Improved gelling properties of whey protein isolate by addition of xanthan gum

The effect of pH (6.5, 6.0 and 5.5), addition of xanthan gum (0.01%, 0.03% and 0.06% v/v) and presence of NaCl (100 and 500 mM) on the formation of whey protein isolate (WPI)–xanthan gum gels was studied in the hope of obtaining a synergistic effect. Small-deformation oscillatory measurements were performed at a frequency of 1 Hz, a strain of 0.5%, and a gradually increasing temperature, to follow gel formation. The elastic modulus (G′) values at the end of the gelling test were compared. Confocal microscopy allowed visual examination of the microstructure. A synergistic effect on the gel strength was observed at pH 6.5 and 6.0 for all concentrations of xanthan gum added. At these pH's, phase separation was also observed between the denatured whey proteins and the xanthan gum when heat was applied. Whey proteins were concentrated in their phase, and consequently mixed gels were stronger than WPI gels. On the other hand, an antagonist effect was observed at pH 5.5 for all concentrations of xanthan gum added. The addition of salt demonstrated both a synergistic and an antagonist effect depending on the pH and salt combination, which would lead to different gel microstructures. Small changes in mixture formulation can result in unstable systems. A better knowledge of the microstructure of these systems and of the type of mixed gels obtained under different conditions (pH, ionic strength, etc.) will allow for a more optimal use of mixed protein–polysaccharide systems in food formulation.