In very dilute solution the viscosity behaviour of polymers is mainly determined by their conformation (size and shape). Experimental evidence supports the earl hypothesis that chain conformation and rigidity can be qualitatively predicted on the basis of viscosity data. Alihough a wide range of rheological experiments, including small deformation oscillatory, steady and transient shear, have been performed at finite concentrations and under well defined experimental conditions (for a reference, Lapasin and Pricl, 1995), still little effort has been done in attempt to correlate the rheological behaviour (slow dynamics) of polysaccharide solutions in terms of chain conformational dynamics. Solution dynamical properties of disordered polysaccharides can be modelled within a reasonable degree of accuracy and their rheological behaviour at finite concentrations is that of a typical network of flexible chains. The lack of ordered sec ondary structure lead to entangled networks in which the topological constrains govern the rheological behaviour. Ordered regions along the polysaccharide chains seem to be the prerequisite to establish intermolecular interactions which lead to non-transient polymer networks. Whenever ordered conformations are stable in solution, both solution properties and rheological behaviour display features that arise not only from the stiffening of the chains in the ordered conformation, but from the likely interchain interactions between the ordered 'ar-ray(s)' of chains. The strength of these interactions reflects in the behaviour of the gel (weak to true gel).
The influence of polymer conformation and rigidity on the rheological properties is especially evident in the semi-dilute concentration regime. The rheologi-cal behaviour of flexible linear polysaccharides in good solvent is that of an entangled network of physically interacting chains, being the Corx-Merz rule obeyed except at very low polymer concentration (Lapasin and Pricl, 1995). In the same concentration regime and under conditions of an ordered conformation, worm-like polysaccharides with low flexibility (i.e. with persistence length > 60 nm) show a remarkable deviation from the behaviour described for flexible molecules. In particular, the viscoelastic spectrum resembles that of a typical gel system with both moduli G" and G nearly frequency independent (G > G' in the whole range of frequency accessible) and the Corx-Merz superposition rule fails.
From a technological point of view, it is relevant to mention the importance played by polysaccharides in the food industry. Polysaccharide thickeners are widely used to modulate and to control texture and mouthfeel of viscous food systems, ranging from sauces and beverages to dairy products. Several in-mouth perceived textural characteristics, like thickness and sliminess of polysaccharide food models, have been successfully correlated to rheological parameters; the correlation, however, resulted to be dependent on polysaccharides conformation. In facts, disorder random coil polysaccharides or rigid ordered polymer like xanthan gave different responses.
In addition to texture perception polysaccharide conformation and related rheology is of crucial importance as far as flavour release is concerned (Morris, 1995). It is well known in the food industry that the quantity of flavouring required to produce the same subjective flavour intensity is often much higher in thickened or structured products than in fluid systems. A similar suppression of perceived intensity is also well established for taste attributes (sweet, sour, bitter, and salty). Random coil polysaccharide solutions, ordered polysaccharide weak gels and true gels show relevant differences in the flavour/taste release. In particular, according to Morris, the increased viscosity in ordered/gelling poly-saccharides hinders the mixing process by which flavour/taste molecules diffuse (Morris, 1995).
Was this article helpful?
Are You Sick And Tired Of All The Fat-Burning Tricks And Trends That Just Don’t Deliver? Well, Get Set To Discover The Easy, Safe, Fast, And Permanent Way To Mega-Charge Your Metabolism And Lose Excess Fat Once And For All! This Weight Blasting Method Is Easy AND Natural… And Will Give You The Hot Body And Killer Energy Levels You’ve Been Dreaming Of.