Contact with espresso can destabilise milk foam. Have you ever noticed the border around a nice latte art design start to break up? This is a very slow-developing phenomenon, but a distinct ring of larger bubbles can often be seen around a drink that has been left untouched for five to ten minutes. The white parts of a latte art design that have not come into contact with coffee will be more persistent than the foam which has mixed with coffee and taken on a brown colour. One possible explanation for this breakup of foam can be the gradual merging of bubbles through the diffusion of gas from smaller bubbles into larger ones. This effect is known as Ostwald ripening (OR):
‘Inert gas diffuses through thin films from the smaller to the larger bubbles because of the difference in their capillary pressures, thus resulting in the growth of larger bubbles at the expense of smaller ones, (Narsimhan 1991). Narsimhan points out that the timescale of interbubble diffusion is much larger than that of drainage, so it is only relevant to beverage presentation after the latte art has been poured and left to stand for a few minutes.
In this simulation, you can see smaller bubbles disappearing as larger ones increase in size. See the video in the next lesson to observe this phenomenon in milk foam.
Professor Abbott points out an important way to avoid Ostwald ripening advancing too quickly in milk beverages:
‘One other message that emerges from the Ostwald modeller (and makes intuitive sense) is that the narrower the original distribution of bubble sizes, the less the ripening — with fewer bigger bubbles in the distribution, there is less of a pressure gradient to power the diffusion process. What that means is that if you successfully produce a very luxurious and finely textured foam, you get twice the value, because it lasts longer too.’