INTERFACIAL ELASTICITY AND THE DESTABILISATION OF BEER FOAMS

Carole Moules
Camtel Ltd., 5 Carrington House, 37 Upper King Street , Royston, Herts SG8 9AZ , UK

ABSTRACT：Foams and emulsions are produced by many industries and the stability of these systems is of vital importance. If the interfacial layer surrounding the dispersed droplets or bubbles is occupied by an adsorbed layer of molecules, proteins or surfactants, stabilisation against coalescence takes place. Proteins form highly elastic 2-dimensional networks around the droplets and this layer provides mechanical strength whereas surfactants, because of their rapid mobility, create a lubricating layer of liquid in the gaps between droplets. Proteins and surfactants are commonly present in many commercial systems e.g. dairy products and beer. When both are present at the interface the mechanism by which each type works to stabilise an emulsion or foam is disrupted. Using an oscillating shear rheometer [1], values of interfacial elasticity of model protein-surfactant systems decreased with concentration of surfactant. The destabilisation of the interfacial layer occurs because the surfactants form domains within the adsorbed protein network as can be seen by atomic force microscopy imaging. For real products such as beer which is protein stabilised, a similar reduction in interfacial elasticity occurs when lipids are added. The degree of foam damage increases with concentration and chain length of the fatty acids. Understanding the stability mechanisms in terms of interfacial behaviour should allow the formulation of lipid resistant beer or for the consumer the retention of the ‘head on a pint', a subject dear to the hearts of the British!