Yeast life, aerobic to anaerobic - when do they transform?

[shanek17]

How do you know when to add an airlock to your fermenter? I have heard different things about this before and honestly iv been just kinda been winging it for now. But I did just make a big expensive batch of clover honey mead and now I feel I Should have some more information on how to do this properly!

I recall hearing that when 80% of the sugar has been eaten then its safe to put on the airlock, or when the fermentation activity is dying down. I just want to know roughly when the yeast are going from their aerobic (oxygen phase) and then to anaerobic (no oxygen phase).

Is there much of a difference between wine yeast and beer yeast on this matter?

 

[diegobonatto]

All Saccharomyces cerevisiae yeast strains (wine/baker/brewing) are metabolically regulated by the amount of carbohydrates present in the wort or dough. In this sense, all yeast are subject to "glucose repression", which allow to convert glucose (the main sugar used by the cell) to alcohol by means of fermentation (some times refereed to "anaerobic metabolism", which is not biochemically correct for yeast). In that context, the cell can ferment glucose in absence of oxygen. Even if oxygen is present in wort, fermentation is active, promoting alcohol to accumulate. When the levels of simple sugars (glucose/fructose) fall to levels below to 1%, alcohol is taken from beer and re-converted to pyruvate, a metabolite that when converted to acetyl-CoA, is channeled to tricarboxylic acid cycle (Kreb's cycle). In this moment, yeast's mitochondria become activated and oxygen is taken to complete the process of biochemical respiration (aerobic phase). During aerobic phase, oxygen (from atmosfere or cylinder) should be supplied for the cell to sustain its metabolism. In absence of oxygen, yeast cell can not use alcohol from wine/beer.

Remember that oxygen is supplied in the initial phase of fermentation to allow yeast to synthesize the components of cellular membrane; however, oxygen itself do not affect fermentation itself.

 

[passedpawn]

All Saccharomyces cerevisiae yeast strains (wine/baker/brewing) are metabolically regulated by the amount of carbohydrates present in the wort or dough. In this sense, all yeast are subject to "glucose repression", which allow to convert glucose (the main sugar used by the cell) to alcohol by means of fermentation (some times refereed to "anaerobic metabolism", which is not biochemically correct for yeast). In that context, the cell can ferment glucose in absence of oxygen. Even if oxygen is present in wort, fermentation is active, promoting alcohol to accumulate. When the levels of simple sugars (glucose/fructose) fall to levels below to 1%, alcohol is taken from beer and re-converted to pyruvate, a metabolite that when converted to acetyl-CoA, is channeled to tricarboxylic acid cycle (Kreb's cycle). In this moment, yeast's mitochondria become activated and oxygen is taken to complete the process of biochemical respiration (aerobic phase). During aerobic phase, oxygen (from atmosfere or cylinder) should be supplied for the cell to sustain its metabolism. In absence of oxygen, yeast cell can not use alcohol from wine/beer.

Remember that oxygen is supplied in the initial phase of fermentation to allow yeast to synthesize the components of cellular membrane; however, oxygen itself do not affect fermentation itself.

So, the oxygen present is only used when the simple sugar supply is exhausted, and the cells begin to convert alcohol?

So, according to this, does a lack of oxygen then cause the yeast to produce excess acetaldehyde?

 

[diegobonatto]

So, the oxygen present is only used when the simple sugar supply is exhausted, and the cells begin to convert alcohol?

So, according to this, does a lack of oxygen then cause the yeast to produce excess acetaldehyde?

Oxygen is used for membrane synthesis during fermentation. On the other hand, simple sugars should be converted to pyruvate and then to ethanol by fermentation. In this case, the concentration of oxygen in wort did not affect fermentation. When occur simple sugar depletion, ethanol is used by yeast as carbon source and, obligatorily, oxygen should be present to allow mitochondria to work. In absence of oxygen, mitochondria is not active, and ethanol is not consumed (exposing beer for a long time to oxygen lead to yeast to consume ethanol from beer. Of course, the rate of ethanol consumption by yeast is slow and take many months to deplete this alcohol from beer/wine.

Acetaldehyde is a by-product of fermentation. However, the levels of acetaldehyde generated by yeast depending upon a series of factors (pitching rate, temperature, wort composition, etc...).

