Effect of copper ions on the drainage stability of foams prepared from eggwhite

In this paper we investigate the effect of copper ions on the stability of foams prepared from egg white. We compare the time of formation and stabili ty of foams prepared from fresh egg whites, with and without added copper i ons. We find that the foams prepared with copper ions take more time to for m, but are more stable. The effect increases upon dilution of the egg white s, which shows that the bulk phase is not responsible for the increase in s tability. Microscopy shows that the initial bubble size distribution of the foam is not affected by the addition of copper ions. Measurements with a r ing trough show that the surface tension of the liquid vapor interface of t he foam films is also unaffected. The results of the ring trough measuremen ts show that the increase in drainage stability is caused mainly by an incr ease in the surface dilatational elasticity of the interface. There is also an increase in the surface dilatational viscosity, but only at frequencies less than 0.1 Hz. The increase in the surface dilatational elasticity affe cts the drainage stability of the foam during the initial seconds of the dr ainage process: whereas the increase in the surface dilatational viscosity affects the long-term drainage stability. (C) 2001 Elsevier Science B.V. Al l rights reserved.