yeast: to ferment or not…?

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Question:

2. Yeasts belong to a group of organisms called "facultative anaerobes" which means they can undergo aerobic respiration in the presence of oxygen to metabolize carbohydrates to CO2 and water, and ferment carbohydrates in the absence of oxygen to produce CO2 and ethanol. I don’t claim to have any expertise on this subject, but if this statement is correct (the CO2 and water part) wouldn’t it be possible to produce low or ‘non’ alchohol beer without dilluting or cooking the finished product. Not that I would want to ;+] But I have seen a few posts about NA beer here. Maybe if you hook up a bubbler to the fermenter and introduce oxygen throughout

Remember if you introduce O2 then the yeast uses a whole bunch of different metabolic pathways, than when it is fermenting. Though I am not certain of the specifics it is possible that these pathways would either produce side reaction end products with unwanted properties (bad taste etc.) A more immediate problem would be that your chances of contaminating bacteria getting into your system would be greatly increased. There are a lot more bugs floating around that love nutrient rich environments with lots of O2, than there are bugs that can grow in an anaerobic environment.

Response:

I don’t claim to have any expertise on this subject, but if this statement is correct (the CO2 and water part) wouldn’t it be possible to produce low or ‘non’ alchohol beer without dilluting or cooking the finished product. Not that I would want to ;+] But I have seen a few posts about NA beer here. Maybe if you hook up a bubbler to the fermenter and introduce oxygen throughout the ferment. just a thought Andy D.

Response:

A very good thought. There were persistent rumors when I was in school (Chem E) that some big brewers were doing just this. I tend to believe it, since if it is possible it would be cheaper than other methods and possible produce beter tasting p-beer ( . –arne – Hide quoted text — Show quoted text – just a thought Andy D.

Response:

To reduce the sugar into water and carbon dioxide, you need to consume oxygen. If oxygen is not available, a less efficient reaction is mandated producing chemicals like alcohol as a byproduct. I use an airlock with water in it, and I would imagine that O2 dissolves in the water, and moves into the carboy down its concentration gradient (though I’m sure it is slow).

Oxygen can’t diffuse very quickly this way. Even if it could diffuse through the entire surface of the carboy, it wouldn’t be enough to support aerobic (oxygen-consuming) life: It’s too big, and has too little surface area to its volume. Aminals larger than a few inches across need a specially-evolved mechanism to get enough oxygen dispersed to the body. In case this isn’t clear, I have two questions: 1. Does oxygenation of the wort reduce the amount of alcohol produced?

No, it actually helps: The yeast grows and thrives until the disolved oxygen is used up. Once the oxygen is consumed, the yeast is still present in great quantity and good health. Therefore you have a large, thriving population which begins anaerobic metabolism and alcohol production. 2. If you were a yeast, why would you ferment when you could respire?

I wouldn’t, but in a fermentor, I wouldn’t have the option. Frank Crary CU Boulder

Response:

Alchohol is the organic product of respiration under anaerobic conditions.

It is nescessary to have oxygen to complete the entire catabolic cycle of

sugar molecules from sugar to CO2. The problem encountered in brewing is

that the presence of oxygen (as air) often carries with it microorganisms

which convert the alchohol to acetic acid (vinegar), or other undesireable

organic compounds. As a microbiologist I just could not let this one pass uncorrected. Fermentation and respiration are two different things completely. Yeast can respire if O2 is present. In this case they will bring sugar down to H2O and CO2. This is a very efficient process, lots of energy is produced and everyone is happy In aerobic respiration O2 acts as the terminal electron acceptor in the energy pathway. There are some organisms that can use other terminal electron acceptors in their respiratory pathways. In this case chemical like NO2 or SO4 play the role. Fermentation does not use an electron transport chain, and do not use oxygen nitrate or sulfate etc. as terminal electron acceptors. Under high sugar concentrations yeast are tricked into believing that they are in an anaerobic environment. Therefore with no oxygen to use as a dumping place for electrons, they ferment the sugars. Sugar is incompletely oxidized (nothing to do with oxygen, just means "loses electrons") to form alcohol and CO2. QED Fintan

Response:

-Just curious (I know this should really go to bio.yeast.physiology, but I couldn’t find such a group:)

Try bionet.molbio.yeast, but last time I checked, the focus there was very strongly on the genetic engineering of yeast, and they appeared to be an unusually intolerant group. — = Martin Lodahl Systems Analyst, Capacity Planning, Pacific*Bell = = If it’s good for ancient Druids runnin’ nekkid through the wuids, = = Drinkin’ strange fermented fluids, it’s good enough for me! (Unk.) =

Response:

In case this isn’t clear, I have two questions: 1. Does oxygenation of the wort reduce the amount of alcohol produced? 2. If you were a yeast, why would you ferment when you could respire?

