Effect of thickeners on the coalescence of protein-stabilized air bubbles undergoing a pressure drop Brent S. Murray * , Eric Dickinson, Cyrille Gransard, Ingrid So ¨derberg Food Colloids Group, The Procter Department of Food Science, The University of Leeds, Leeds LS2 9JT, UK Received 13 September 2004; revised 1 February 2005; accepted 22 March 2005 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 K3 to 2.4 s K1 , respectively. Pure b-lactoglobulin, a commercial whey protein isolate (WPI) consisting primarily of b-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 b-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 K1 . 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%. q 2005 Elsevier Ltd. All rights reserved. Keywords: Bubbles; Coalescence; Thickeners; Proteins; Sucrose; Pressure drop 1. Introduction Many important commercial products are foams (Wilson, 1989), with the dispersion of bubbles commonly contributing to the desired density, texture and stability. Important food examples include bread and sponge-cakes (solidified foams or sponges), ice cream, whipped products and mousses (Campbell, Webb, Pandiella, & Niranjan, 1999). In foods the predominant foam stabilizing molecules are proteins (Turan, Kirkland, & Bee, 1999; Walstra, 1989). Controlling coalescence in foams is critical, as this can quickly lead to complete loss of the desired foam properties. Usually, inhibition of coalescence is viewed from the point of view of the Gibbs–Marangoni stability, where the depletion of the surface-active material protecting the two adjacent interfaces from coalescing is prevented by interfacial tension gradients arising from local variations in the surface concentration of adsorbed species. Additionally, with proteins, the surface rheology of the adsorbed film itself may provide a mechanical barrier to coalescence. Most adsorbed proteins form, in effect, a thin three- dimensional protein gel at the interface that can have considerable resistance to deformation (Murray, 2002). Clearly, the greater the extent of deformation of the interface, then the more likely it is that coalescence will occur. Instances of foam processing where there can be a significantly high rate of bubble expansion are the exiting of foam from a mixing or aeration chamber, and the extrusion of an aerated product from a nozzle in dispensing and filling, etc. In the manufacture of many foamed products, the gas is introduced under pressure, and so the foam exits the aerator at an even higher pressure than that induced by the impeller motion alone. Under such circumstances, the exit from the aerator will be accompanied by simultaneous expansion of the bubbles as the pressure within the foam falls back to Food Hydrocolloids 20 (2006) 114–123 www.elsevier.com/locate/foodhyd 0268-005X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2005.03.010 * Corresponding author. Tel.: C44 113 343 2962; fax: C44 113 343 2982. E-mail address: b.s.murray@food.leeds.ac.uk (B.S. Murray).