Influência de parâmetrosfísicos e químicos e da adição de íons no comportamento reológico de goma xantana

Abstract

Several strains of Xanthomonas campestris are able to produce a bacterial biopolymer called xanthan which is widely used in the food industry. In order to have an effective use of the xanthan in the industry, not only the studies concerning the properties of the solutions should be considered, but also the studies related to its solubility, the addition of electrolytes, and its characteristics related to temperature variations and pH. There is not a unique oficial methodology that establishes the preparation of aqueous solutions of xanthan to evaluate the rheological behaviour. When a new bacterial biopolymer is sinthetized, new rheological behaviours appear, so it is necessary to know the existing methodologies in order to apply or adopt them as laboratorial procedure. This study aims at evaluating the commercial xanthan solubility by using different methods, as well as the rheological behaviour of the commercial xanthan and of the one produced by Xanthomonas campestris pv pruni strain 24 when facing the addition of salts to the aqueous solution and temperature variation and pH, by using as a parameter the apparent viscosity of the different solutions. The samples for the rheological analysis were prepared at 1% (m/v); the concentrations of 0.01mol L-1, 0.1mol L-1 and 1mol L-1 of potassium and sodium chloride were tested with a temperature variation of 25°C and 45°C and pH at 3.0, 4.0 and 5.0 by using Haake rheometer RS 150. According to the results, the best xanthan samples solubilization method for the analysis of the rheological behaviour is the one proposed by Zhang Xuewu et al. (1996), and the adequate solubilization time for the comparative rheological analysis is of 24 hours at room temperature. It was also determined that the chemical composition of the commercial xanthan and of the one produced by pv pruni strain 24 affects directly the viscosity of the aqueous solution. After being dialyzed, the highest viscosity was reached with the addition of potassium chloryde at 0.1 molL-1 showing that the temperature influences the apparent xiv viscosity of the solutions decreasing the viscosity with the increase of the temperature, and that the pH also has influence on the apparent viscosity, increasing the viscosity with the increase of the pH. However, the solution of xanthan pv pruni strain 24 presented higher apparent viscosity of its aqueous solution in pH equal to 5.0