If you have ever made a salad dressing, you will have witnessed the oil floating on top of the vinegar. Similarly, the fats in milk are less dense than the water and will float to the surface. Traditionally, milk was left to rest to allow this natural separation process to occur, and the floating cream was then decanted. It is now common to use centrifuges known as separators for this purpose.
Rupert does not skim his milk at all before dispatching it out to cafes in and around London. His cows’ milk fat content hovers around 4%. Different breeds of cows can produce milk with a higher fat percentage. Notably, the Jersey and Guernsey breeds produce milk with as high as 5.2% fat content.
Baristas often debate the virtues of higher or lower fat content in milk. They witness differences in the foamability of skimmed milk compared with that of semi-skimmed or whole milk. ‘Foams produced from skim milk and whole milk differ considerably in their appearance and bubble size distributions with whole milk foams showing smaller sized bubbles and higher rates of bubble rupture as a result of coalescence during storage’ (Kamath, S., Huppertz, T., Houlihan, A. V., & Deeth, H. C., 2008). Coalescence means the bubbles merge into each other, forming larger bubbles that feel less silky.
This is both good and bad news for folks who love whole milk (aka full-cream milk) in their coffee. Foam from whole milk features small bubbles and a more luxurious-feeling texture than foam produced from skimmed milk, but it doesn’t last as long as skim milk foam. Whole milk foam is more likely to coalesce as the fats in the foam cause it to break down faster.
The following images have been drawn to illustrate research results from Kamath et al., 2008. In spite of the expected increase in bubble rupture of whole milk foams, at the half-life, the bubbles are still considerably smaller and numerous than those of skimmed milk.