monkeymath
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In the thread Maturation rest for lagers we sort of went off-topic and started discussing yeast starters, and I backed my position - which is not to cold crash your yeast starter - by an appeal to authority, claiming this was supported by advice from Ulrich Preise, head of the yeast bank of Weihenstephan. Since the article I derive this from is entirely in German, and it might be of general interest, I thought I'd share a sort of transcript/summary here. To those of you who do speak German: please do not point out the parts where deviate from the original. These are deliberate lies which tie into a much bigger evil master plan.
I hope it can be of use to some of you.
The article itself is not written by Ulrich Preise. A couple of homebrewers from Munich had the opportunity to visit the yeast bank and ask Ulrich numerous questions. The article arose from these questions, but probably the author - Moritz Gretzschel, who is himself a wealth of knowledge on homebrewing - filled in some parts on fundamentals of yeast physiology etc. where he found it fit. This is just a guess on my part. Either way, I am certain that Ulrich Preise double-checked all the claims made in that article and that no major misinformation is contained therein.
The article is openly available online, so I hope it is fine to try and make its content available to a broader audience. I possess no rights to the article.
This is not a reproduction of the article, just an attempt to make the information contained therein available to a broader audience. I omitted some parts on closely related topics such as hygiene and yeast storage.
So let's jump right into it.
Fundamentals
There are two main aspects to consider for a starter. Yeast quality and yeast quantity. Let's talk about yeast quality first.
To find the optimal procedure for yeast propagation, we need to first take a look at the different stages of yeast reproduction. In terms of the number of active yeast cells, we can divide the fermentation into four stages:
In terms of yeast age, on the other hand, we may distinguish the following four stages:
If we were to retrieve our yeast pitch from currently fermenting wort, it'd be advisable to do so when fermentation has just started rolling at full power (beginning Hochkraeusen), typically after 48 to 72 hours [your mileage may vary (greatly)]. For bottom fermenting beers, take some fermenting wort with yeast in suspension; for top fermenting beers the yeast swimming atop.
From that it is also apparent that, also when making a starter, one should not wait for the yeast to settle out and should not, as is often recommended, decant the supernatant: yeast only fully drops out after fermentation has finished. The material at the bottom contains a high percentage of middle-aged and old yeast cells, near the end of their activity. Instead, keep your starter on the stir plate and pitch it entirely into the next stage once it has reached the end of log phase (or the beginning of stationary phase). That way, the yeast quality will be at its peak.
To aerate or not to aerate?
With the method outlined above, continuous aeration is not necessary, in particular since that always harbours a risk of infection. If you own a stir plate, it is sufficient to run it at a high speed at the beginning, so that the vortex is audibly pulling in air. Afterwards [no mention of the precise timing is made, but I assume it should be after some 2-4 hours], the speed can be reduced, so that a vortex of only 2-3cm depth remains, which signals healthy mixing.
With an unaerated starter, it is not quite as crucial to pitch at exactly the right time as it is when aerating your starter. Moreover, should you wish not to pitch the entire starter - e.g. due to the color of the starter wort - , then given enough time you can wait for the yeast to drop out, in spite of the above.
Pitching rate
For bottom fermenting yeast, aim for 1-1.2*10^6 cells/ml per degree plato. For top fermenting yeast, aim for 0.5-1*10^6 cells/ml per degree plato.
Propagation steps
As homebrewers, we cannot truly work in a sterile environment, so we should ensure a quick take-off at every step of the propagation to leave no chance to any unwanted microbes. So let's apply the same pitching rate(s) as above to the starter wort used in the propagation and shoot for 10*10^6 yeast cells per ml. Under the right conditions (stir plate, 20 degrees celsius, sufficient nutrients) yeast should reach a concentration of 100*10^6 cells/ml within 24 hours, so this naturally suggests to always pitch at a ratio of 1:10 between each step (e.g. let a starter of 100ml wort work for 24 hours, then pitch into 1l of wort). Again, it is recommended not to decant the individual starters, but to pitch them entirely into the next stage.
To play it safe, homebrewers might be better off at a propagation ratio of 1:8. When working with older yeast or bottle dregs, you should start with much smaller steps of 1:2 or 1:3. Only the final step before pitching should always adhere to the ratio of 1:10.
Propagation wort
The ideal medium for yeast growth is regular wort. Of course, not everybody wants to produce a separate batch a couple of days prior to brewing just for propagation (although a simple no-sparge, 1 hour at 68 C, batch would be entirely sufficient). Left over (and then immediately frozen) wort from the last brew day is a great option: just dilute to the right strength and boil to sterilize. While boiling, you may pitch some yeast nutrient or even old yeast.
Otherwise, you may also use malt extract to create a wort of 6-8 degree plato, ideally with some yeast nutrient. Those that cool over night ["no chill"] might also perform the last (large!) propagation step with diluted and boiled first runnings while the wort is slowly cooling.
I hope it can be of use to some of you.
The article itself is not written by Ulrich Preise. A couple of homebrewers from Munich had the opportunity to visit the yeast bank and ask Ulrich numerous questions. The article arose from these questions, but probably the author - Moritz Gretzschel, who is himself a wealth of knowledge on homebrewing - filled in some parts on fundamentals of yeast physiology etc. where he found it fit. This is just a guess on my part. Either way, I am certain that Ulrich Preise double-checked all the claims made in that article and that no major misinformation is contained therein.
