Is open vat fermentation really anaerobic

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.

as odours.

There is a lot of controversy concerning this due to the partial pressure of gbuttes. 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 gbuttes 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