 

[Malticulous]

Once yeast begin producing CO2 any remaining O2 will be scrubbed out anyway. During the lag phase yeast use the O2 to make sterol and unsaturated fatty acid. These are needed for reproduction and are passed on to daughter cells.

Different strains will use different amounts of O2. All of the O2 yeast use has not been scientifically accounted for.

There are ways to help keep O2 in reach of the yeast after the lag phase. Kai made a simple one for starters.
http://braukaiser.com/wiki/index.php?title=Yeast_Propagator

 

[shanek17]

All Saccharomyces cerevisiae yeast strains (wine/baker/brewing) are metabolically regulated by the amount of carbohydrates present in the wort or dough. In this sense, all yeast are subject to "glucose repression", which allow to convert glucose (the main sugar used by the cell) to alcohol by means of fermentation (some times refereed to "anaerobic metabolism", which is not biochemically correct for yeast). In that context, the cell can ferment glucose in absence of oxygen. Even if oxygen is present in wort, fermentation is active, promoting alcohol to accumulate. When the levels of simple sugars (glucose/fructose) fall to levels below to 1%, alcohol is taken from beer and re-converted to pyruvate, a metabolite that when converted to acetyl-CoA, is channeled to tricarboxylic acid cycle (Kreb's cycle). In this moment, yeast's mitochondria become activated and oxygen is taken to complete the process of biochemical respiration (aerobic phase). During aerobic phase, oxygen (from atmosfere or cylinder) should be supplied for the cell to sustain its metabolism. In absence of oxygen, yeast cell can not use alcohol from wine/beer.

Remember that oxygen is supplied in the initial phase of fermentation to allow yeast to synthesize the components of cellular membrane; however, oxygen itself do not affect fermentation itself.

wow! theres alot of details in there, alot of new words for me too haha. I am trying to understand all this and im sure it will catch on in time, but perhaps someone cnan explain to me whens a good time for me to put my air lock on my fermenter? I understand some people just throw it on the fermenter at first signs of yeast activity, but I know the yeast also like oxygen at the beginning of their life so Im trying to allow them to have it by keeping my fermenter open. But when do I throw air lock on to keep oxygen out?

 

[MalFet]

wow! theres alot of details in there, alot of new words for me too haha. I am trying to understand all this and im sure it will catch on in time, but perhaps someone can explain to me whens a good time for me to put my air lock on my fermenter? I understand some people just throw it on the fermenter at first signs of yeast activity, but I know the yeast also like oxygen at the beginning of their life so Im trying to allow them to have it by keeping my fermenter open. But when do I throw air lock on and oxygen out!

Generally you just put your airlock on when you pitch your yeast. If you are relying on leaving the thing off to get oxygen exchange, you'll barely get any. Typically, you want to oxygenate your wort/must beforehand.

 

[Onlooker]

I'm getting the feeling that you should post this in the mead section, as what you're describing isn't anything I've heard around here (or read anywhere) for the fermentation of beer.

 

[diegobonatto]

You should put your airlock after the active phase of fermentation is finished (when you reach your expected FG). After that, yeast will beginning to use oxygen in order to metabolize the alcohol formed previously.

 

[shanek17]

You should put your airlock after the active phase of fermentation is finished (when you reach your expected FG). After that, yeast will beginning to use oxygen in order to metabolize the alcohol formed previously.

Really? Its okay to wait for it to ferment completly out? iv never heard this before. most people say to attsch it near the end of fermentation. I am a memeber on wine making forum and robie said to adx the air lock when fermentations is almost done. ill post his words below. please give me some feedback as i am confused at the different answers im getting.

" No matter what is said, there are two distinct phases to fermentation.1) Aerobic phase:Oxygen is required for healthy yeast. This phase lasts until the SG is about 1.010 to 1.020. If you get the rotten egg smell during this first phase, it is likely because the yeast are either starved for oxygen or they need a nutrient. Most (but not all) wine kits do not require a nutrient to be added.2) Anaerobic phase:Oxygen should be kept away from the wine, as the yeast no longer need oxygen and the wine surely doesn't need it, either. This phase starts at about SG 1.010 to 1.020, right after the previous stage.

