Five Foam Factors, Revisited
At the start of this chapter we talked about Professor Abbots Five Foam Factors that were conducive to great foam for latte art. First, we wanted foam that was wet, with a relatively low aeration percentage. The sweet spot for Barista Hustle was around a 33% aeration level. Less than 20% aeration is too wet for latte art purposes, and foam above a 50% aeration level is too dry.
Second, we wanted fine bubbles with a small diameter. The whey protein found in milk contains β-lactoglobulin, a surfactant that helps us achieve this second foam factor. Only a surfactant at the interface of bubbles will lower surface tension. Lower surface tension results in smaller, finer bubbles.
Third, surfactants in milk foam help not only with size but also with stability — bringing us into compliance with the first half of Foam Factor Three. Milk foam above 37°C will remain stable, satisfying the second half of Foam Factor Three.
Fourth, Foam Factor Four calls for latte foams having moderate viscosity. We obtain this, in particular, with unhomogenised whole milk. The micelle structures of the milk fat globules and the casein proteins enhance the luxurious mouthfeel of milk foam.
Finally, for Foam Factor Five, we want a low yield stress to allow for easy pouring of latte art. The more time that has passed since you’ve aerated your milk, the more your yield stress increases. Pouring within 10 seconds of aerating your milk, or spinning your milk pitcher, will combat this increase in yield stress.