Effect of milk homogenisation and foaming temperature on properties and microstructure of foams from pasteurised whole milk

Differently homogenised and HTST-heated milk (3.5% fat) was foamed at temperatures between 4 and 60 °C. Foaming was achieved by air injection through fritted glass. Initial foam density, drainage and corresponding bubble size were analysed. Transmission electron microscopic (TEM) images completed the study. The studies showed that whole milk was better foamable between 50 and 60 °C than at lower temperatures. This was mainly due to the completely liquid milk fat and the increased protein adsorption at the air–serum interface. The resulting bubbles of these two foams maintained their spherical shape also for 20 min of draining. However, the average bubble diameter and the drainage mass in relation to the initial foam mass increased from about 20 g/100 g after 1 min to about 80 g/100 g after 20 min. It was surprising to learn that milk homogenisation and corresponding fat globule size had only marginal effect on foam formation and stability. TEM images suggested that the air–serum interface consisted mainly of protein monomers and oligomers, while casein micelles were not directly adsorbed. The membrane of the homogenised fat globules was destroyed near the interface and coalesced liquid fat formed a restricted film on the bubble that was obviously of minor importance for the foam properties.