Size-Dependent Rheological Variability of Levan Produced by Gluconobacter Albidus TMW 2.1191

Abstract

Levan is a fructan-type exopolysaccharide, which is produced by many microbes from sucrose via extracellular levansucrases. The hydrocolloid properties of levan depend on its molecular weight, while it is unknown why and to which extent levan is functionally diverse in dependence of its size. The aim of our study was to get deeper insights into the size-dependent, functional variability of levan. For this purpose, levans of different sizes were produced using the water kefir isolate Gluconobacter albidus TMW 2.1191 and subsequently rheologically characterized. Three levan types could be identified, which are similarly branched, but significantly differ in their molecular size and rheological properties among each other. The smallest levan (< 107 Da) produced without adjustment of the pH exhibited Newton-like flow behavior up to a specific concentration of 25% (w/v). On the contrary, larger levans (> 108 Da) produced at pH ≥ 4.5 were shear-thinning and showed a gel like behavior at ≥ 5% (w/v). A third (intermediate) levan variant was obtained via production in buffers at pH 4.0 and exhibited the properties of a viscoelastic fluid at ≥ 5% (w/v). Our study reveals that the variable size and composition of levan are controllable and more decisive for its functionality than the amount of exerted levan.