Effect of thickeners on the coalescence of protein-stabilized air bubbles undergoing a pressure drop

The effect of some common food thickeners on the coalescence stability of protein-stabilized air bubbles at a planar air–water interface has been studied, as the bubbles undergo expansion due to a pressure drop. The fraction of bubbles coalescing (Fc) has been measured as a function of bulk protein concentration (Cb) and rate of area expansion. The ranges of protein concentration and rate of area strain studied were 0.01–0.3 wt% and 10−3 to 2.4 s−1, respectively. Pure β-lactoglobulin, a commercial whey protein isolate (WPI) consisting primarily of β-lactoglobulin, and pure ovalbumin were used. The thickeners used were sucrose (50 wt%), xanthan gum, guar gum, gelatine and pectin. In the absence of thickeners, Fc was slightly lower for ovalbumin than for WPI but slightly higher than for pure β-lactoglobulin at the highest Cb. It was found that stability with thickeners present was less dependent on expansion rate than for the proteins on their own. The effect of the concentration of thickeners on Fc was therefore studied at a fixed area strain-rate of 0.13 s−1. Sucrose had a general stabilizing effect with all the proteins, though this stabilization was less marked at higher protein concentrations and expansion rates. Overall, gelatine had the most marked effect on Fc, at concentrations as low as 0.01 wt%. For xanthan and guar gum, effects on Fc were significant at concentrations between 0.025 and 0.1 wt%, but were variable, increasing or decreasing the value of Fc, depending on the protein and its concentration. Pectin had a less noticeable effect than xanthan or guar gum, and then only at a considerably higher concentration, i.e., 0.25–1 wt%.