AlaskaWortChiller
Member
This question originates from a discussion with the guy at the local homebrew store. I had a yeast starter culture of about 1100 mL. I normally pitch my starters into the wort about 18-30 hours after inoculating the starter flask. However due to a house flood the morning after I made my starter, I was unable to brew for several days. By the time I got around to pitching, the starter culture had been sitting around for approximately 72 hours. I assumed this would just increase the size of the yeast cake. I have often read on this forum about people making 2-3 day starters in order to increase yeast cell count for high gravity brews.
When I mentioned this at the shop, the gentleman replied that in fact, after three days the yeast would be less vigorous than at 24 hours and the cell count would not increase after the intial 24 hours or so. He said that after 24 hours, the yeast in the flask are just fermenting rather than consuming oxygen and reproducing.
I always thought that the trigger for fermentation was lack of oxygen. I did not expect that the yeast in the culture flask, which is covered by tinfoil but not airtight, would begin fermenting until placed in the airlocked fermenter, after which they would consume all oxygen in the vessel, thus creating anaerobic conditions which would trigger the shift to fermentation. I assumed that if the yeast were kept in a vessel such as the culture flask which allows for gas exchange, they would continue to consume oxygen as needed and happily continue reproduction, but would not ferment.
The homebrew store guy did not agree. He reminded me that some beers are brewed using open fermentation systems in which the vessel is not airtight. I have no experience with such a system. Are the yeast in this case still engaged in anaerobic metabolism (fermentation), even though the fermenter is open to the air?
"In the aerobic wort, the yeast grows and reproduces very rapidly, using energy obtained from available sugars. No ethanol forms during this stage, because the yeast, amply supplied with oxygen, oxidizes the pyruvate formed by glycolysis to CO2 and H2O via the citric acid cycle. When all the dissolved oxygen in the vat of wort has been consumed, the yeast cells switch to anaerobic metabolism, and from this point they ferment the sugars into ethanol and CO2." (Lehninger, Principles of Biochemistry, 4th Ed. pp. 542).
The passage above seems to indicate that fermentation is triggered by lack of oxygen as I always believed. But clearly, in an open fermentation system, the yeast has access to oxygen yet still switches to anerobic fermentation at some point.
My question is, if lack of oxygen is not the trigger for this switch from repiration/reproduction to anaerobic fermentation, what is? Is it related to the amount of time in culture or the quantity of yeast cells present? If a starter culture is made in a non airlocked vessel that allows for gas exchange, will the yeast keep reproducing for several days or will they experience a short reproductive burst within the first 24 hours and then switch to fermentation after that point? If the latter case is true, all this stuff you hear in posts about making 3 day starters for high gravity beers is bogus.
More generally, why do the yeast in an open homebrew system such as a simple plastic bucket switch to fermentation, an anerobic form of metabolism, if they have access to oxygen from the environment? Please cite sources if possible.
When I mentioned this at the shop, the gentleman replied that in fact, after three days the yeast would be less vigorous than at 24 hours and the cell count would not increase after the intial 24 hours or so. He said that after 24 hours, the yeast in the flask are just fermenting rather than consuming oxygen and reproducing.
I always thought that the trigger for fermentation was lack of oxygen. I did not expect that the yeast in the culture flask, which is covered by tinfoil but not airtight, would begin fermenting until placed in the airlocked fermenter, after which they would consume all oxygen in the vessel, thus creating anaerobic conditions which would trigger the shift to fermentation. I assumed that if the yeast were kept in a vessel such as the culture flask which allows for gas exchange, they would continue to consume oxygen as needed and happily continue reproduction, but would not ferment.
The homebrew store guy did not agree. He reminded me that some beers are brewed using open fermentation systems in which the vessel is not airtight. I have no experience with such a system. Are the yeast in this case still engaged in anaerobic metabolism (fermentation), even though the fermenter is open to the air?
"In the aerobic wort, the yeast grows and reproduces very rapidly, using energy obtained from available sugars. No ethanol forms during this stage, because the yeast, amply supplied with oxygen, oxidizes the pyruvate formed by glycolysis to CO2 and H2O via the citric acid cycle. When all the dissolved oxygen in the vat of wort has been consumed, the yeast cells switch to anaerobic metabolism, and from this point they ferment the sugars into ethanol and CO2." (Lehninger, Principles of Biochemistry, 4th Ed. pp. 542).
The passage above seems to indicate that fermentation is triggered by lack of oxygen as I always believed. But clearly, in an open fermentation system, the yeast has access to oxygen yet still switches to anerobic fermentation at some point.
My question is, if lack of oxygen is not the trigger for this switch from repiration/reproduction to anaerobic fermentation, what is? Is it related to the amount of time in culture or the quantity of yeast cells present? If a starter culture is made in a non airlocked vessel that allows for gas exchange, will the yeast keep reproducing for several days or will they experience a short reproductive burst within the first 24 hours and then switch to fermentation after that point? If the latter case is true, all this stuff you hear in posts about making 3 day starters for high gravity beers is bogus.
More generally, why do the yeast in an open homebrew system such as a simple plastic bucket switch to fermentation, an anerobic form of metabolism, if they have access to oxygen from the environment? Please cite sources if possible.
