[narcout]

On the last starter I do I think I’m going to let the starter ferment a bit and use the fermenting starter as my pitch.

A lot of people do that, and it's fine.  The only thing I would say is that sometimes starter wort that's been on a stirplate can develop some undesirable flavors.  I think most people that pitch the full volume of an actively fermenting starter will skip the stir plate and just aerate/oxygenate the starter right after pitching the yeast into it. 

When I’ve been aerating and turning the starter off at night there has been a slightly larger yeast cake, but no signs of fermentation.

That's not unusual, starters tend to ferment out pretty quickly.

Usually my starters grow a layer of head/foam. Why might that be? Is it the large presence of oxygen inhibiting fermentation in the starter?

That sounds like krausen, and is a sign of active fermentation.  Unless you've overoxgenated to the point where it's toxic to the yeast (not possible if you are only using air and not O2 from a tank), oxygen isn't inhibiting the fermentation of the starter.

[Saccharomyces]

AFAIK, Crabtree is only about the presence of glucose, not whether it's aerobic.

The Crabtree effect is based on percentage of glucose in the medium.  Below the Crabtree threshold and in the presence of O2, reproduction is respirative. Above the Crabtree threshold, all reproduction is fermentative, regardless of O2 level.  The difference is lies ergosterol and unsaturated fatty acid (UFA) reserves when maximum cell density is reached as well as the usage of adenosine triphosphate (ATP).  Cells produced under the Crabtree threshold in the presence of O2 have fully-charged ergosterol and UFA reserves.  Cells produced above the Crabtree threshold share the ergosterol reserves of their mother cells; therefore, a mother cell shares a portion of its ergosterol and UFA reserves every time it buds a daughter cell.  Dry yeast is produced using a continuous process that holds the medium in a steady state below the Crabtree threshold, which means that dry yeast cells have fully-charged ergosterol and UFA reserves.  That is why dry yeast can be pitched into poorly aerated wort.

As I have mentioned many times, a stir plate does not oxygenate wort after it is offgasing. All it does is keep cells in suspension.  One should be pitching or stepping a culture at high krausen, which means that a stir plate does not buy one much when propagating yeast cells.

When propagating a culture from dregs, one should start with about 25 to 50mls of 1.020 wort.  That culture can be stepped by a factor of 10, but a factor of 4 to 5 is more foolproof (50 -> 250ml -> 1L).  The culture should be stepped as soon as it reaches high krausen.  High krausen signals the transition from exponential growth to the stationary phase where reproduction is for replacement only.  I cover this area of yeast management in the following blog entry: https://www.experimentalbrew.com/blogs/saccharomyces/yeast-cultures-are-nuclear-weapons



Saccharomyces (a.k.a. S. cerevisiae)

[Robert]

Saccharomyces, I arrived while you were away, but given your reputation, I'm glad you're back, and of course well,  and I have the opportunity to avail myself of your knowledge.    I understand why the method of propagation from dregs you've just detailed is ideal.  Nonetheless, I have always had success with the simple (crude? ) method I recommended above:  a few steps, decanting in between (use of stir plate for initial oxygenation and maintaining yeast in suspension purely optional.)  It is convenient for those limited in equipment or the practical ability to precisely time steps to high kräusen.  So, my question:  does it actually do any harm to the resultant pitch and/or first generation fermentation, or is it simply inefficient in terms of yield over medium supplied?

[James K]

Ok so, this beer has been in the carboy for 10 solid days, it is still fairly active. After I switched from a blow off valve to airlock, I still had to replace the airlock twice because the beer was still blowing off.
The airlock is burping once every second-three seconds and the inside of the carboy still has movement.

Do you think this is the Brett working away? Or the farmhouse strain? I raised the temp from high 70s into the 80s yesterday but I do not see any signs of the beer slowing down.

I made another farmhouse beer 3 days after this experiment and it finished out completely and is already cleared up.  Also. Usually when I have made a beer with Brett I transferred into secondary and then pitched the Brett. Am I ok with leaving this beer on the trub for a long duration of time? I could transfer to secondary but don’t really care how clear the beer gets, I just don’t want to impart off flavors like autolyzing from leaving the beer on the yeast too long.

Thanks for the input. For now I just plan on letting this beer sit in the carboy another week-2weeks.

Update: current gravity is 1.002, so I think the Brett is working away, I taste the Brett.