About Levan: Rheology
EXCEPTIONALLY LOW INTRINSIC VISCOSITY: Unlike most polysaccharides, levan has a compact spherical structure and does not swell in water. The intrinsic viscosity of levan is only 0.14 dl/gm. This compares with an intrinsic viscosity around 1 dl/gm for flexible chain polysaccharides such as dextrose or compact coils like amylose. Extended chain polymers such as alginate or the galactomannans have an intrinsic viscosity around 20 dl/gm (Kasapis a). Materials used as thickeners such as CMC may approach 100 dl/gm intrinsic viscosity.
EXTREME VISCOSITY SENSITIVITY TO CONCENTRATION OF LEVAN: The zero-shear specific viscosity for levan plotted against the concentration times the intrinsic viscosity on a log-log scale, falls into four linear sections (Arvidson). The low concentration region (below 4%) has a gradient typical of dilute solutions of unbranched polymer coils and a slope of 1.4. Initial contact between individual molecules is responsible for the first transition. Here the swept-out volume occupied by the spheres and the branches sticking out of the surfaces, is equal to the total volume of the solution. Around 45% solids, spheres are sufficiently close together for branches to become entangled. At this point, the viscosity begins to increase dramatically. By 55% solids, levan has the consistency of chewing gum.
POLYMER EXCLUSION AND PHASE SEPARATION: Levan has been shown to have striking exclusion effects with pectin and locust bean gum. This property can be used to create a biphasic system and a large reduction in viscosity. The effective concentration of the individual polymers within their respective phases can be raised well above their nominal concentration. It has been speculated that levan may be used to generate mixed systems with versatile rheology (Kasapis a and b).
Kasapis S, Morris E, Gross M, Rudolph K. Solution properties of levan polysaccharide from Pseudomonas syringae pv. phaseolicola, and its possible primary role as a blocker of recognition during pathogenesis. Carbohydrate Polymers 23: 55-64 (1994). (a)
Kasapis S, Morris E. Conformation and physical properties of two unusual microbial polysaccharides: Rhizobium trifolii CPS and Levan. In Food Hydrocolloids: Structures, Properties and Functions, ed by K. Nishinari, Plenum Press, New York, pp 97-103 (1994). (b)
Arvidson S, Rinehart B T, Gadala-Maria F. Concentration regimes of solutions of levan polysaccharide from Bacillus sp. Carbohydrate Polymers 65: 144-149 (2006).