The article is openly available online, so I hope it is fine to try and make its content available to a broader audience. I possess no rights to the article.
This is not a reproduction of the article, just an attempt to make the information contained therein available to a broader audience. I omitted some parts on closely related topics such as hygiene and yeast storage.
So let's jump right into it.
Fundamentals
There are two main aspects to consider for a starter. Yeast quality and yeast quantity. Let's talk about yeast quality first.
To find the optimal procedure for yeast propagation, we need to first take a look at the different stages of yeast reproduction. In terms of the number of active yeast cells, we can divide the fermentation into four stages:
- The lag phase or latency phase. Broadly speaking, the yeast is adapting to the wort. It analyzes the nutrients available and prepares its metabolism through the activation of enzymes. Countless processes happen within the yeast cell, but they do not yet impact the wort. The brewer waits for fermentation to start.
- The log phase. The yeast is fully active, there is still an abundance of nutrients and oxygen. The yeast reproduces at a constant rate over several generations. Fermentation starts.
- The stationary phase. The wort is depleted of oxygen, nutrients are diminishing, the yeast cells feel it's getting crowded. We find an equilibrium of yeast reproduction and yeast death. The brewer observes a continued, but slowed down, alcoholic [= anaerobic] fermentation and a decrease in extract content.
- The lethal phase. The wort is depleted of nutrients, the yeast starves to death or dies of the products of its own metabolism. Fermentation comes to a halt.
In terms of yeast age, on the other hand, we may distinguish the following four stages:
- "children", that have just been created, but are still unable to reproduce,
- "young adults" in the prime of their youth, reproducing like bunnies,
- "adults" whose reproduction rate is slowly declining due to "bud scars" [no idea if that's the right word] which inactivate their surface,
- "seniors" which have a reduced metabolism and will die soon.
- in terms of yeast age, we want to have as many "children" and "young adults" as possible (and consequently few seniors)
- in terms of yeast activity, we want the starter to be at the transition from log phase to constant phase (for maximum yeast quantity at maximum activity)
If we were to retrieve our yeast pitch from currently fermenting wort, it'd be advisable to do so when fermentation has just started rolling at full power (beginning Hochkraeusen), typically after 48 to 72 hours [your mileage may vary (greatly)]. For bottom fermenting beers, take some fermenting wort with yeast in suspension; for top fermenting beers the yeast swimming atop.
From that it is also apparent that, also when making a starter, one should not wait for the yeast to settle out and should not, as is often recommended, decant the supernatant: yeast only fully drops out after fermentation has finished. The material at the bottom contains a high percentage of middle-aged and old yeast cells, near the end of their activity. Instead, keep your starter on the stir plate and pitch it entirely into the next stage once it has reached the end of log phase (or the beginning of stationary phase). That way, the yeast quality will be at its peak.
To aerate or not to aerate?
With the method outlined above, continuous aeration is not necessary, in particular since that always harbours a risk of infection. If you own a stir plate, it is sufficient to run it at a high speed at the beginning, so that the vortex is audibly pulling in air. Afterwards [no mention of the precise timing is made, but I assume it should be after some 2-4 hours], the speed can be reduced, so that a vortex of only 2-3cm depth remains, which signals healthy mixing.
With an unaerated starter, it is not quite as crucial to pitch at exactly the right time as it is when aerating your starter. Moreover, should you wish not to pitch the entire starter - e.g. due to the color of the starter wort - , then given enough time you can wait for the yeast to drop out, in spite of the above.
Pitching rate
For bottom fermenting yeast, aim for 1-1.2*10^6 cells/ml per degree plato. For top fermenting yeast, aim for 0.5-1*10^6 cells/ml per degree plato.
Propagation steps
As homebrewers, we cannot truly work in a sterile environment, so we should ensure a quick take-off at every step of the propagation to leave no chance to any unwanted microbes. So let's apply the same pitching rate(s) as above to the starter wort used in the propagation and shoot for 10*10^6 yeast cells per ml. Under the right conditions (stir plate, 20 degrees celsius, sufficient nutrients) yeast should reach a concentration of 100*10^6 cells/ml within 24 hours, so this naturally suggests to always pitch at a ratio of 1:10 between each step (e.g. let a starter of 100ml wort work for 24 hours, then pitch into 1l of wort). Again, it is recommended not to decant the individual starters, but to pitch them entirely into the next stage.
To play it safe, homebrewers might be better off at a propagation ratio of 1:8. When working with older yeast or bottle dregs, you should start with much smaller steps of 1:2 or 1:3. Only the final step before pitching should always adhere to the ratio of 1:10.
Propagation wort
The ideal medium for yeast growth is regular wort. Of course, not everybody wants to produce a separate batch a couple of days prior to brewing just for propagation (although a simple no-sparge, 1 hour at 68 C, batch would be entirely sufficient). Left over (and then immediately frozen) wort from the last brew day is a great option: just dilute to the right strength and boil to sterilize. While boiling, you may pitch some yeast nutrient or even old yeast.
Otherwise, you may also use malt extract to create a wort of 6-8 degree plato, ideally with some yeast nutrient. Those that cool over night ["no chill"] might also perform the last (large!) propagation step with diluted and boiled first runnings while the wort is slowly cooling.