Is open vat fermentation really anaerobic?

We have all see the pictures (and perhaps the real thing) of a red grape cap floating on the surface of an open vat and of winery workers either punching down the cap or pumping-over the must.

I showed a couple of those pictures at a recent wine class and one of the students wondered out loud whether the fermentation that was going on was really anaerobic, given the cap's exposure to air. She got me wondering too. Obviously the brew is producing CO2 and alcohol, but what about the presence of the oxygen?

I understand that yeast may need some oxygen at some point in their life cycle and that they can carry out plain old aerobic respiration (i.e., can turn sugar and oxygen into CO2 and water instead of CO2 and alcohol). Are they both respiring and fermenting simultaneously?

Will greatly appreciate an explanation (or reference to a suitable book or article, no matter how technical) about what is going on in an open fermentation vat from the yeast's perspective.

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Reply to
Leo Bueno
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Leo,

I don't know if yeast carry out respiration & fermentation simultaneously, but I doubt they do to any significant degree.

As I understand it, yeast ferment preferentially over respiration, even though fermentation is a far less efficient energy producer per sugar molecule. The reason seems to be that they can ferment at a much faster rate than they can respire, and this more than compensates for the lower ATP yield. I suppose it's a "Fast nickels vs. slow dimes" kind of thing.

I had always assumed that yeast would switch to respiration if oxygen were present, because of the higher energy yield per sugar molecule, but a very knowledgeable researcher type informed me of the above.

Hope this helps.

Reply to
Mike McGeough

Leo, While I have no particular knowledge about yeast metabolism, I doubt that there's much opportunity for aerobic respiration even in an open fermenter. The reason I suggest this is that the surface-to-volume ratio of something the size of a typical fermenter is so low that the rate of diffusion of oxygen into solution would be too low to have much effect on the yeast. Analogous situations arise in wine aging (e.g., double magnum vs. half-bottle) and in industrial chemistry, where pilot plant workers can add very oxygen-sensitive pyrophores to 1000 L reactors in the open air because of the low surface-to-volume ratio.

Mark Lipton

Reply to
Mark Lipton

Mark Lipton wrote in news:e96dndhsOYjH7mDcRVn- snipped-for-privacy@comcast.com:

the CO2 tends to "hang over" the tanks as well creating somewhat of a barrier.or so it was explained to me once upon a time.

Reply to
jcoulter

workers

Leo, open vat fermentation (or any other kind, for that matter) is usually intentionally *not* anaerobic. In fact, one of several good reasons for either pumping over or punching down is to aerate the must. As has been pointed out by others, between the cap and the blanket of CO2 that results from fermentation, little oxygen would be present without some intervention. Anaerobic fermentation can cause yeast to start consuming sulfur from the must instead, resulting in reductive byproducts, typically starting with the production of hydrogen sulfide.

Once fermentation is complete, as I'm sure you're aware, winemakers go to some lengths to exclude oxygen and so prevent oxidation. During primary fermentation, though, not only is oxygen not excluded but its presence is actually generally positive for the end product.

- Mark W.

Reply to
markw

Sorry, but you are wrong

Sorry, again, wrong. COMPLETELY.

Hi

if you did not have anerobic conditions you would not get significant alcoholic fermentation. You would get mostly CO2 and water, and some ethanol. When making bread you want aerobic conditions, not when making wine.

Another nasty effect under the presence of oxygen is that if there is any acetobacter you will get acetic fermentation (aerobic by definition).

The alluded to aeration is done before the fermentation kicks in, in order to provide nourishment to the yeasts and favour their growth, as well as after the fermentation is over, in order to get rid of odours.

During fermentation the CO2, being heavier than air, "sits" on top of the cap and prevents any oxygen from reaching it.

Mike Tommasi, Six Fours, France email link

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Reply to
Mike Tommasi

While fermentation in an anaerobic process, it will occur in the presence of O2 if there is sufficient sugar present. Wine yeast will prefer fermentation over respiration due to the speed of fermentation. Additionally, alcohol will suppress respiration so as the sugar is depleted the yeast will continue fermenting even in the presence of O2.

Yes, but the yeast usually overwhelms any other organisms by competing for the same nutrients and by producing compounds that will suppress the growth of other organisms. Yeast cells are known to be an oxygen scavengers even though they aren't using it for respiration. They will use it to make compounds that maintain cell integrity and will "stock pile" the O2 even if it is not currently needed. Of course if the yeast do not reproduce quickly for some reason, there is an increased risk of spoilage organisms taking hold.

odours.

There is a lot of controversy concerning this due to the partial pressure of gasses. If we're talking about a closed container that only allows gas to escape, this is probably true, but if O2 has a means of entering, O2 will dissolve in the wine/must even if it's blanketed with CO2. The gasses will mix and the O2 will dissolve in the wine because it has a lower concentration.

Some wine makers are using oxygenation during fermentation in order to make softer wines. They will pump over the wine and at the same time remove the seeds which they believe contribute the harsh tannins. Andy

Reply to
JEP62

workers

Isn't punching the chapeau done for red wines to increase the contact between the solids and the juice leading to colouration? And of course it will also help Malthusian colony of yeast to get energy to multiply, is it not?

Once the dissolved oxygen in the must is used up, there frankly there will be no oxygen available to the poor yeast cells _inside_ the must. I mean, if I run up 10 flights of stairs in 2 minutes flat, my body is surrounded by all the oxygen one can imagine, but the muscles have none and have to resort to anaerobic respiration (a.k.a. fermentation).

The aeration towards the end of the process is perhaps to cleanse the "wine" of excess carbon dioxide from fermentation. The oxygen will eventually help "age" the wine by oxidising non-sugar extracts. Cheers

Reply to
TB

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