Effect of foaming temperature and varying time/temperature-conditions of pre-heating on the foaming properties of skimmed milk

The objective of this study was to investigate the effect of foaming temperature and impact of heat on the foaming properties of skimmed milk. Increasing foaming temperatures from 4 to 60 °C led to a decreased average foam bubble diameter (d10) from 0.26 to 0.18 mm. Temperature increase also led to increased foam density values (0.14–0.22 g cm−3). Overall, the skimmed milk foam that was prepared at 50–60 °C was the most stable, while minimal drainage was detected in the range of 30–40 °C. Pre-heating of skimmed milk was performed by high-temperature-short-time (HTST), high temperature (HT)- or ultra-high-temperature (UHT)-heating. The (d10)-values of the milk foam increased from non-heated to HT-heated milk. Foam density decreased in a continuous fashion from unheated to UHT-heated milk. The composition of non-denatured whey proteins in milk samples and the respective foams differed only marginally. Foam from unheated milk was the most stable. Transmission electron microscope-images revealed differences in foam microstructure in relation to the age of the foam and the heat treatment applied. The air–serum interface in the foam of skimmed milk consists of casein and whey protein monomers to oligomers. Casein micelles do not seem to be an integral part of the air–serum interface in skimmed milk foam.