Kind of a technical question about fermentation.

But I'm curious...I've just recently (as in today) gotten interested in Homebrewing so I've been reading the Wiki on the steps.

One step that caught my eye was aeration, adding O2 to the wort. Perhaps I misunderstood but I thought alcohol was the product of anaerobic glycolysis? Where as adding the oxygen would result in (with the presence of pyruvate dehydrogenase, which the yeast may not have) citric acid?

Now, obviously the oxygen works, has for years, but does anyone know why the yeast turn it into alcohol (ethanol if you want to get technical) instead of citric acid?


Or perhaps lactic acid is the anaerobic product where as ethanol and citric acid are the aerobic products dependent on presence of enzymes. So yeast would have alcohol dehydrogenase (and pyruvate carboxylase) instead of pyruvate dehydrogenase?

Anyone know? It's been a while since I've learned this stuff.

There are some people on here that are chem engineers and will be able to answer your question. As far as my response..... uhhhhh...... relax have a homebrew?

Um..... the answer is Tuseday

O2 isn't used for fermentation. Your aerate your wort because yeast use O2 to multiply and make more east. Once the O2 is used up, then the yeast go about fermentation. That's what happens during the lag time between pitching and start of fermentation.

MikeFlynn74 said:

Um..... the answer is Tuseday

Click to expand...

Mike, the answer is always 42!

O2 isn't used for fermentation. Your aerate your wort because yeast use O2 to multiply and make more east. Once the O2 is used up, then the yeast go about fermentation. That's what happens during the lag time between pitching and start of fermentation.

Click to expand...

Hrm, but to have them multiply wouldn't that require the consumption of oxygen for energy? That's what the sugars are for. I'm just wondering what pathway they use and why the oxygen is necessary.

We need oxygen for the end electron acceptor in the electron transport chain. I'm just wondering why they need it.

relax have a homebrew?

Click to expand...

Can't No homebrew stuff yet. Accepting donations though.

I think this is what you're looking for:

MB Raines, Ph.D. - Guide to Yeast Culturing for Homebrewers - Maltose Falcons Home Brewing Society (Los Angeles Homebrewing)

From wikipedia

Yeasts are chemoorganotrophs as they use organic compounds as a source of energy and do not require sunlight to grow. The main source of carbon is obtained by hexose sugars such as glucose and fructose, or disaccharides such as sucrose and maltose. Some species can metabolize pentose sugars like ribose,[14] alcohols, and organic acids. Yeast species either require oxygen for aerobic cellular respiration (obligate aerobes), or are anaerobic but also have aerobic methods of energy production (facultative anaerobes). Unlike bacteria, there are no known yeast species that grow only anaerobically (obligate anaerobes). Yeasts grow best in a neutral or slightly acidic pH environment.

Click to expand...

Schoffleine said:

Hrm, but to have them multiply wouldn't that require the consumption of oxygen for energy? That's what the sugars are for. I'm just wondering what pathway they use and why the oxygen is necessary.

We need oxygen for the end electron acceptor in the electron transport chain. I'm just wondering why they need it.




Can't No homebrew stuff yet. Accepting donations though.

Click to expand...

But if you turn up the fluxcapacitor to 1.21 jiggawatts you will have your answer! :cross:

One step that caught my eye was aeration, adding O2 to the wort. Perhaps I misunderstood but I thought alcohol was the product of anaerobic glycolysis? Where as adding the oxygen would result in (with the presence of pyruvate dehydrogenase, which the yeast may not have) citric acid?

Now, obviously the oxygen works, has for years, but does anyone know why the yeast turn it into alcohol (ethanol if you want to get technical) instead of citric acid?

Click to expand...

You're overthinking a little bit.

Yeast do not turn oxygen into alcohol. Yeast metabolize malt and other sugars into carbon dioxide and ethanol.

ChshreCat said:

O2 isn't used for fermentation. Your aerate your wort because yeast use O2 to multiply and make more east. Once the O2 is used up, then the yeast go about fermentation. That's what happens during the lag time between pitching and start of fermentation.

Click to expand...

There are two main phases to fermentation: Aerobic and Anaerobic. During the aerobic phase, yeast use oxygen dissolved in the wort to [FONT=Verdana, Arial, Helvetica, sans-serif]facilitate their growth processes. [/FONT][FONT=Verdana, Arial, Helvetica, sans-serif]Yeast use oxygen and other materials dissolved or suspended in the wort - lipids and proteins and minerals - to synthesize[/FONT] materials essential to reproduction.
[FONT=Verdana, Arial, Helvetica, sans-serif]
An oxygen-rich wort permits the yeast to quickly reproduce to levels that will ensure a good fermentation. When the oxygen is used up, the yeast switch to anaerobic metabolism, cease reproducing as vigorously, and begin the process of metabolizing sugars into alcohol.

Make sense?

Bob
[/FONT]

Fermentation (sugar into alcohol and CO2) is just the "main" thing yeast do. There is a series of actions/reactions leading up to actual fermentation. For example, glycogen, oxygen, ADP -> ATP, etc. are all involved. There's a great explanation of it in the book New Brewing Lager Beer (Greg Noonan). In short, oxygen is essential for yeast growth. (That is to say, it uses oxygen to make a fatty acid that it needs for yeast growth, which can be given to the yeast directly with olive oil, but that's another topic.) By the time actual fermentation starts, oxygen is used up and/or has come out of solution.

