Stability, temperature dependence, and microstructure of high pressure jet-treated dairy foams

The objective of this work was to study the effects of high pressure jet (HPJ) processing on the foaming of whole milk at different temperatures. High pressure jet-treated whole milk (200 and 400 MPa) was foamed at 5, 20, 40, and 50 °C using a custom-made air injection device and the foam expansion and foam stability was compared to a non-HPJ-treated control. Aliquots of the foams (formed at 5 and 50 °C) were extracted for proximate analysis and visualization using confocal scanning laser microscopy (CSLM). The relative foam expansion values for the non-HPJ-treated sample at 5, 20, 40, and 50 °C were 45.5 ± 11.3%, 0 ± 0%, 79.6 ± 9.7%, and 88.2 ± 2.8%, respectively while the 400 MPa-treated sample foam expansion values were 36.4 ± 10.8%, 60.8 ± 5.7%, 50.0 ± 5.3%, and 71.7 ± 3.0%. Notably the HPJ-treated sample foamed at 20 °C while the control milk did not foam at all. Furthermore, the 400 MPa-treated foam was stable for >2 h at all foaming temperatures, unlike the non-HPJ-treated whole milk foam, which completely collapsed in 60 s and 1490 s after being foamed at 5 °C and 40 °C, respectively. CSLM and proximate analysis of the non-HPJ-treated sample revealed differences in foam composition based on foaming temperature with more fat present in the foam formed at 50 °C (3.2 ± 0.3%) than at 5 °C (2.5 ± 0.1%), likely due to the ability of liquid oil (at 50 °C) to migrate to the air-water interface during air incorporation. In contrast, there were no compositional differences in the foams from 400 MPa-treated milk based on foaming temperature, reflecting that there are structural differences, particularly in the fat, compared to the non-HPJ-treated foams. This was supported by evidence of fat-protein complexation and micron-size bubbles in CSLM images of the 400 MPa-treated foam.