It’s not as if the yeast flipped a switch from respiration to fermentation. There are chemical processes, and they occur at different rates. If there’s lots of oxygen, respiration dominates, and the yeast reproduce a lot and belch a lot of CO2. If there isn’t much oxygen, fermentation dominates, and the yeast produce oxygen and a little CO2. In practice extra oxygen during fermentation isn’t a problem because it gets used up quickly, the krausen tends to shield the gyle from absorbing more oxygen, and CO2 settles in the container, also shielding the gyle from oxygen. I suppose vigorous aeration during the ferment could significantly reduce the amount of alcohol in the beer. The only breweries that I am aware do this brew English ordinary bitter, which tends to be quite low in alcohol anyway. Florida State University SPAN: scri::pepke Disclaimer: My employers seldom even LISTEN to my opinions. Meta-disclaimer: Any society that needs disclaimers has too many lawyers.

Response:

Well, they give us *both*. Isn’t that nice of them? I use an airlock with water in it, and I would imagine that O2 dissolves in the water, and moves into the carboy down its concentration gradient (though I’m sure it is slow). I also know that some people do open fermentation and still get beer. Maybe the large number of yeast in the wort keep it anaerobic by rapidly using any O2 that does get into the wort.

Well, mostly the fact that CO2 is heavier than O2 helps keep the O2 away from the beer. In case this isn’t clear, I have two questions: 1. Does oxygenation of the wort reduce the amount of alcohol produced?

No, in fact it can *increase* it. Why? Because without sufficient oxygenation, the yeast may not reproduce and respire. This can result in an incomplete fermentation. 2. If you were a yeast, why would you ferment when you could respire?

Well, they basically go through a respiration phase before beginning fermentation. I have to admit that the biology/chemistry of fermentation is something I have read about but don’t fully understand. Sort of like the biology I took in school <g. Check out George Fix’s book, called something like _Principles_Of_Brewing_Science_. You can find out what the yeast do and why. And what the yeast can do that you don’t want them to do if your environment is far from optimum. John DeCarlo, MITRE Corporation, McLean, VA–My views are my own

Response:

du writes: Yeast have mitochondria, and are therefore capable of respiration. Energetically, reducing sugars to CO2 taps 80% more of the potential than fermentation. Why do our yeast friends give us alcohol instead of pure CO2?

Alchohol is the organic product of respiration under anaerobic conditions. It is nescessary to have oxygen to complete the entire catabolic cycle of sugar molecules from sugar to CO2. The problem encountered in brewing is that the presence of oxygen (as air) often carries with it microorganisms which convert the alchohol to acetic acid (vinegar), or other undesireable organic compounds. I use an airlock with water in it, and I would imagine that O2 dissolves in the water, and moves into the carboy down its concentration gradient (though I’m sure it is slow). I also know that some people do open fermentation and still get beer. Maybe the large number of yeast in the wort keep it anaerobic by rapidly using any O2 that does get into the wort.

More likely is the fact that CO2 is heavier than O2, and tends to lie in a blanket over the wort, protecting it against oxidization (to some degree). In case this isn’t clear, I have two questions: 1. Does oxygenation of the wort reduce the amount of alcohol produced? 2. If you were a yeast, why would you ferment when you could respire? -Just curious (I know this should really go to bio.yeast.physiology, but I couldn’t find such a group:)

1. If you mean reduce the amount of alchohol to produce other compounds, Yes. 2. Yeasts belong to a group of organisms called "facultative anaerobes" which means they can undergo aerobic respiration in the presence of oxygen to metabolize carbohydrates to CO2 and water, and ferment carbohydrates in the absence of oxygen to produce CO2 and ethanol. -David

– T.L.Schadler AV/ID Services–TLS1 Xmas Soucon Annex #14A

Response:

Yeast have mitochondria, and are therefore capable of respiration. Energetically, reducing sugars to CO2 taps 80% more of the potential than fermentation. Why do our yeast friends give us alcohol instead of pure CO2? I use an airlock with water in it, and I would imagine that O2 dissolves in the water, and moves into the carboy down its concentration gradient (though I’m sure it is slow). I also know that some people do open fermentation and still get beer. Maybe the large number of yeast in the wort keep it anaerobic by rapidly using any O2 that does get into the wort. In case this isn’t clear, I have two questions: 1. Does oxygenation of the wort reduce the amount of alcohol produced? 2. If you were a yeast, why would you ferment when you could respire? -Just curious (I know this should really go to bio.yeast.physiology, but I couldn’t find such a group:) -David

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