At Moosehead we use our Lager yeast for a maximum of eight generations. That means we harvest the yeast after the first fermentation is complete, and use it again in the next batch – and so on for seven more fermentations. (We’ll cover this process in detail in our next yeasty post.)

When our current batch of yeast enters its fourth or fifth generation, we need to start the process of growing our replacement culture. This ensures we are always using a pure and healthy yeast strain.

Our yeast – the “mother culture” if you will – is stored frozen in a lab, and two slants are sent to us six times a year.

It comes to us like this:


It looks like a very small beer, doesn’t it?

But in fact it isn’t liquid at all. What you’re seeing is white yeast cells growing on UBA (Universal Beer Agar), a solid growth medium designed to mimic the sugar content and nutrients present in wort.

Inoculating a full 525 Hectolitre batch of wort isn’t as simple as shaking out those few yeast cells and letting Mother Nature take its course. Not by half.

Our lab techs are responsible for propagating – or growing – our yeast, step by step, until the new culture is large enough to be handed off to the brewing team.

The first step is to pull a few of those original yeast cells out of the test tube and paint, or “streak” it onto a larger dish of UBA. Twenty millilitres of sterile wort is added to the dish, and it is left to grow for 18-24 hours.

When it’s ready, it looks like this:

yeast colonies

Three to five colonies of yeast (the little round guys on the dish above) are removed, and then added to 200 mls of sterile wort in a larger container. This is allowed to grow, and is then in turn transferred to a vessel containing a litre of sterile wort. After one further growth stage, the yeast is transferred to a vessel called a Carlsberg Flask.

Carlsberg flask

The Carlsberg Flask is a great tool because it is a self-contained sterile propagation system. This means the yeast can leave the lab and still stay sterile.

Here’s how it works:

  • The flask is filled to 80% capacity with wort, which is then sterilized using a burner or autoclave. Steam is allowed to escape the flask during this process.
  • The wort is cooled by refrigerating the flask.
  • The wort is oxygenated by injecting sterile air into the flask using an aeration lance.
  • Next, the yeast culture is injected into the flask using a syringe.
  • The yeast begins to propagate. CO2 is allowed to vent during this phase.

When it is ready, the new yeast culture is used to inoculate an even larger propagation vessel containing 30 hectolitres of sterilized wort. Aeration and yeast growth continues in the 30 hl self-contained propagator for three 3 days, which is the time required to achieve sufficient yeast mass to pitch a full-sized batch of wort.

Yeast propagator     yeast propagator2

Because we don’t want to stress this new yeast culture, we give it a set of training wheels for its first time in the fermenter – allowing it to go to work on wort with a low original gravity. (Just a baby-sized amount of sugars to convert.) To further protect the yeast, we also add a small amount of yeast food to help bolster the new cells. It’s a little like giving nutritional supplements and vitamins to your growing kids.

After 24 hours in the fermenter, during which time the yeast culture has doubled in size, we add a second batch of wort and more sterile air, allowing yeast growth to continue. After another 24 hours we do it again, adding a third batch of wort and more sterile air, allowing the yeast to double in size again. This process is called “doubling”, and what it does is create a big, healthy, well-concentrated yeast culture we can use in successive generations.

We allow these three brews to ferment and attenuate normally, cropping the yeast after the fermenter goes into cool down. The yeast cropped from this fermenter is now of sufficient volume and concentration to pitch into successive brews.

Because the doubling process involves much more yeast growth than we would normally see in a regular fermentation, the flavours produced by the yeast will be slightly different (for example, different levels of esters and sulfur compounds will be produced). It is for this reason that we choose to blend the beer resulting from the doubling process with batches produced using a regular fermentation. We do this for consistency reasons – the propagation beer tastes just fine, but it might taste just slightly different than a non-propagation beer. Blending will reduce the flavour impact of these slight variations down to negligible amounts.

And so ends our propagation story!


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