Apparent yield stress estimation in xanthan gum solutions and fermentation broths using a low-cost viscometer

The apparent yield stress, a rheological property important in determining the extent of mixing in xanthan fermentations as well as the suspending ability of the gum solutions, was determined for xanthan solutions (10-50 g l-1) and fermentation broths containing 12-30 g l-1 of xanthan, by model fitting and stress relaxation methods. By using a low-cost viscometer, a rapid relaxation technique proved to be reliable and preferable to the fitting of theoretical models. The yield stress increased with xanthan concentration, but different behaviour was observed for different xanthan sources and salt concentrations. Different apparent yield stresses were measured for fermentation broths and solutions of commercial xanthans of identical concentration. Apparent yield stress values were used successfully for the prediction of the size of the well mixed region in xanthan and Carbopol solutions in a stirred tank.