Why do yeast produce alch and then co2?

Hello,
I have always wondered about this: Why does yeast convert sugar to alcohol at first (until it makes its environment toxic and goes dormant), but if given sugar again (priming sugar), produce Co2 instead.

Maybe part 2 of my question is: Is there any way to boost the alcohol content once the yeast has gone dormant already?

Thanks!

The yeast is producing alcohol and CO2 simultaneously - that's why your airlock bubbles during fermentation. And when you add your priming sugar, you are increasing alcohol slightly while carbonating the beer.

hammy48 said:

Hello,
I have always wondered about this: Why does yeast convert sugar to alcohol at first (until it makes its environment toxic and goes dormant), but if given sugar again (priming sugar), produce Co2 instead.

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the short answer is, it doesn't. The premise there is incorrect.

They produce both alcohol and CO2 at first. I assume they still produce alcohol with the priming sugar. Also, if you were to dump a bunch of malt extract into the fermenter after primary fermentation was complete, you would get more alcohol.

The malt sugar has a more complex sugar profile than say the refined corn sugar, which is just glucose. Maybe the ratio of alcohol/CO2 production is a function of the type of sugar the yeast are converting.

If you pull a sample of the beer during fermentation you will see bubbles of co2 in there. It happens simultaneously. In winemaking they do something to remove the built up co2 during the process (called degassing) but agitating out beer that way could cause it to oxidize.


That is what makes an airlock bubble. OR blows the lid off a fermenter if the airlock gets blocked.

What happens when we add more sugar in the bottle is we TRAP the co2 in the vessel strong enough to contain it without exploding and it build up then infuses the beer with co2. THEN we can say a beer is carbed up.

Okay I think I get it... so during fermentation excess Co2 outgases through the airlock so none is trapped. But if you seal the container and re-start the fermentation the Co2 is trapped, thus carbonating the beer. So a beer that is bottle fermented and big, like a belgian quad, must have a ton of captive Co2 in the bottle right? But they aren't typically highly carbonated in my experience...

Similar to when I eat beans.. converts to sugars first.. followed by an outgassing of methane. arrgghh

buzzbromp said:

They produce both alcohol and CO2 at first. I assume they still produce alcohol with the priming sugar. Also, if you were to dump a bunch of malt extract into the fermenter after primary fermentation was complete, you would get more alcohol.

The malt sugar has a more complex sugar profile than say the refined corn sugar, which is just glucose. Maybe the ratio of alcohol/CO2 production is a function of the type of sugar the yeast are converting.

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just searched and corn sugar is dextrose , d-glucose, or glucose. I didn't know it had multiple names.

The type of yeast or bacteria has an effect on the amount of co2 release. Saccharomyces cerevisiae (regular beer yeast) vs others like brettanomyces.

hammy48 said:

Okay I think I get it... so during fermentation excess Co2 outgases through the airlock so none is trapped. But if you seal the container and re-start the fermentation the Co2 is trapped, thus carbonating the beer. So a beer that is bottle fermented and big, like a belgian quad, must have a ton of captive Co2 in the bottle right? But they aren't typically highly carbonated in my experience...

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Almost, but I think you mean "bottle conditioned" as opposed to "bottle fermented". The carbonation level is going to depend on the amount of fermentable sugars (and the presence of viable yeast) in the bottle at the time that you cap it. Typically, us homebrewers who bottle our beers wait till the fermentation is complete, then we add priming sugar at bottling time. The amount of priming sugar that we add determines our final carbonation level. I would do the same for a Belgian quad or any other beer.

Even yeast gotta fart now and then

isurf said:

Even yeast gotta fart now and then

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Makes you wonder what the is really working out, doesn't it?

as has been said...the yeast produce both alcohol and co2 during both fermentation...and bottle conditioning.

If you split a fully fermented batch of beer, and kegged half with artificial carbonation(co2 added from a pressurized bottle)...and bottled the other half using bottle conditioning (adding a measured amount of sugar then bottle)

The bottle conditioned beer would be "about" 0.25% stronger from the alcohol created during bottle conditioning.

hammy48 said:

Okay I think I get it... so during fermentation excess Co2 outgases through the airlock so none is trapped. But if you seal the container and re-start the fermentation the Co2 is trapped, thus carbonating the beer. So a beer that is bottle fermented and big, like a belgian quad, must have a ton of captive Co2 in the bottle right? But they aren't typically highly carbonated in my experience...

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The alcohol content of the beer does not affect the carbonation level because the co2 during fermentation is escaping. After the beer is finished you bottle with a measured amount of sugar and that determines your carbonation level.

Belgian quad is refermented in the bottle not primary fermented, also called bottle conditioning.