Understanding Oxygen Help

Ingredients
1

I’ve done a little reading trying to get my head around this whole oxygen thing. I’ll post what I think I have figured out so maybe some of you smarter folks can set me straight if I’m wrong. Here goes.

Sacchromyces don’t need O2 to make beer. They can make alcohol and esters and (mysteriously to me) CO2 anaerobically.

But healthy cell membranes are needed for reproduction, for cell protection, and for controlling what passes in and out of the cell.

Saccharomyces with weakened membranes can produce more esters, more phenols, more diacetyl. They can also have less ability to absorb diacetyl, less ability to resist and survive in high alcohol %, and less ability to attenuate to their potential, or do so more slowly.

They need sterols to build healthy membranes. Sterols are steroid alcohols, and ergosterols are unique to fungus. So, saccharomyces can’t build cell membranes from plant or animal sterols but they can get them from dead yeast (like in Wyeast Yeast Nutrient) or they can synthesize them if they have fatty acids (present in wort) and O2 (not present in wort because of boiling)

Yeast that come from a fresh stirplate starter are less in need of O2 because they should have gotten enough from the starter aeration process. Yeast that has been harvested from a finished beer has not been exposed to O2 since the stirplate. New generations created in that beer have never been exposed to O2. The only sterols they have are from their parents or from dead relatives.

Each strain of yeast has its own desired amount of O2 for healthy membrane production, but generally range from 8ppm to 12ppm. With aeration you generally can only obtain about 8ppm of dissolved O2 in wort. To get above 8ppm, injection of pure O2 is required.  Also, the higher the gravity of the wort the harder it is to retain dissolved O2.

In summary, if you use fresh yeast from a stirplate, and or yeast nutrient that contains dead cells, and or the beer style doesn’t need cleaner esters and reduced phenolics, or doesn’t need to fully attenuate or attenuate rapidly, then you can be less concerned about aeration.

If you want to harvest and repitch without a stirplate starter, you might think about your aeration.

If you want a cleaner, or bigger, beer that fully attenuates to that strain’s potential, you might want to think about your aeration.

If you don’t want to shake 5 gallons for several minutes, or use a pump, or risk contamination from a splashing device and whatever is in the air, or if you need more than 8ppm you might think about injecting O2.

It is possible to get too much O2 in the wort. One symptom could be fusel alcohol (hot solvent flavors). Way way too much O2 could kill yeast as not much lives in pure O2.

But, you don’t NEED O2 to make beer.

Am I on the right track?

2

I think your close at least. And no, you don’t need o2 to make great beer. I’m sure it helps. Particularly if you move from kettle to fermenter without contact with air.

3

To clarify, when we talk about O2 it can get confusing. There’s oxygen in aeration without using pure O2. So I guess I could have been more specific. Aeration vs Oxygenation. I think you can make great beer without oxygenation, so long as there is aeration at least in the yeast starter. But as you add other issues (repitching, desiring controlled ester phenol and fusel production, etc) the need for oxygenation increases. At least that’s what I believe I am understanding from the stuff I’m reading.

4

Jim, it looks like you have a good handle on it.

The CO2 from fermentation comes from the sugar.

C6H12O6 → 2 C2H5OH + 2 CO2
Glucose.        ethanol.        Carbon dioxide.

From experience I have found that Higher levels of O2 in some beers, like a bitter, leads to a disappointing beer due to lack of esters. I just aerate those by pumping into the bucket from the kettle.

5

Well at least it’s not a total mystery to me now. I have zero chemistry knowledge, so I would pretend that I understand how CO2 is made, but at least it’s not magic to me now.

Regarding when I will oxygenate vs aerate vs not worry about it… I probably won’t oxygenate if I’m pitching a fresh stirplate starter. Or if I do it will be like half.

6

Yes, the question should be: aeration vs oxygenation. While similar, they have differing application.

I prefer to aerate my yeast starter since we want the yeast to have a consistent O2 supply to promote conditions that create sterols and build yeast cells. Unless you have an oxygenation system that supplies a really low, constant flow to the starter vessel, it would probably be a waste of oxygen. Using filtered air is a good alternative to supply the constant and low oxygen content to a starter.

For freshly boiled and chilled wort, you want to bring the wort O2 content up quickly and you only get to do it once (typically). So using oxygen is best then.

But it’s questionable if you really need to oxygenate at all if you pitch big enough with yeast that has high sterol content. For us homebrewers that aerate their large yeast starters, it seems reasonable that you could get away without oxygenating the wort. For major brewers that are repitching yeast, the yeast probably doesn’t have a sterol reserve and oxygenating the wort seems imperative.

7

Thanks Martin,  I do believe I’ve got this figured out enough for my purposes.

I do both ways. Fresh activator on a stirplate in 2L of starter wort… but I run that yeast in about four or five repitches without starters. I now see the differences and different requirements.

8

Pretty much.