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

Abstract

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 ( F c) has been measured as a function of bulk protein concentration ( C b) 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, F c was slightly lower for ovalbumin than for WPI but slightly higher than for pure β-lactoglobulin at the highest C b. 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 F c 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 F c, at concentrations as low as 0.01 wt%. For xanthan and guar gum, effects on F c were significant at concentrations between 0.025 and 0.1 wt%, but were variable, increasing or decreasing the value of F c, 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%.