The term barm, what Germans call Kräusen, itself is mostly historical outside of traditional baking and brewing circles, but the underlying practice continues to this day, under modern names and tighter control.

Historically it was used to inoculate the next batch of beer and to leaven bread before pure, packaged yeasts were available, before people even knew what yeast was. In fact, medieval English speakers were known to call active barm “Godisgood”, reflecting the sense that this invisible agent was a kind of providential gift.

What is Barm?

In short, barm was a method (and the material) for yeast re-use before pure cultures became available in the late 19th century after Dr. Emil Christian Hansen isolated a pure cell of yeast.

During the course of fermenting an ale, a thick foam forms on top. This substance is the "barm," and it's full of active yeast, a bit of the wort, and CO₂ produced by the microbes. You'd collect it by skimming it off the top while you had a batch of beer actively fermenting.

Barm's Historical Role

There were two main historical roles for barm. They both have to do with the live yeast present in the substance, and they form the core of the brewer-baker link.

The Standard Way to Pitch Beer

How else are you going to pitch a beer other than with part of the previous batch? Well, of course there must be a way. How else could the first batch have come to be?

You'd have to create a barm the same way you'd create a bread levain from scratch, which is done by constantly refreshing of a flour/water mixture until lactic acid bacteria and yeast come to flourish in the slurry. This can take several days to weeks, and it's extremely impractical to do it often. It's much easier to keep a culture alive than start a new one.

So, you just skim the foam off the top of the current batch for use with the next batch. It wasn't just easy and thrifty, either: it also selected for yeasts that rose and formed a good head, reinforcing the character of the yeast and the resulting beer over generations.

The Brewer-Baker Connection

Because barm was abundant and lively, bakers prized it as leaven for bread and cakes. Just like brewers, bakers also kept a piece of their latest batch to inoculate the next one. If they didn't have such a substance, they'd either have to make it themselves, with some inconvenience as we discussed in the prior section, or find another leavening substance. Barm perfectly fits the bill for such a substance: it contains lively yeast and it's made of mostly the same stuff bread is.

Of course, in modern times the bakers have moved away from using barm. They overwhelmingly use commercial baker’s yeast or sourdough starters instead.

The Modern Echoes of Barm

The essence of this old practice is alive and well, but nobody calls it "barm" anymore. Most breweries now pitch pure, lab-propagated strains of yeast for the sake of consistency, essential when producing at such large scale.

Despite this, they do not pitch brand new yeast with every batch. Rather, they re-pitch cropped yeast (top-cropped from open fermenters or harvested as slurry from the tank cone) for many generations under tight quality control. It's usually called “cropped yeast,” a “yeast harvest,” or a “slurry.” This is essentially the same yeast-saving method as barm, with the same theory behind it, but with added attention to modern hygiene and record-keeping.