Here's some info from John Palmer that may have a bit of relevance...
100-150 years ago, fermentation was open, followed by maturation in a wooden cask. The beer
was prone to contamination. This could be mitigated by heavy hopping and long warm
maturation to wait for the bitterness to die down, or by long cold maturation (lagering) to use
temperature to keep the contamination down.
Yeast have 3 phases in their life cycle: Adaptation, High Growth, and Stationary. (See Yeast by
CW and Jamil) They do not have a maturation phase where they clean up byproducts. Adaptation
phase is where they take in oxygen and build sterols and other lipids, assess the sugar
composition and build enzymes, etc. Once those activities are done, they start the High Growth
Phase, eating and reproducing. The number of cell divisions is limited by their lipid reserves they
made during Adaptation. These reserves are shared with each daughter cell. When those lipid
reserves are exhausted, the cell stops reproducing. In addition, when those reserves are
exhausted, the cell is old and cannot eat or excrete waste efficiently across it’s cell membrane. A
yeast cell typically can reproduce about 4 times during a typical fermentation, after that it is old
and tired and tends to enter Stationary phase where it shuts down most of its metabolism and
flocculates, waiting for the next batch of aerated wort. Stationary phase is essentially an
inactivity phase, resting on the bottom.
Like I said, no conditioning phase as far as the yeast are concerned. Byproducts can be consumed
at any point during the high growth phase, but they are a lower energy source than sugar, so
guess what? Byproducts are not a biological priority. The brewer therefore needs to plan his
pitching rate and fermentation conditions such that the yeast run out of fermentable wort sugar
before their lipid reserves are exhausted and they go into stationary phase. Now you have a
majority of vigorous yeast that have only undergone 2 reproductions (for example), the sugar is
gone, and they are still hungry, so they turn to acetaldehyde and diacetyl as alternate energy
sources and maturate the beer. You can help this by doing a diacetyl rest by raising the
temperature a few degrees after the first half of fermentation, to keep the yeast active and eating.
Where in the fermentation? after the first half, 2/3 to 3/4, when most of the attenuation has
occured and raising the temperature is not going to cause rampant growth and the off-flavors
associated with it.
Today, we have closed stainless steel tanks which allow us to prevent oxidation, pull the yeast,
and control the temperature. This plus our understanding of the yeast cycle above changes the
way we ferment lagers, so now lager beer fermentation is started cooler to control yeast growth
and allowed or controlled to rise during fermentation to the diacetyl rest, such that ALL of the
fermentation and maturation is complete before the beer is cooled to lagering temperature. The
effect of temperature at this stage is strictly physical, increasing the strength of hydrogen bonds
to coagulate beer haze and help it settle out. The yeast are still susceptible to temperature shock
and lipid excretion, so the cooling to lager temperature 35-38F still has to be slow, i.e. 5F per
Please note that this behavior and fermentation technique is applicable to ALL beers, not just