures of biopolymers are commonly used to economic advantage. However, the mechanism responsible for these effects and their relation to the functionality of the individual biopolymer still remains poorly understood. The main objectives of this study are two-fold—to develop methods for enhancing the functionality of a common biopolymer, guar galactomannan and to subsequently characterize structure-property relations of the modified galactomannan in synergistic blends with xanthan. Rheology and confocal scanning laser microscopy are employed as the principal techniques for characterizing the macroscopic parameters as well as the associated microstructural changes in these systems.; In the first part of this study, enzymatic hydrolysis is used to tailor the molecular architecture of guar gum to produce galactomannans with different galactose contents. The synergistic effects in mixed blends of modified guar with xanthan gum are found to be directly proportional to the extent of enzymatic hydrolysis. Rheological evidence points to the existence of two different mechanisms of interaction in the mixed blends depending on the galactose content. At high galactose contents (33.6%), mixed blend rheology is controlled mainly by the galactomannan and is insensitive to conformational changes in xanthan. In contrast, at lower galactose contents (25.2%), the dynamic moduli of the blends are directly related to the level of disorder in xanthan molecules and are sensitive to changes in blending temperature and ionic strength which mainly affects the conformation of xanthan.; In the second part of this study, time-dependent properties of modified guar and its synergistic blends are investigated using a combination of rheology and confocal scanning laser microscopy. Solutions and blends of guar containing 25.2% galactose are found to be stable over a period of time and show no changes in either their rheology or microstructure. In contrast, at even lower levels of modification (16.2%), phase separation is observed due to the competing process of self-aggregation of guar molecules which leads to the formation of a macromolecular gel. Similar aggregation processes are seen in the mixed gels, though to a lesser extent, suggesting that the formation of mixed junction zones retards self-association.; The diffusion of macromolecular probes in biopolymer solutions and gels is addressed in the last part of this study. Diffusion coefficients of fluorescein isothiocyanate probes measured in solutions of xanthan or modified guar by a technique known as fluorescence recovery after photobleaching, decrease uniformly as the concentration of biopolymer is increased or as the radius of the probe increases. Scaling theory is used to collapse the data into a single mastercurve which allows a priori prediction of diffusion behavior of macromolecules. In the case of the enzyme-modified guar/xanthan mixed blends, diffusion processes are dependent on the actual size of the probe used, with mobility of small probes being unhindered by the matrix. A minimum critical probe size is necessary before concentration changes begin to have an effect on diffusion. Although the elastic modulus of the gel increases significantly with an increase in total biopolymer concentration, a corresponding sharp decrease in diffusion coefficient is not observed, suggesting that the macromolecular properties are not always indicative of microstructural changes.