MadSci Network: Biochemistry

Re: how does yeast produce both alcohol and CO2?

Date: Fri Aug 20 15:19:48 1999
Posted By: Carl Custer, Staff, Office Public Health & Science, Scientific Research Oversight Staff , USDA FSIS OPHS
Area of science: Biochemistry
ID: 934512476.Bc

Good question, bad spealing, but most uv us aren’t perfeck. ;^)
Short answer: 
Glucose (from starch and other sugars) is a six carbon molecule.  The 
enzymes in yeast (as well as bacteria, people, and other organisms) break 
the six carbon molecule in half, tack on oxygen molecules to pick up energy 
as NAD and end up with two molecules of pyruvate (Pyruvate looks kinda like 
this: H3C-CO-COOH). 
For alcohol (specifically ethanol), the COO is snipped off yielding: 
COO and H3C-CHO.   COO is carbon dioxide, H3C-CHO is acetaldehyde, which is
then reduced to CH3CH2OH, ethanol.  
NAD: Nicotinamide adenine dinucleotide (energy packet carrier)
Longer answer:
Sugars have the general formula of CH2O (that’s why they’re called 
The most common sugar is glucose (A.K.A. dextrose, blood sugar).  It’s a 
six carbon mono-saccharide (sounds fancier than one-ah sugar-ah) and is the 
building block for many poly-saccharides such as starch and cellulose.  
Cane sugar, sucrose, is a di-saccharide containing a glucose and a fructose 
(another six carbon sugar) hooked together. 
Second, common brewers (& bakers) yeast cannot break down starch, only 
sugar.  So, you either have to break down the starch with a process, such 
as malting or mashing (used by brewers), or by just using sugar in the mix. 
 (Interestingly, brewers-bakers yeast is called Saccharomyces cerevisiae.  
Interesting because an important word in Spanish is “cerveza” as in 
“Quisiera una cerveza, por favor” see: bablefish  http:/
Breaking down the glucose:
The yeast takes in the six carbon glucose, splits it in half, and adds 
oxygen to each half in a series of several steps known as “glycolysis” or 
the Embden-Meyerhoff-Parnas pathway (more often as  Embden-Meyerhoff 
pathway.  Parnas, alas, was the graduate student that made the connection 
between the Embden’s and Meyerhoff’s work [sigh]).
The end result is a three carbon molecule known as “pyruvate” or pyruvic 
acid.  Picture at:   http:// 
Pyruvate is important as it is at the cross roads to several pathways 
including lactic acid (yoghurt, pickles, salami) and the Krebs cycle 
(that’s why you have to breathe) which leads to amino acids and so forth.  
On to ethanol:
The next step is to snip off the oxidized end carbon from the pyruvate 
(H3C-CO-COOH) -- which being connected to two oxygen molecules -- is 
carbon dioxide (OCO).  This leaves a two carbon piece which is reduced 
to ethanol ( CH3CH2OH).   tah dah.  
BTW you already know from inorganic chemistry that OH- is an alkali but in 
organic chemistry, -OH is an alcohol. 
Thus, with yeast, you can’t get ethanol without carbon dioxide - but by 
taking another route from pyruvate (to acetyl CoA) you can get carbon 
dioxide without ethanol. 
Want more?: http:// 
Want something else?: 
Tequila, well actually pulque.  The original fermentation of agave produced 
an alcoholic drink called pulque.  Unique thing about this fermentation is, 
it isn’t a yeast but a bacterium Xymomonas lindnerii!! It uses the Entner 
Douderoff Pathway or another way to convert glucose to pyruvate. Try 
AltaVista to learn more.  For Tequila, a form of 
mescal, the agave is baked to release the sugars, fermented with yeast, 
then distilled, yielding mescal.  

Admin Note:  As alluded to above, brewers can alter the alcohol and carbonation
of the substance they're brewing (whether it be beer, wine, or soft drinks) simply
by adjusting the amount of oxygen available to the fermenting microbes - longer
aerobic fermentation yields carbonation with little ethanol; longer anaerobic
fermentation yields lots of ethanol with a little fizz.  Similarly, brewing in a closed
container is essential for maintaining good carbonation.

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