The Ten-Second Rule: In this slo-mo video watch closely as milk is poured in the espresso to see the liquid slowly draining out of the foam. To successfully pour latte art, you much complete you pour before the drainage has advanced too far — in practice we advise you to start pouring within 10 seconds of completing the steaming process.
When the bubbles in a freshly poured latte appear to be falling down instead of floating up, what you are actually seeing is liquid draining out of the bubbles. This is due to the process of drainage. From a taster’s sensory perspective, slow draining can be very pleasant, imparting a velvety texture. From a practical, latte art perspective, the 10-second rule is something to aim for — i.e., begin pouring in no longer than 10 seconds after steaming is finished. Otherwise, the drainage of your foam will have advanced too far and your milk will separate into a liquid layer and a dry foam layer.
An experiment compared the impacts of casein protein and lactoglobulin in the magnitude of milk foaming (Borcherding et al 2009). The data revealed significant differences. The elastic nature of the lactoglobulin foam was such that the drainage was not complete even after 24 hours of observation. The researchers also noted that drainage rates were affected by the casein-to-whey ratio:
‘Foams prepared from samples with whey:casein [ratio] of 20/80 exhibited the lowest drainage values after 10 min’.
This represents roughly a 10% higher whey protein level than is commonly found in commercial whole milks. If you want to play with this idea, you could try adding a whey protein isolate with a high lactoglobulin content to your milk. Those who want slow drainage without changing the milk properties can explore one clear rule that is described in this foam drainage app:
‘halving the bubble size quadruples the draining time.’