Ferment Like a Yeast Starter?

Utilizing a stir plate when making a yeast starter increases your yeast cell count (due to improved oxygenating). While this is a benefit in terms of amount of yeast growth the consequence is that the yeast starter wort (beer) becomes oxidized:

“Decanting is required since the wort has been oxidized and fermented at a higher temperature than is optimal for most yeasts (75F-80F recommended). Though this temperature is excellent for yeast growth, it does not leave a very favorable flavor in the starter liquid.”

https://www.brewersfriend.com/2009/10/25/stir-plates-and-growing-brewing-yeast-quickly/

Needless to say but when you ferment the wort in the primary to produce beer you do not want the beer to have an oxidized flavor profile. It is best to just ferment as usual (e.g., 5+ days for an ale).

Cheers!

 

In addition to the oxidation problem, yeast starters have an inoculation rate (pitch rate) that is typically 5-20 times what you would use in a primary. In a starter the objective is to produce the most healthy yeast . . . pitching in a primary has an objective of making the best beer. So the starter is biased to what's best for the yeast (pitch-rate, temp, oxygen), the resulting beer is a by-product to be tossed. Your primary has a much lower pitch rate, typically a lower controlled temp, and an oxygen free environment (once it takes off) which is optimal for the best tasting beer.

When I harvest yeast dregs the first step is normally very small, (100ml). Once the yeasties come to life they can start and finish this tiny starter in a couple of hours (very easy to miss entirely). So the starter vs primary have different objectives and criteria.

 

I agree with the points that have been made, but I think they aren't completely responsive to the question. Sure, a starter typically has oxygen because the goal is to culture up healthy yeast. But that's not a necessary aspect of agitating the yeast. You could easily do it without the oxygen. Similarly with the pitch rate.

The question is what you would achieve by agitating yeast in the absence of oxygen, and I don't know the answer to that. Some have speculated that the purpose of agitation is not really oxygenation but rather getting carbon dioxide out of solution, or moving the yeast away from their waste products and toward nutrients. I think those things would happen with agitation even in an oxygen-free fermentation, so I'm not sure we can say a priori that it wouldn't work.

So in other words, I think it's an interesting question, and I don't think I've ever seen it answered anywhere. One potential downside for commercial brewers is that any improvement in fermentation time might be negated by increased time required to let the yeast settle. It also may disrupt the normal yeast settlement pattern, making it harder to harvest yeast for the next batch. These may not be concerns for some homebrewers. (These thoughts should be regarded as completely speculative by the way.)

 

^^I don't get it. What's with the picture?

 

The following discussion relates to yeast starters.

Kai Troester conducted a study to quantify the impacts of a yeast starter access to air (oxygen); in other words is yeast growth impacted by oxygenating the starter wort. He studied yeast starters under four conditions:

• Airlock
• Covered with foil
• No cover
• Air injected

The test results indicate that more access to air (oxygen) increases yeast growth.

"It is apparent that increased access to air results in more yeast growth."

http://braukaiser.com/blog/blog/2013/03/19/access-to-air-and-its-effect-on-yeast-growth-in-starters/

Cheers!

 

It is @PortLargo with the pics again. My assumption is it is wet cardboard i.e. oxidation. Will have to say I have been missing the double post pics lately

 

Got it. A little slow on the uptake lately. Blaming the cold meds.

 

I figured he ordered yeast with an ice-pak, and forgot to specify the "-pak" part.

 

You and I were both thinking too literally. Sometimes you need to think outside the box.

 

Some may have speculated but it's fairly clear that agitation does all the above. Increasing oxygen definitely increases yeast growth (see link from @JackHorzempa, et al), driving out CO2 has the same effect as increasing oxygen exposure, and keeping yeast in contact with wort logically speeds the process. In a starter this is all desirable. In the primary you certainly want to avoid continued oxygen exposure to prevent oxidation. As the oxygen is metabolized the CO2 hanging around has no real disadvantage (yeast is now growing anaerobically so what's the harm of CO2 exposure?). Although not normally done, agitation of the primary would speed things up a bit (who hasn't roused a primary to get the yeasties back in the game?).

For those on the scientific bent, the Pasteur Effect is the phenomenon that describes the two different metabolic pathways. Here's a pretty good description of how oxygen affects the process. The abbreviated version is aerobic fermentation is ideal for growing yeast with less alcohol created versus anaerobic fermentation being ideal for creating ethanol alcohol with increased yeast growth a side effect. I really believe that's the crux to the OP's question and the posts here fairly explain the applicable Brewer's Law.

