Carbonic Maceration - chemistry help needed

[Lucienne]

Ok, so basically, because the grape doesn't have oxygen, it goes and starts attacking other things? Or something like that?

CO2 doesn't alter the taste as far as I know, I guess since the vine has been producing it this whole time it's not really avoidable regardless. There is a lot of stuff that's generally put into wine anyway actually, like SO2.

[Chun Liroy]

Nah, like I said Yeast is doing all the fermentation. The Grape is there for taste and fuel the production of alcohol.
I'm fairly sure fruits don't do any metabolic processes at all.

There's two modes of producing energy. Think of the presence of oxygen as a switch.

If there is no oxygen, fermentation happens, and the ETC doesn't happen.
If there is oxygen, ETC happens, and fermentation doesn't happen.

[Psion]

Yeah, basically, the grapes are the yeast's food, and they normally breathe O2 like us, converting the sugars in the grapes into food for themselves. (I've not reached more than first semester in inorganic chem, so don't ask me how. XD )

But if you took a human, dumped him in a room full of CO2 instead while giving him grapes for food, he'd obviously die in minutes because he has no O2 to use to convert food into sugars the body can use. on the other hand, yeast is able to go "oh shit, there's no oxygen, swapping to CO2 mode" and uses a different process to convert the grapes into something they can use, and in the process alchohol is produced and whatever things go into wine. I think the alchohol is a waste product to them, and they use the other part of the reaction with CO2 as food.

As far as the skins bursting, that I've no clue. maybe the skins are just being broken down by the yeast as food as well, and so the skin gets weak enough to tear open?

But the grape isn't doing anything, just the yeast. Think of the grapes as a raw steak and the yeast as a chef cooking it with CO2 in some way to produce a tasty steak, and the bones and fatty crap it didn't want and trimmed off is leftover, and that leftover crap is stuff that humans find tasty.

I probably made things worse, didnt I? lol

[Priran]

I'm going to assume the skins are breaking because a mole of glucose occupies about 100mL and a mole of carbon dioxide occupies ~22.4L

[Lucienne]

I probably made things worse, didnt I? lol

Absolutely not! That was amazing. lol

[kazi]

After placing the grapes in a vat the whole thing is pumped full of CO2. The weight of the grapes placed on top of each other also helps to crush the bottom 10-15% of grapes. The sugar in these grapes is transformed into alcohol, via the natural yeast on the skin. The upper non crushed grapes go through a miniature fermentation inside the grape, the grape absorbs the co2 and the enzymes inside the grape convert the sugars to alcohol and more co2. at 2-3% alcohol the mini fermentation stops and the cells begins to breakdown and the fluid is drained to the bottom of the vat.

After Carbonic maceration and fermentation, the alcoholic grapes slurry is put threw a pneumatic press and finally threw filtration pads then bottled.
This form of maceration creates a lighter wine and has low tannins.

-from my class taught by a master sommelier

[Lucienne]

I had no idea about the stopping at 2-3%. Did he give you any reason for this?

[kazi]

At 2-3% the cell begins to breakdown and it can no longer create alcohol inside its own cell walls. But i do believe that when more yeast is added, or the residual yeast in the liquid, it continues to ferment in the liquid/mash in the bottom of the vat. it should continue to get reach 5-14ish % before the yeast dies, or is stopped.

[Lucienne]

At 2-3% the cell begins to breakdown and it can no longer create alcohol inside its own cell walls. But i do believe that when more yeast is added, or the residual yeast in the liquid, it continues to ferment in the liquid/mash in the bottom of the vat. it should continue to get reach 5-14ish % before the yeast dies, or is stopped.

Yeah, there is still a lot of sugar left in the grapes at that point. What I guess might happen is that the grape is so filled with all the things it has produced at that point that there just isn't enough space for more production and it bursts, but that's just a guess really. Do you know anything about the length of the maceration? Of what I understand some producers, like the new style Riojas, choose to have longer maceration times. Would they simply prolong the process of leaving the crushed grapes to ferment on the bottom before pressing them? Or is there a way to regulate the speed of the fermentation inside the grape by I don't know, the amount of CO2 added or something?

[Priran]

Or is there a way to regulate the speed of the fermentation inside the grape by I don't know, the amount of CO2 added or something?

If there is, it would probably be by temperature. CO2 isn't a reactant in the reaction so its concentration wouldn't affect the rate of fermentation unless it got so low aerobic respiration resumed.

[Lucienne]

Thanks, that makes a lot of sense actually.