I've been doing a little more digging on this topic and I found an article from the American Journal of Enology and Viticulture (
Am. J. Enol. Vitic., Mar 1980; 31: 46 - 52 ) entitled "Cultivation of Yeast Under Carbon Dioxide Pressure for use in Continuous Sparkling Wine Production."
The abstract reads as follow:
Batch fermentations of Montrachet wine yeast under carbon dioxide pressure in a wine medium containing 5% added glucose indicated the feasibility of cultivation of yeast in this manner as the initial tank in a continuous fermentation production of sparkling wine ("champagne"). At 0.6 atm (gauge) carbon dioxide pressure, there was a fourfold inhibition of growth rate, and the final maximum cell concentration was about half of the control. The onset of fermentation was delayed by carbon dioxide pressure; however, the maximum fermentation rate was not greatly affected.
Continuous fermentation, with dilution rates in the range of 0.015 hr-1 (with a constant volume of four liters and with flow rates in the range of 1 mL/min) showed the typical relationship between steady state cell concentration and dilution rate. The yeast were able to maintain steady state growth at each of the carbon dioxide pressures tested [up to 0.6 atm (gauge)], in spite of the inhibitory effect. From these results it was suggested that a dual-tank system, maintained at 0.3 and 5 atm (gauge) carbon dioxide pressures, with the second tank having six times the capacity of the first, could be used for continuous production of sparkling wine.
In the steady state condition, higher utilizations of glucose were found at the higher carbon dioxide pressures, even though a lower steady state cell concentration was maintained. The extra requirement for substrate at the increased carbon dioxide pressure was explained as an increased maintenance energy requirement at the higher carbon dioxide pressure.
The yeast cultivated in continuous fermentation at increased carbon dioxide pressures were used as inocula for bottle fermentations. Although no adaptation or mutation of the yeast to carbon dioxide pressure was evidenced in the continuous fermentations, the bottle fermentations with the yeast precultivated at the medium carbon dioxide pressure [0.3 atm (gauge)] showed a definite increased fermentation rate (as evidenced by rate of pressure development). The wines from the bottle fermentations were subjected to sensory evaluation and were found to be indistinguishable.
There is a neat chart in the article (a JPEG file) that I would like to display but I'm not able to attach it here (if someone else can, I'll be happy to forward it). It show that there is an impact of CO2 on the growth rate of yeast at relatively low pressures. While it may take 7 atmospheres (or more) of CO2 pressure to stop yeast outright, as little as 0.3 atm CO2 pressure can retard the growth rate. The chart shows that at 0 atm (like under airlock) the maximal yeast count is achieved at about 100 hours. With 0.3 atm pressure, it takes about 200 hours, and under 0.6 atm, roughly 320 hours to reach the same cell count. So even though the yeast are still able to achieve the maximal cell count, under pressure, it takes much longer.
This test was done with Montrachet yeast, not a Champagne yeast, so it is possible that Champagne strains might show much less sensitivity to CO2 pressure. I expect the sensitivity to CO2 pressure varies greatly between strains as most other things do.
The question still remains does active degassing allow better performance than fermenting at 0 atmospheres. Well this data suggests that if you could keep negative pressure on a batch, that it might be possible for yeast to achieve maximal cell counts in less than 100 hours, but that is by no means certain. Vigorous stirring will briefly lower the CO2 saturation, though it will rapidly build up again, so unless you stirred it continuously, I'm skeptical that it will have significant impact. Certainly any impact would be measured in hours, not weeks or months.
I'd like to see someone step up and test this out in a controlled trial. If anyone has any better data, please share it with us.
Medsen
