Fermentation Theory: Oxygenate the starter, not the wort

I've been looking into yeast fermentation performance and the research literature indicates that once yeast that have undergone respiration and have built up their reserves of sterols, they should be able to transition directly to anaerobic fermentation. This implies that if a sufficient number of yeast are propogated in an optimally oxygenated starter, it should be possible to pitch those yeast into an un-aerated wort with no undesirable effects, since the yeast have already synthesized all the necessary sterols and unsaturated fatty acids. Provided that an appropriate amount of yeast have been pitched, the fermentables in the wort should be exhausted before the yeast's sterol reserves. This would produce an extremely clean fermentation, since a host of undesirable compounds, including diacetyl, require oxygen in order to form.

So this brings up the practical question of how to pitch yeast at the height of their reserves, before they start exhausting those reserves in anaerobic fermentation. Since yeast will fully scavenge all dissolved oxygen before transitioning to anaerobic fermentation, it should be possible to oxygenate the starter to optimal levels (say 15 ppm), and then use a dissolved oxygen meter to determine precisely when the yeast have consumed all of the oxygen. At that point I could then pitch the whole starter into 5 gallons of wort.

Has anyone tried this approach? Can anyone think of any potential pitfalls that it would encounter?

Wouldn't you need a huge starter to pitch enough yeast for them not to have to reproduce?

You would need to pitch somewhere around 336 - 672 billion cells in a 5 gallon wort.

Normal pitch rate is about 168 billion cells (Mr Malty). Yeast normally double 1 to 2 times during the aerobic growth in the fermentor (wyeast powerpoint).

One doubling = 168 X 2 = 336 billion.
Two doublings = 336 x 2 = 672 billion

Mrmalty suggests that for 674 billion cells, you would need a continuously aerated starter of 3.9 L (about 1 gallon). The yeast are going to keep growing until they run out of nutrients, but if you keep them aerated, they should not enter fermentation cycle. So I do not think any special monitoring is needed - just give them time to grow to saturation.

But - your beer would lack flavors associated with the growth phase. For some styles this might be OK. The wyeast ppt suggests that for an German Alt in a Pale wort, a high pitching rate (less growth) was preferred. However, in an Amber wort for a Weihenstephan wheat, a lower pitching rate gave more flavour and was preferred.

Yeah, that is something I thought about too. The short answer is that the starter wouldn't have to be larger than normal, but the number of yeast you pitch into the starter would. So, ultimately, this method would be more expensive, in terms of the cost of the yeast, because you would have to buy more Activator packs in order to obtain the desired pitching rate, since you are not relying on the starter for yeast PROPOGATION. Rather, the starter is used to prepare the yeast for cell division, which can happen in the nutrient and fermentable rich environment of your primary fermenter.

So the starter would just have to be large enough to accomodate the OXYGEN needs of the yeast. For instance, if I wanted a pitch rate of about 16 million cells per ml, that would be about 302 billion cells per 5 gallon batch, or 3 Wyeast Activator packs. What I am uncertain about, at this point, is how much optimally oxygenated starter wort would be sufficient to allow all 300 billion cells to fully synthesize all of their necessary sterol reserves. I haven't found any research that gives me a number for this. If anything it would be LESS than the amount that you would normally use for a starter. So, to be safe, I would probably use 1 liter for every 100 billion cells.

But a secondary question is whether the starter could be cold-crashed without affecting the yeast's need for further oxygen. If it COULD be cold-crashed and the yeast slurry could be extracted from the starter, then the size of the starter would not be an issue.

However, even if the starter were NOT cold crashed, pitching the starter into the primary would not be as much of an issue, because the wort being pitched would be mostly un-fermented, since we are pitching the starter at the end of the yeast's lag phase.

BioBeing, thanks for that info. I posted before reading your reply. I was assuming that no actual cell division took place during the aerobic phase. So this would reduce the number of Activator packs that I would need to pitch into the starter.

