The Brewing Continues

Both fermenters are bubbling like crazy today; I don't have a rotameter hooked up, but I would guess that carbon dioxide production is peaking in excess of 100mL per minute.  Everything in the must is now suspended, buoyed by the bubbles rising from the yeast, which are now floating in a cloudy layer near the surface.

It's obvious enough that yeast produces a ton of carbon dioxide during anaerobic growth... but how?  Molecular oxygen is not present to act as the final electron acceptor in oxidative phosphorylation, so the TCA cycle (the source of the carbon dioxide produces by you right now) certainly isn't active.  For a closer look, we go to the basic (unbalanced) bioreaction stoichiometry:

C₆H₁₂O₆ (sugar) + CH_1.9O_0.51N_0.23 (protein) -> CH1_1.77O_0.49N_0.24 (biomass) + C₂H₆O (ethanol)

Most available protein is going to biomass (which has almost the exact same stoichiometric ratios as protein), along with some of the sugar.  Most of the sugar will go toward generating reducing equivalents (NADH) and energy carriers (ATP), in which it will be almost stoichiometrically converted to ethanol.  If you take a look at that stoichiometry:

C₆H₁₂O₆ (sugar) -> C₂H₆O (ethanol)

Glucose on the left has a 1:1 C:O ratio, whereas ethanol has a 2:1 C:O ratio.  Carbon dioxide has a 1:2 C:O ratio, so this is a likely solution for closing the mass balance!  Of course, this is all nasty global inference, which should be left to systems biology (fake science).

What really happens?  Pyruvate decarboxylase (using thiamine pyrophosphate [pictured] in an acid-catalyzed reaction) hacks pyruvate into acetaldehyde and carbon dioxide.  The acetaldehyde is converted into ethoxide by reaction with NADH, and is then protonated to yield ethanol.  This is 'Ethanolic Fermentation'.

(Isn't mechanistic biochemistry so much more satisfying?)