Must aeration - a question I haven't seen answered

[henrik.peytz]

Hey.

I know there are a lot of threads on the subject of must aeration, but I haven't seen this particular question asked nor answered:

So as I understand it, it is recommended aerate your mead regularly in the beginning phase down to the 1/3 sugarbreak, especially if you're doing a high ABV-mead. This sort of confounds me, because fermentation is an anaerobic process that goes C6H12O6 (sugar) --> 2x C2H5OH (alcohol) + 2x CO2. If you introduce oxygen into the mix wont the yeast-bacteria simply switch to ordinary aerobic respiration (C6H12O6 + 6x O2 --> 6x CO2 + 6x H2O) which is (from the bacteria POV) preferable as it's a far more efficient way of generating energy for the cell compared to fermentation?

I can understand the initial aeration, you're interested in multiplying the yeast-cells to a sufficient number; but going all the way to 1/3 sugar-break seems like you're wasting a lot of sugar on nothing (well, water and carbondioxide). I ask because it seems like a sneaky trap to fall into: the hydrometer doesn't measure the alcohol-content, but the denseness of the mixture; it doesn't care whether your sugars got turned into alcohol or water (well a little bit), it only cares that the concoction has become less thick. Has anyone tried chemically establishing the alcohol-content of their heavily-aerated meads through titration or some other method other than hydrometer-readings?

Eagerly awaiting a reply.

 

[Squatchy]

Yeast consumes the O2 add in very short order. Less than an hour and 15 ppm is gone from suspension. The yeast assimilates it at such a fast pace. I am no chemist. And maybe you are. Do you think this would be such a short time it doesn't cause them to switch over? The 1/3 break is not found in any of the literature I have found. I have found that one publication says to add a final dose at the .5 break. I have been using pure O2 adds for about two years now. And I have no scientific proof of anything. But I never have any troubles getting to the end of my fermentations with strong kinetics. I believe this is one of two pieces we will see with O2. The other as I understand it speeds up replication during the growth phase

 

[henrik.peytz]

It's not really a "mode" per se that they're in. It's more like serving up a plate of broccoli and a plate of icecream to a 7-year-old, they're gonna go for the icecream if they get the chance . Aerobic respiration (the icecream) provides a lot more energy which the bacteria stores as adeninetriphosphate (ATP), which it then can use to replicate and transcribe its genetic material with. Anaerobic respiration (the broccoli) on the other hand, generates a lot less ATP (though still some). Unless the bacteria is a purely anaerobic organism (such things exist) which is killed by the presence of oxygen, the bacteria will always prefer aerobic over anaerobic respiration as it gives it a leg up in the evolutionary race. Both processes will consume the sugar and lower the gravity.

That's the theory of it at least. I have no idea about the dosage, or if it has any significant impact on the final alcohol-level, though in theory it should.

Kinetics? What's that?

 

[bernardsmith]

Also not a a chemist but my understanding is that in aerobic environments the yeast consume the sugar but do not then produce ethanol. So there can be a significant reduction in alcohol production with the same reduction of sugar (gravity). Am I wrong about that?

 

[henrik.peytz]

Also not a a chemist but my understanding is that in aerobic environments the yeast consume the sugar but do not then produce ethanol. So there can be a significant reduction in alcohol production with the same reduction of sugar (gravity). Am I wrong about that?

Exactly the thing I was wondering. You could easily be led to believe that the fermentation was "done" since the airlock bubbles have ceased and the gravity has gotten to where you expect it to be; but that's not a guarantee you've hit the ABV you were shooting for. If I had to resort to gross hyperbole: it'd be possible to end up with only water + unfermentables if you aerated the must enough, allowing the yeast to respirate instead of ferment.

 

[Stasis]

I think the oxygen is consumed too quickly to have that much of an impact on abv. My final abv has always been very near my predicted abv.
Yeast do not always produce pleasant alcohol. During the growth phase they produce a lot of alcohol and possibly fusels. Perhaps it might even be a good thing that through aeration yeast take a break from producing alcohol for a while. I have noticed that meads are much smoother (have less fusel alcohol) when the must is properly aerated. This is also due to a larger yeast population and healthier yeast though.
When aerating by rocking/shacking with air a lot of yeast do not even have access to enough oxygen. In such a case I go for multiple aerations to counter this. With pure oxygen you can go for two doses of oxygen if sufficiently diffused, perhaps 3 if you're going for a high abv mead.

Btw it has been mentioned many times that you should only ideally aerate until the 1/3 sugar break. This does not mean you should always aerate until the 1/3 break, but if you really must aerate you should do it until this point. I used to aerate with air until the 1/3 break, although at that point it's a mix of degassing/keeping yeast in suspension and aerating. The focus is much less on the aeration and much less thorough than earlier aerations (unless something seems off, such as sulphur smells). By time I've cut down on the amount I aerate a bit at a time to see where the cut off point is where things start getting detectably worse. If things go wrong or you have a particularly high abv mead you can aerate later than the 1/3 break.

By no means am I saying that this is the best possible protocol. I'm just saying what I have done multiple times and that there does not seem to be any real ill effects of doing this

 

[budgies]

The key here is called the Crabtree effect. Yeast will engage in only a tiny amount of aerobic respiration, even with ample oxygen available to them, in the presence of sugars. It's possible they evolved this way so that the ethanol poisons their potential competitors. So when you oxygenate a must or wort, the yeast aren't interested in breathing -- they are deliberately fermenting, the same as they would without any oxygen at all. They consume oxygen, rather, for the purpose of biosynthesis, in order to help build their cell membranes and, as a result, reproduce. Therefore, adding any amount of oxygen before or during fermentation does not have a significant effect on ABV (other than enabling a more robust fermentation, that is).

