Two strains of X. campestris from a collection and a wild strain isolated from infected cabbage were cultured in a defined medium in a 25 dm 3 fermenter. The rheological characteristics of the broth were measured using a Weissenberg rheogoniometer. The rheological behaviour over a wide range of shear rates could be described with the power law model and a yield stress was obtained using the Herschel-Bulkley equation. As expected, the broth's flow behaviour is highly influenced by gum concentration. In general, the rheological parameters during the fermentations varied within the following ranges: K (2-210 dyn-sn/cm2), n (0.2 0.9), rr(0.02-50 dyn/ cm2), with strain 1459 reaching the maximum values as well as the highest gum concentration.
Differences in flow behaviour could be associated with different molecular configurations of the polymers produced by the different strains. Heat treatment of the broths change the rheologica! characteristics, increasing K and decreasing n. Reconstituted solutions of all, sterilized gums exhibited similar rheological characteristics and, at low concentrations, also similar to those of a commercial xanthan.
List of symbols
Di
m impeller diameter Dt m tank diameter A, B constants in equation (4) of Thompson and Ollis [4] K dyn-sn/cm z consistency index n flow behaviour index T dyn/cm z shear stress s-1 shear rate zy dyn/cm 2 yield stress KuB -Herschel-Bulkley equation constant NHB Herschel-Bulkley equation constant