I tried it once, and it worked.

But I heard Jamil saying, that he tried it with some lagers and he didn't like the results. Beer was too clean, too sterile. You need some yeast growth, even in lagers.

Piotr, I'd love to hear the details about your experiment. What was your pitching rate, how many yeast packets did you use, did you monitor the dissolved oxygen, did you pitch the entire starter or just the slurry, etc.

Yeah I can see how this might produce a beer that is even TOO clean. But it is always easy to adjust the conditions to produce a less clean beer -- increase fermentation temp, add a little dissolved oxygen to the primary, reduce pitching rate, etc.

BioBeing, I finished reading that Wyeast powerpoint, which is awesome. The guy who wrote it, Greg Doss, is the same guys who answers questions that you send to Wyeast. Very helpful guy.

Anyway, the powerpoint doesn't seem to indicate how much cell growth actually occurs during the aerobic stage. The experiment Wyeast performed just indicates the terminal cell growth as a result of different levels of oxygenation. So it takes into account cell growth during the aerobic and anaerobic phases.

In general, unless the starter is CONTINUOUSLY aerated, I would think that little cell growth would occur during the aerobic stage. Yeast prefer aerobic conditions and will definitely multiply at a fast rate in an aerobic environment after their sterol/glycogen reserves have been filled, but cell division cannot occur until these reserves are built up. So I am unclear as to how long and how much oxygen and what starter sizes are necessary just to provide the cells with an opportunity to build up their reserves.

However, if the starter was continually replenished with oxygen, it would seem that this would result in cell division without an appreciable depletion of reserves. So if you had a stir plate and oxygenated the stater once an hour, you should be able to build up your yeast count in the starter and ALSO pitch into un-aerated wort. But, in this scenario, the wort would be substantially fermented (i.e. yucky tasting) and you would want to cold-crash and decant off the spent beer.

stoutaholic said:

BioBeing, I finished reading that Wyeast powerpoint, which is awesome. The guy who wrote it, Greg Doss, is the same guys who answers questions that you send to Wyeast. Very helpful guy.

Anyway, the powerpoint doesn't seem to indicate how much cell growth actually occurs during the aerobic stage. The experiment Wyeast performed just indicates the terminal cell growth as a result of different levels of oxygenation. So it takes into account cell growth during the aerobic and anaerobic phases.

Click to expand...

Hmmm - true. But I assume the majority of the growth occurs during aerobic growth rather than anaerobic - it is so much more efficient for the cells.

stoutaholic said:

Piotr, I'd love to hear the details about your experiment. What was your pitching rate, how many yeast packets did you use, did you monitor the dissolved oxygen, did you pitch the entire starter or just the slurry, etc.

Click to expand...

Oh, it wasn't a real experiment. I had a batch of small beer 1.032 and a packed of dry yeast US05, I calculated that whole packet is too much and I taught "what if I pitch them all but don't oxygenate" and it worked - fermenation started in <24h, and fermented nicely to 1.008.

EDIT: BTW: even with no oxygen yeast can multiply, using sth (streols? lipids?) from trub

Right Piotr, I have pitched dry yeast in aerated wort and unaerated wort and had a slightly longer lag time with the unaerated to the tune of about 8 more hours. Results were based on my observations of when the airlock started ticking away. My fermenters are airtight.

My last batch of APA I did not aerate (on purpose) and I noted that when I woke up in the morning that the airlock still had no movement - no CO2. I thought it was odd since normally they are going strong within 12 hours of pitching, so I went off to work, and when I came home it was already ticking away.

Nice research by the way. I thoroughly enjoy topics like this!

I was asking myself the same question and ran into this old thread.
http://maltosefalcons.com/tech/yeast-propagation-and-maintenance-principles-and-practices
talks a lot about yeast propagation. Even with a small amount of yeast, it appears that you could get about half the needed population in a 1 L starter with a stir plate. Wondering if anyone else has tried this.