 

[bernardsmith]

um... I may be very mistaken but I thought the Crabtree effect was when yeast ferment in oxygen rather than use the O2 to produce more yeast cells. Typically, yeast ferment anaerobically. That is a less efficient use of their processes - less efficient but preferred. Crabtree showed that there are occasions when yeast ferment aerobically. Again, my ignorance here, biology was not a topic I studied in high school or university, but I thought "respiration" has nothing to do with "breathing" although that is how non biologists think of that word. Isn't "respiration" simply the ability to extract energy from the environment - and yeast can successfully do this either in the presence of O2 (like us) or anaerobically.

 

[budgies]

Sorry, I was getting a bit carried away with my turn of phrase, you're right that breathing has nothing to do with it. I think we're saying the same thing here. Let me try to be a little more specific in my wording.

Crabtree effect is only that yeast seem to choose not to perform aerobic respiration in the presence of sugar -- even though it is a more efficient use of energy. They instead prefer anaerobic respiration. During anaerobic respiration, they still use up the oxygen, but not for respiration. They consume it as a building block for their cell wall. Therefore, due to the Crabtree effect, when we aerate our musts there is no danger that the yeast will simply produce water and CO2 instead of ethanol and CO2. That was what the OP was speculating.

 

[henrik.peytz]

Sorry, I was getting a bit carried away with my turn of phrase, you're right that breathing has nothing to do with it. I think we're saying the same thing here. Let me try to be a little more specific in my wording.

Crabtree effect is only that yeast seem to choose not to perform aerobic respiration in the presence of sugar -- even though it is a more efficient use of energy. They instead prefer anaerobic respiration. During anaerobic respiration, they still use up the oxygen, but not for respiration. They consume it as a building block for their cell wall. Therefore, due to the Crabtree effect, when we aerate our musts there is no danger that the yeast will simply produce water and CO2 instead of ethanol and CO2. That was what the OP was speculating.

Ah jeez, I actually read about this some months ago, and managed to completely forget it again. But yeah Saccharomyces Cerevisiae is special in that way, probably, as you say, because it uses ethanol as an area-denial weapon against other organisms once it finds a desirable environment to propagate in, instead of straight up competing on aerobic respiration. So aerate away it is.

Somewhat related I found this: https://www.bio-conferences.org/articles/bioconf/pdf/2016/02/bioconf-oiv2016_04003.pdf . It has a nice chart about all sorts of critters you want to avoid creating. It put my mind at ease about spoilage since the pH these things propagate at is way higher than any regular mead-pH (lowest one being Salmonella at pH 3.7).

 

[bernardsmith]

So what is the "bottom line" here? If yeast are provided with O2 during active fermentation will they then shift from fermenting sugar to creating more yeast biomass? And if that is what they are likely to do - do they then use up that sugar without pissing out ethanol?

 

[budgies]

Ah jeez, I actually read about this some months ago, and managed to completely forget it again. But yeah Saccharomyces Cerevisiae is special in that way, probably, as you say, because it uses ethanol as an area-denial weapon against other organisms once it finds a desirable environment to propagate in, instead of straight up competing on aerobic respiration. So aerate away it is.

Somewhat related I found this: https://www.bio-conferences.org/articles/bioconf/pdf/2016/02/bioconf-oiv2016_04003.pdf . It has a nice chart about all sorts of critters you want to avoid creating. It put my mind at ease about spoilage since the pH these things propagate at is way higher than any regular mead-pH (lowest one being Salmonella at pH 3.7).

They are a magical little critter, surely. Their survival mechanism is our pleasure-drug. Also, a lot of spoilage organisms are aerobic, and the yeast grab the oxygen so quickly it denies them that resource as well.

So what is the "bottom line" here? If yeast are provided with O2 during active fermentation will they then shift from fermenting sugar to creating more yeast biomass? And if that is what they are likely to do - do they then use up that sugar without pissing out ethanol?

The bottom line is, no. The yeast consume sugar to create energy for themselves. The uses for that energy are to transform and combine molecules into other molecules that strengthen their little yeast bodies, one of those molecules being oxygen; and then to reproduce. I believe they do pause from metabolizing to build and reproduce when the must is aerated, but I'm not sure. However: at no point are they consuming sugar without producing ethanol as a byproduct.

 

[henrik.peytz]

Here's a whitepaper on the subject: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4429655/

Short story: They do both aerobic and anaerobic respiration, oftentimes concurrently if oxygen is available. The reason (which the whitepaper talks about) is a mix of the previously stated "ethanol as chemical warfare" and "because I can". Last one means that even though aerobic respiration generates a lot more ATP (18 per glucose-molecule in aerobic respiration vs 2 ATP per glucose-molecule in fermentation) the yeast cell will always do fermentation either solely or concurrently with aerobic respiration if the environment allows for it. This is because (as I understand the paper) the yeast-cell does not concern itself with efficient usage of glucose, but rather the rate at which it can process it. It has all the enzymes needed for fermentation, so no need for them to sit idle just because it happens to also be doing aerobic respiration at any given time.

It also mentions that yeast can eat alcohol in the presence of oxygen if in low-sugar conditions, basically it switches over to oxidizing the ethanol in order to derive ATP from it. It doesn't really define low-sugar conditions, but I guess this is why we don't aerate the must all the way to the end.

 

[bernardsmith]

Many thanks for that citation, Henrik.