Image by Sean McGrath (http://www.seanmcgrath.ca).

When we last looked in on our friend yeast – the tiny engine that creates the alcohol in our favourite beverage – it had just completed the fermentation process.

But before we leave the fermentation topic entirely, let’s rewind a little bit and talk temperature.

Lager yeasts enjoy fermentation temperatures between 7 o and 13 o Celsius (45o - 55 o F).

Ale yeasts ferment at a much wider temperature range – between 15 o and 21 o Celsius (59o - 70o).

Temperature must be tightly controlled during the fermentation process – because fermentation itself creates heat – and too much heat = yeast death.

Because lagers ferment at cooler temperatures, the process takes longer. Fermentation for ales takes 4-10 days, while for lagers it takes 1-2 weeks. (Note: homebrew recipes talk about longer fermentation times – but that’s because primary and secondary fermentation times are combined. At Moosehead we conduct one fermentation process, remove the yeast, and then allow the beer to mature during a conditioning phase.)

Near the end of the process, when the beer is 2-5 gravity points away from the desired final gravity, many brewers do what’s called a diacetyl rest, to ensure the remaining yeast cells reabsorb diacetyl compounds in the beer. It isn’t an absolute process – and not everyone does it. For lagers, many European brewers use a long and slow fermentation to accomplish the task, while others raise the temperature to 18 o – 20 o Celcius (65 o – 68 o F) for 24-72 hours. Ales, on the other hand, since they are already fermented at warmer temperatures, do not require a temperature boost for the diacetyl rest. They are simply allowed to rest for the same period of time.

Here is where we’re going to split our story into two threads.

 

The lager story

First, to the lager fermentation tank!

This tank is a cylindroconical tank – a tall cylinder, tapering to a cone shape at the bottom.

This type of tank takes good advantage of the fact that lager yeast is a bottom-fermenting yeast, which means the yeast clumps up and flocculates to the bottom of the tank.

Once our beer has arrived at the desired final gravity (for example, 4o Plato), we apply the cooling jackets to cool the beer temperature, and once it reaches our desired setpoint, perform a cold trub.

This is where we cool down the fermenter, which helps yeast, hop remnants, and coagulated proteins sediment out of suspension.

These items collect in the cone, creating a bottom layer that is largely composed of dead yeast, trub, and older yeast cells that flocculated out first. This layer is darker in colour than the rest.

The middle layer in the cone is largely composed of flocculated healthy yeast. This layer is lighter in colour, a soft yellow, and has a putty or custard-like texture.

The top layer is a thin yeast/beer mixture.

 

YEAST LAYERS2

Brewers will often harvest the middle layer for re-pitching into future batches of lager.

This is called “cropping”.

Cropping timing is pretty simple when it comes to lagers – once the yeast is in the bottom of the tank, it’s time.

How is it done? Easy. The brewer opens the tap at the bottom of the tank, and lets ‘er rip.

I know what you’re going to ask next: how does the brewer know what to save and what to dispose of?

This is a little more complex.

The brewer needs to run off and dispose the bottom layer, save the middle layer for
re-pitching, run off and dispose of the top layer, and then stop the flow so the remaining beer can enter the conditioning phase of the brewing process.

How does a brewer know when they’ve moved from one layer to the next?

They use a variety of tools:

  • They can evaluate the colour of the yeast coming out of the tank. The undesirable bottom layer is darker than the desirable middle layer.
  • They can measure it out – pouring off the bottom layer for a specified amount of time or a specified amount of volume, before switching to collection mode.
  • They can use a tool called a turbidity meter, which measures the concentration of suspended solids in a liquid. This is particularly valuable when it comes to deciding when the desirable layer has been poured off, and the brewer has moved into the undesirable top layer, which is thinner in composition.

 

Here’s what it boils down to:

With lagers, we take it all. First we open the hose and run off the trubby parts. Then when we see the good stuff coming down the line, we grab it and store it. When it thins out a bit – we turn off the tap and let things settle a bit. Then we open the hose again and take a little more. When the good stuff is done, we pour off the rest.

 

The ale story

Now, to the ale fermentation tank!

This is where it gets tricky.

Ale yeast, as you know, flocculates to the top of the tank.

The same three layers are created there, just in reverse.

 

YEAST LAYERS

 

There is a trubby top layer, a lovely middle layer, and a thin bottom layer. But here’s the thing…the brewer needs to harvest the yeast while it’s hovering there and fermentation is nearing completion. If they wait too long, the fermentation will slow, the tank will cool, and the yeast layers will precipitate down to the bottom of the tank.

The brewer needs to watch the gravity of the brew to determine when the exact right time to harvest the yeast. At Moosehead we target 4o Plato for “the save” as we call it. This can be measured with a hydrometer or a saccharometer.

Our brewers actually eyeball the tank as well, because it’s possible to see the drop start to happen.

In the ale fermentation tank, the harvesting process is called “skimming”. To do it, we use a mechanism that looks like a cone attached to an arm. The cone is dropped into the middle layer, and the desirable yeast is sucked up and out of the tank. We pump what we need into a storage tank, leaving the rest behind to complete the fermentation, at which point it will flocculate down to the bottom of the tank during cooling.

 

Here’s what it boils down to:

With ales, we don’t take it all. We just take what we need for storage and repitching. Then we let the fermentation finish, and allow the drop to happen. The trubby bits and leftover yeast then gets filtered out.

 

Planning the harvest

I’ve just made this sound very precise, but it needn’t be.

Basically, we decide how much brewing we’re going to be doing in the coming week, calculate how much yeast slurry we’ll need to inoculate our wort, and then pull off the right quantity during cropping.

The rest – even the good stuff – can be disposed of.

 

Quality assurance

Once we’ve harvested the yeast, we run quality assurance tests such as viability and cell count to be sure the yeast is healthy and ready for repitching.

 

 

Storing the yeast

Our cropped yeast is stored in a large tank called a yeast brink, at 2o-4o Celcius – to keep it dormant and reduce autolysis (cell death) – until it’s time to repitch.

 

Serial repitching

Our lager yeast is pitched and repitched (which is called serial repitching) up to 8 or 10 times, before the batch is destroyed and we start again. After 6 pitches, we repropagate  the colony, starting with a slant.

 

 

With notes from:

http://byo.com/equipment/item/739-harvesting-yeast-techniques

http://howtobrew.com/book/section-1/fermentation/secondary-or-conditioning-phase

 

 

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