I admire your knowledge on the subject, and curiosity is normally a good thing. However, you are complicating matters to the point that if I had been forced to understand it before I started brewing, then I would still be drinking Bud.
Increasing your depth of understanding is a good thing, but the most important thing you need to know is: you pour the cooled sweet liquid into a bucket, shake the crap out of it to aerate, then toss the yeasties into it and trust them to do what they do best..... make beer.

Schoffleine said:

Hrm, but to have them multiply wouldn't that require the consumption of oxygen for energy? That's what the sugars are for. I'm just wondering what pathway they use and why the oxygen is necessary.

We need oxygen for the end electron acceptor in the electron transport chain. I'm just wondering why they need it.

Click to expand...

My cats breath smells like cat food

Mike, the answer is always 42!

Click to expand...

doh stole my thunder!

Schoffleine said:

We need oxygen for the end electron acceptor in the electron transport chain. I'm just wondering why they need it.

Click to expand...

Yeast need it for exactly the same reason you do. The basic metabolism between a yeast and humans is pretty much the same. Glycolysis, gluconeogenesis, beta-oxidation, etc. are pretty much the same in both. It is the specialized biochemical pathways that make each unique.

Yeast do not turn oxygen into alcohol. Yeast metabolize malt and other sugars into carbon dioxide and ethanol.

Click to expand...

Well I realize that, I was just wondering what the oxygen was used for in the conversion of pyruvate to ethanol. (assuming that's what it's used for; I would assume so unless it's used in another pathway all together).

I think the problem is that I was stuck in thinking that Pyruvate -> Acetaldehyde -> Ethanol was an anaerobic process when I guess it's really aerobic. I could've sworn that of the three primary fates of pyruvate - citric acid, ethanol, and lactic acid - that ethanol and lactic acid were both anaerobic processes. But I reckon lactic acid is the only one that's truly anaerobic.

Edit: OK it seems that the oxygen isn't even used for fermentation. So fermentation is indeed an anaerobic process. So I reckon the oxygen is just used in another energy pathway of theirs.

Yeast need it for exactly the same reason you do. The basic metabolism between a yeast and humans is pretty much the same. Glycolysis, gluconeogenesis, beta-oxidation, etc. are pretty much the same in both. It is the specialized biochemical pathways that make each unique.

Click to expand...

Like pjj said.

However, you are complicating matters to the point that if I had been forced to understand it before I started brewing, then I would still be drinking Bud.

Click to expand...

Lol well I was forced to understand this stuff for my degree. I'm just applying it to beer making instead. Makes the degree all the more worthwhile.

Is this a test? Because if it is I didn't study, and I'd only be guessing, but I'd pick C.

Schoffleine said:

Edit: OK it seems that the oxygen isn't even used for fermentation. So fermentation is indeed an anaerobic process. So I reckon the oxygen is just used in another energy pathway of theirs.

Click to expand...

Right. The pathway from pyruvate to ethanol is not the "best" pathway. Ethanol is toxic to the yeast but they need to convert the pyruvate for 2 reasons: accumulation of pyruvate will slow the metabolism of glucose (which generates some energy) and that they need to oxidize the NADH to NAD+ to "refresh" the NAD+ which is needed for one of the intermediate steps in the conversion of glucose to pyruvate.

If oxygen is present the pathway for pyruvate to enter the citric acid cycle is preferred as oxygen is the final "sink" for that pathway. This all happens in humans too. But, we do not have the enzyme to make the ethanal, we convert the pyruvate to lactate. This happens in muscle under high loads when oxygen depleted.

Also, oxygen is needed to build the cell walls (unsaturated fatty acids, sterols, etc.) of daughter cells. You need this since yeast reproduce by budding.

MikeFlynn74 said:

Um..... the answer is Tuseday

Click to expand...

not on a leap year its Monday.

DutchK9 said:

But if you turn up the fluxcapacitor to 1.21 jiggawatts you will have your answer! :cross:

Click to expand...

Cue Doc Brown: "1.21 jiggawatts!!!!!!!!!!"

Cue Marty McFly: "What the hell is a jiggawatt????"

Cue commercial that always happens after that line...

Right. The pathway from pyruvate to ethanol is not the "best" pathway. Ethanol is toxic to the yeast but they need to convert the pyruvate for 2 reasons: accumulation of pyruvate will slow the metabolism of glucose (which generates some energy) and that they need to oxidize the NADH to NAD+ to "refresh" the NAD+ which is needed for one of the intermediate steps in the conversion of glucose to pyruvate.

If oxygen is present the pathway for pyruvate to enter the citric acid cycle is preferred as oxygen is the final "sink" for that pathway. This all happens in humans too. But, we do not have the enzyme to make the ethanal, we convert the pyruvate to lactate. This happens in muscle under high loads when oxygen depleted.

Also, oxygen is needed to build the cell walls (unsaturated fatty acids, sterols, etc.) of daughter cells. You need this since yeast reproduce by budding.

Click to expand...

Makes sense.

Mystery solved y'all. You don't have to go home but you can't stay here.