As a new brewer it took me quite a while before I was able to understand what was going on inside the LBKs during 'the fermentation'. I see posts here all the time from new and conditioned brewers alike about off flavors in their beer and how or why they have them. So this morning I decided to sit down with my morning coffee, since I couldn't sleep anyway, and try to set the record straight and try to explain the 4 things every brewer should know about yeast.
Here's how I view the relationship between yeast, off flavors and
fermentation temperatures throughout the typical beer fermentation.
While I'm writing this with brewing an Ale in mind the same principals
apply equally to Lager fermentations too.
Phase 1 begins as soon as you pitch your yeast and is referred to as the lag phase,
which we brewers want to keep as short as possible. The yeast are using
up the sugars and oxygen in the wort to load up their food reserves,
they won't ferment anything until they've been well fed. Stressing the
yeast out with too high temperatures or too low numbers of viable cells
will prolong the lag phase and the fermentation will take longer to
complete while increasing the amounts and types of off flavors like
diacetyl that may or may not ever condition out.
Phase 2 starts as soon as the lag phase ends because now the
yeast have enough energy stored up to start multiplying, this is
referred to as the growth phase. This is where you begin to see a
bit of foam floating at the surface the wort from the production of Co2
and the pH and oxygen levels of the wort will start dropping. If you've
pitched enough healthy yeast at the right temperatures into well
aerated wort the lag time should have been 6-12 hours and the yeast are
now full of energy and off to a very healthy start.
Phase 3 begins as soon as the growth phase is done and is triggered by a lack of oxygen in the wort, this is known as the fermentation phase.
This is the phase where the production of Co2, alcohol and your beer's
flavor is at it's peak and the wort temperature rises 3-5F higher than
the ambient air outside the fermentor. The yeast will stay in
suspension, so they come in contact with as many sugars as possible,
over the next 3-7 days before they run out of sugars to eat and
flocculate out to the bottom of the fermentor. Higher temperatures
during this phase will produce more esters or fruity flavors and aromas,
like the banana flavors in a hefeweizen. It's interesting to note that
another cause of ester production is wort that hasn't been aerated
Phase 4 is the final phase of the fermentation process and it's referred to as the sedimentation phase
where the yeast begin consuming and converting any remaining flavor
precursors in the wort like diacetyl that will produce off flavors in
your beer. During this phase the yeast cells are preparing themselves to
go dormant and storing up energy reserves for their deep sleep, even
though this is where most of us flush them down the drain. I'd like to
point out that the amount of cleanup work left for the yeast to do is
dependent on how well we treated, or mistreated, our yeast during the
first 3 fermentation phases.
During the sedimentation phase I raise the temperature of my fermentors
3-5F and hold it there for at least 3 days before bottling or kegging my
beer. I do this because the yeast will only convert the flavor
precursors it created earlier if it's warmer than it was when they
created them. There is a limit to this rule though since the yeast can
only do so much cleanup before they go dormant. Once the yeast go
dormant any remaining flavor precursors will be left in your beer to
produce off flavors.
The moral of this post is to always use fresh healthy yeast in
sufficient quantity pitched into well aerated wort at the recommended
temperature and you will produce some great tasting beer.