 

I think this is mostly right, but I don't follow your point about oxidation. In a fermenter under an airlock what is the source of the oxygen you are worried about? To my mind agitating yeast in an oxygen-free environment does not present any risk of oxidation.

I also don't follow your point about CO2. CO2 is a waste product as far as the yeast are concerned, and I assume a steep gradient from inside the cell to outside the cell helps the yeast get rid of it quickly. (In other words, by driving off CO2 you make it easier for the yeast to respire.) Maybe not, but I don't see why CO2 would be undesirable in a starter but not in primary fermentation.

 

Any confusion is just the way I put the words together. In a normal primary there is no oxygen (after yeast consume the initial). The OP's thoughts about re-circulating with a pump would most likely add a degree of oxygen which would be very undesirable from a taste point of view and less efficient as aerobic fermentation. I agree agitating yeast in a primary is riskless and even sometimes an advantage, we've all given a primary a shake to help the yeast finish.

This is explained in my link above regarding Pasteur effect. Here's another good explanation of the difference in aerobic/anaereobic fermentation.

Bottom line is aerobic fermentation (i.e. starter) is ideal for producing the most yeast and is relatively inefficient for ethanol production. Anereobic fermentaion is much more efficient for producing ethanol and less efficient for producing yeast. This is what drives our decision to make a starter aerobic (stir plate/loose top) and a primary anereobic (sealed with air lock). In a starter you want to drive off the CO2 with agitation to allow more oxygen contact with each yeast bud. This is the perfect environment for yeast growth. In a primary there is no advantage of extracting the CO2 as the entire environment is oxygen-free after the lag phase. Those little yeast buds are happy without O2 and will produce ethanol more efficiently (but with comparatively less yeast growth). As CO2 is non-reactive to beer we just let it hang around till bottling.

The Kahn Academy has a very detailed explanation of all of this:
https://www.khanacademy.org/science...llular-respiration/v/lactic-acid-fermentation

This is long and tedious, but the videos do a great job explaining fermentation and how each atom is accounted for during the process. The concept is applicable to all cellular respiration and helps explain why you feel differently after a 100 yard dash versus a 5K jog.

 

This is where you lose me. I don't think dissolved CO2 shields the yeast from oxygen contact. I think the two gases operate independently—you could have a solution with lots of O2 and little CO2, lots of CO2 and little O2, lots of both, or little of both. I wouldn't view driving CO2 out of solution as an effective way to increase O2 exposure.

 

I would think only temperature or sugar content could do this. I like to give the fermenter a few swirls as a habit but i'm not sure if it helps since it's only 5 gals. My last starter i decided to skip the stir plate,added a little pure o2 and a airlock and it's working great.Watched a few videos where the just pass yeast back and forth in pales to aerate so i'm thinking a stir plate may not be needed.

 

From Yeast (White/Zainasheff): "The yeast do best when the starter setup continuously releases the carbon dioxide they create, keeps them in suspension and evenly distributed throughout the solution, and provides them with access to reasonable amounts of oxygen."

The chart and narrative from JackH's post (Results and Discussion), reinforces this concept.

I can not explain the interaction of CO2 and O2 at the mitochondrial level (that's where the action takes place) Not sure I understand your independent concern, but can state fairly certain that the gasses never operate independently, they always move to an equilibrium state. The only question is how fast (many variables). You'll never have an enclosed container with gasses that do not co-mingle if given enough time. However, in a starter there is only a matter of hours from start to finish. So getting the show on the road is of importance.

 

I don't think we need to overcomplicate it. The question is simply whether it's useful to remove CO2 from solution when there's no O2 in the environment. The way I understand your point is that CO2 removal is valuable because it makes O2 more accessible to the yeast. I don't think that's right as a matter of chemistry (the solubility of one gas isn't a function of the amount of any other gas dissolved in the fluid). As long as the yeast is excreting CO2, I would assume it's helpful to prevent its buildup. Of course maybe that's wrong, maybe it doesn't matter, but I don't see why it would be more helpful to remove CO2 in the presence of O2 than in its absence.

 

I know how some folks here feel about Brülosophy, but they did exactly what you’re proposing. You might want to take a look.

http://brulosophy.com/2016/10/17/th...tion-during-fermentation-exbeeriment-results/