Formation and Stability of Milk Foams

Foamed beverages, such as cappuccino, have become very popular with consumers in recent years. Though the main appeal of such products lies in their milky frothy top, there is a lack of understanding on the liquid properties and process parameters driving froth quality. There are also no reliable methods to evaluate foam characteristics, since most studies to date focus on avoiding milk foaming during packaging and heat treatment, or on the behaviour of pure protein foams. The present study aims to understand the mechanisms of foam formation in milk, with a view to producing a froth that is more appealing to consumers. A methodology to characterize and distinguish between milk foams generated by two main methods—mechanical agitation and steam injection—is presented. The methodology involves characterizing the foam in terms of initial bubble hold-up, foam destabilization mechanisms and kinetics. Observation on foams formed by steam injection revealed that steam injection time does not have a significant effect on initial bubble hold-up even though it increases general foam stability. The increase on foam stability is most probably related to the greater extent of protein denaturation induced by increasing steam injection time. In the case of mechanically whipped milk foams, it was observed that higher milk temperatures resulted in dispersions having a lower bubble hold-up. The overall stability of the higher temperature foam expressed in terms of foam-to-liquid ratio was also lower as initial milk temperature increased. Higher milk temperatures induced by increasing steam injection time didn't affect foam stability, whereas whipping at higher temperatures had a strong destabilizing effect. This suggests that mechanisms of interface formation by the two methods differ and reinforces that an understanding of the changes occurring in milk during aeration by steam injection is essential to establish the differences between foams generated by both processes.