Yeast and foam: how Protease A can break down your beer's head

Why does a beer's foam sometimes collapse after a few weeks? A detailed article on Hobbybrouwen.nl points to an unexpected culprit: the yeast itself. That was an interesting angle to me, because we usually look at malt, hops and clean glassware.

Foam is carried by proteins from barley malt, mainly LTP1 and Protein Z, together with bitter compounds from hops. Yeast, however, contains an enzyme, Protease A, that can cut those proteins. In a healthy cell it stays locked in the vacuole. Under stress, such as a shortage of nutrients, high temperature, high starting gravity or too little yeast, the cell releases more of it, and when cells die and their walls break down, the whole contents are released.

The tricky part is that the enzyme keeps working after the yeast is gone. Racking or filtering does not remove the dissolved enzyme, and in unpasteurised beer it slowly cuts proteins in the bottle. The effect is therefore most visible in beers that sat long on the yeast cake or were stored for a long time.

Shedding differs per yeast strain, sometimes by a factor of two to three. Clean American ale strains, highly flocculent English strains and lager yeasts are known for good foam retention. Saison yeasts need extra attention, because they ferment warm and deep and run short of nutrients towards the end. The author honestly says that manufacturers do not publish figures per strain, and that his table is a guide, not a ranking.

The practical lesson: pitch healthy and in ample quantity, feed the yeast, don't ferment too warm, and rack off in time. As a homebrewer you have a lot of control over those things.

Source: Hobbybrouwen.nl