|MadSci Network: Biochemistry|
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 carbo-hydrates) 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:/ /babelfish.altavista.com/cgi-bin/translate?) 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:// www.johnco.cc.ks.us/~pdecell/cellresp/pyruvate.html 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:// www.microbiology.med.umn.edu/CoursesF97/MicB5105-4.html 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 http://www.altavista.com 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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