Some kit instructions say to seal the wine during both phases. This is more because they are concerned the wine will get oxidized, since a wine, such as a weak mist kit, can survive the aerobic phase sealed. (Better to starve the yeast than have oxidation, I guess.) This, however, is not the best for your wine. Learn to protect your wine from oxygen without having to starve your yeast of oxygen during that first phase.*During the first stage, very little oxygen can get to the wine, anyway. The CO2 is bubbling out very strongly and is heavier than oxygen, so it protects the wine by pushing out almost all the oxygen. During the second stage there is much less CO2 to displace the oxygen. Hence, you seal the carboy with an air lock that lets out the excess CO2 and won't let air in.Kits with grape packs can usually survive being sealed during the first phase, because you have to open the lid at least twice a day to push down the grape pack cap. This generally lets in enough oxygen to "get by".If you want a good fermentation, don't seal the lid down during aerobic fermentation. put the lid on in such a way as to keep flies out. Place a cloth over the air lock's hole and maybe even place a towel over the top of the lid to protect it.__________________Robie"

 

[diegobonatto]

OK, I think that I understood the confusion....the aerobic/anaerobic phase described regards the process of wine making and not yeast metabolism.

During fermentation, wine/beer will have a CO2 blanket that prevent the entry of oxygen (protecting beer in open fermenters and also wine). Again, oxygen is consumed by yeast in this phase to build up membrane and not for glucose metabolism.

Aerobic phase of wine making = anaerobic phase of yeast metabolism. Grape must should contain sufficient oxygen to sustain yeast growth by allowing membrane formation, like wort in brewing.

Anaerobic phase of wine making = aerobic phase of yeast metabolism. Wine should be protect against the contact with oxygen, like those observed in beer making. If beer/wine get oxygenated, yeast activate its aerobic metabolism and initiate to consume ethanol. In addition, beer/wine components get oxidized, resulting in a flaw product.

Thus, when fermentation is approaching the end, you should seal the carboy to avoid oxygen to get contact with wine/beer.

 

[Peppers16]

Since fermentation is a very 'overlappy' process there's not really a set point when all the yeast switch over, but I think the rough answer is: at the end of the lag stage, where after a few hours (to a day) of apparent inactivity you start seeing some bubbles.
I'm not really sure that leaving the airlock off will make much difference. If you fully aerate and pitch enough yeast, you should have enough oxygen. Some mead makers aerate twice daily for the first 2-3 days (until the 2/3 break) but I don't think much O2 will passively diffuse into the must once the yeast starts producing even a little CO2.

If you do choose to aerate take it slow. You'd be amazed how much CO2 can gas out even early on.

 

[shanek17]

OK, I think that I understood the confusion....the aerobic/anaerobic phase described regards the process of wine making and not yeast metabolism.

During fermentation, wine/beer will have a CO2 blanket that prevent the entry of oxygen (protecting beer in open fermenters and also wine). Again, oxygen is consumed by yeast in this phase to build up membrane and not for glucose metabolism.

Aerobic phase of wine making = anaerobic phase of yeast metabolism. Grape must should contain sufficient oxygen to sustain yeast growth by allowing membrane formation, like wort in brewing.

Anaerobic phase of wine making = aerobic phase of yeast metabolism. Wine should be protect against the contact with oxygen, like those observed in beer making. If beer/wine get oxygenated, yeast activate its aerobic metabolism and initiate to consume ethanol. In addition, beer/wine components get oxidized, resulting in a flaw product.

Thus, when fermentation is approaching the end, you should seal the carboy to avoid oxygen to get contact with wine/beer.

So during fermentation there is a co2 blanket protecting the wine/beer from oxygen. So is there even a point in leaving the fermenter open to the air during fermentation? i thought leaving it open to oxygen would help everything out.

 

[diegobonatto]

Yes, is possible to keep your fermenter open during fermentation. In fact, some brewers describes the benefices of open fermentation (less sulfuric smell, esthers, etc...) when compared to closed carboys/fermenters. But this is a long discussion....

Just put your carboy in a room with little circulating air/people (to maintain the CO2 blanket during active fermentation's phase) and you're done.