Carbon dioxide production and fermentation

[Tbaldwin]

I'm a comparative novice to home brewing, and I have a question about CO2 production and the need (?) to aerate the wort prior to pitching the yeast. Initially, I blindly followed instructions, but as I think about what's happening, I wonder "why aerate?".

Here's my reason. Alcoholic fermentation does NOT need oxygen. If there is oxygen available, the yeast will convert the sugar entirely to carbon dioxide. They get a LOT more energy from this aerobic metabolism, so will do this in preference to the anaerobic process, which yields ethanol. When you pitch the yeast, there will be some oxygen, regardless. It will be used to make carbon dioxide. When the oxygen is used up, the yeast will switch offer to alcoholic fermentation, and will produce vastly less carbon dioxide.

So, to my question. What you are wanting to do is make ethanol. Why not hasten the process by minimizing exposure to air after boiling the wort. This will minimize the time required for the yeast to use up the oxygen and get down to the business of making ethanol. There are two reasons I can think of...1. To give the yeast a running start to increase cell numbers dramatically to out-compete any contaminating microbe you might have let in. (Could compensation by using larger innocuous). Or, 2. You want a drier beer, so you let the yeast eat up some of the sugar, thereby making the beer less sweet.

Any ideas out there??

Thanks,

Tom

 

[kh54s10]

The yeast do need oxygen to reproduce to cell counts necessary to ferment the wort properly. You are not only looking to produce alcohol you are looking the get the right flavors. If there is not enough oxygen the yeast, even if you pitch more, will be stressed in doing their work and produce off flavors in your beer.

I have no science and cannot quote any studies to support this, but it is considered common knowledge.

 

[dnslater]

You are making a tasty beverage, not a biofuel.

 

[govner1]

I'm a comparative novice to home brewing, and I have a question about CO2 production and the need (?) to aerate the wort prior to pitching the yeast. Initially, I blindly followed instructions, but as I think about what's happening, I wonder "why aerate?".

Here's my reason. Alcoholic fermentation does NOT need oxygen. If there is oxygen available, the yeast will convert the sugar entirely to carbon dioxide. They get a LOT more energy from this aerobic metabolism, so will do this in preference to the anaerobic process, which yields ethanol. When you pitch the yeast, there will be some oxygen, regardless. It will be used to make carbon dioxide. When the oxygen is used up, the yeast will switch offer to alcoholic fermentation, and will produce vastly less carbon dioxide.

So, to my question. What you are wanting to do is make ethanol. Why not hasten the process by minimizing exposure to air after boiling the wort. This will minimize the time required for the yeast to use up the oxygen and get down to the business of making ethanol. There are two reasons I can think of...1. To give the yeast a running start to increase cell numbers dramatically to out-compete any contaminating microbe you might have let in. (Could compensation by using larger innocuous). Or, 2. You want a drier beer, so you let the yeast eat up some of the sugar, thereby making the beer less sweet.

Any ideas out there??

Thanks,

Tom

You need to oxygenate AFTER pitching your yeast (see Papazian). This is important for healthy yeast budding in the early (aerobic) stages. Then you'll have a healthy yeast to ferment your wort (anaerobic stage) producing CO2 & alcohol & avoiding off flavors.

 

[kh54s10]

You need to oxygenate AFTER pitching your yeast (see Papazian). This is important for healthy yeast budding in the early (aerobic) stages. Then you'll have a healthy yeast to ferment your wort (anaerobic stage) producing CO2 & alcohol & avoiding off flavors.

Really??

I would bet that most oxygenate THEN pitch the yeast. I haven't read the Papazian article/book that you refer to, but most of everything else I have read suggests oxygenation then adding the yeast, sealing it up and leaving it alone until it is done fermenting.

The oxygen is still in there when the yeast get to work fermenting the wort.

In some high gravity beers people will start fermentation then oxygenate again.

 

[ACbrewer]

Bio/Chem lesson - btw, this come from Yeast by White and Zainasheff.

Initially there is a limited amount of yeast pitched into the wort. The yeast will multiply to take advantage of the food. While doing this multiplication, they need Oxygen to build more yeast. Without it, they can't build them. They also need some nitrogen, which is why in mead we put in nutrient. Wort has plenty of that, but still need O2. Now you can pitch more yeast so you don't grow any, but the problem with this is that while making yeat (budding) the yeast produces other chemicals that I like to call 'flavor.' It varies depending on 1, type of yeast, 2 temp of ferment, 3 pitch rate and there is probably a 4 and 5 I can't remember right now. If you can't grow the yeast (because you lack 02) often you will have a stalled ferment, or the flavors will be off.

There are studies about this, in fact it was brewing and vinting that Louis Pasteur made his living. Rabies and making milk shelf stable longer were... side hobies. And while he is one of the first, they still do this sort of stuff today. There was a brewery in Belgium that experimented (and wrote up) using olive oil in the wort, it works out to about 1/2 a drop in a 5 gallon batch, so not practical for home brewers. This was in substitution for oxygenation - which is why I mention it.

And in general, you oxygenate before pitching, although they could be done in either order as long as they are both timely to one another. You do not want to oxygenate after 1 day. Big beers are to get a 2nd blast of oxygen at the 12 hour mark. After a certain point as the OP pointed out, you'd destroy the alcohol and flavor by allowing the yeast to continue to work.