Simple qualitative interfacial shear rheology measurements on coffee samples published by Anton Paar confirm the close correlation between foam stability and interfacial shear properties.
Many food products are either in the form of emulsions or foams for which the knowledge of interfacial properties is important to achieve product stability. Coffee is a complex dispersion, which in the case of espresso coffee is topped by a foam structure of tiny bubbles, e.g. the espresso cream or 'crema'. The 'crema' is a polyphasic system consisting of dispersed tiny gas bubbles of carbon dioxide and water vapour surrounded by surfactant films, droplets of emulsified oils, and solid fragments.
Interfacial rheology does not probe the foam itself, but measures the adsorption of the amphiphilic ingredients and their network formation at the liquid surface. Higher values of the interfacial properties and a faster film formation are expected to correlate with a better foam stability.
Different techniques have been used to measure interfacial shear properties. However, a combination of a suitable geometry with a rotational rheometer, which is both extremely sensitive in torque and angular resolution, offers the largest flexibility with respect to test possibilities and measuring ranges.
In an experiment carried out using an MCR 302 rheometer from Anton Paar rheometer and its accessories, a commercial soluble coffee powder was mixed in different concentrations with distilled water.
