Yeast Metabolism: Where does Alcohol Come From?

A few posts ago, I wrote about how the human body breaks down alcohol.  But, how is alcohol formed in the first place?  Sure, sugary water plus yeast equals ethanol, but there’s a bit more to it than that.

The first step in the yeast metabolic cycle is to get sugar into the yeast cell.  Logical enough.  The cell does so through little “portals” in the cell wall; glucose enters through the glucose transporter, while maltose enters through a symport at a slower rate.  The enzyme maltase then does the work of converting the maltose into two glucose molecules.  Fructose is treated the same as glucose, while maltotriose is taken up last, or not at all.  Once glucose is in the cell, the magic can happen.

Yeast (saccharomyces cerevisiae) viewed with a DIC microscope

Glycolysis

Glycolysis is the process of transforming glucose into pyruvate (aka pyruvic acid), which is used in the next step of yeast metabolism.  The complete reaction is summarized here (though there are many intermediate reactions that must take place for this process to occur- 8 by my count):
glucose + 2 NAD⁺ + 2 ADP + 2 P_i → 2 NADH + 2 ATP + 2 pyruvate.
NAD (nicotinamide adenine dinucleotide) is a helper molecule which scuttles electrons back and forth in many bio-chemical reactions, and exists in the NAD⁺ form, capable of receiving electrons (an oxidizing agent), and NADH form, capable of giving electrons (a reducing agent).  P_i is inorganic phosphate.  ATP (adenosine triphosphate) is just a unit of cellular energy, and allows the cell to carry out many functions.

So, the result of glycolysis is pyruvate, two units of energy, and two units of NADH.  The yeast cell now has pyruvate to continue the metabolic cycle, some extra energy, and NADH.  Given that there is a limited supply of NAD in the cell, the next step in the process must use NADH and produce  NAD⁺ to keep things in balance, as we’ll see it does.  Glycolysis is anaerobic, which means it does not require oxygen, useful since wort is fermented without oxygen for most of the time.

After glycolysis, the pyruvate can be used in either the Krebs cycle (aerobic), or anaerobic respiration.

Krebs Cycle

The Krebs cycle, also known as the tricarboxylic acid (TCA) cycle or the citric acid cycle, is a circular and repeating set of reactions which requires oxygen.  In beer making, this would occur in the first stage of fermentation when the yeast is pitched into a well aerated wort, and carries on until all oxygen is used up.

Pyruvate (are you tired of this word yet?) is first converted to acetyl-CoA (pronounced “Co-A”) in the following reaction:
pyruvate + 2 NAD⁺ + CoA-SH → acetyl-CoA + CO₂ + NADH,
with the help of the pyruvate dehydrogenase (PDH) complex.  Note that this is the first time CO₂ is produced, and yet more NADH is generated.

This acetyl-CoA then enters into a cycle of reactions which nets two molecules of CO₂, one GTP (guanosine triphosphate, another unit of energy equivalent to ATP), three NADH, and one FADH₂ (flavin adenine dinucleotide, which functions similarly to NADH).  After the cycle completes, another acetyl-CoA molecule enters and the cycle repeats itself.

But wait, this just made more NADH, and we need to regenerate NAD⁺ so glycolysis can continue.  Both the NADH and FADH₂ now donate their electrons to a process called the electron transport chain/ oxidative phosphorylation.  The result is a return of NAD to the NAD⁺ state, and a large amount of ATP cellular energy.

Because the Krebs cycle is so efficient at producing ATP energy units, this is the yeast’s preferred pathway.  But, you’ll notice a rather conspicuous absence: ethanol.  This is only formed in the absence of oxygen.

Anaerobic Fermentation

This is a relatively simple process compared to the Krebs cycle.  Once again we start with pyruvate, but this time we have no oxygen.  An enzyme called pyruvate decarboxylase transforms pyruvate into acetaldehyde and CO₂.  Acetaldehyde is toxic to both humans and yeast.  Fortunately, just like humans, yeast have the alcohol dehydrogenase (ADH) enzyme.  In humans, this enzyme breaks ethanol into acetaldehyde; in yeast, the reverse process happens, and acetaldehyde is converted (reduced) to ethanol.

Ethanol is also toxic to yeast, but brewer’s yeast can handle a much higher concentration than other microorganisms.  The yeast actually use ethanol to kill off competing organisms.

In the process of forming ethanol from acetaldehyde, NADH is converted to NAD⁺ returning balance to the force, and allowing glycolysis to continue, but without the extra ATP generated by the Krebs cycle.

[2014.06.30 UPDATE: Thanks to Matt of A Ph.D. in Beer for minor corrections in this section.]

Summary

The yeast metabolic process starts with glycolysis which forms pyruvate from sugar.  From this point, the yeast can continue with the aerobic Krebs cycle or anaerobic respiration.  The yeast will prefer the Krebs cycle if oxygen is present because of the greater levels of energy produced; lacking oxygen, the yeast will produce ethanol. CO₂ is produced either way.

This is just one reason to aerate the wort when pitching yeast: they are able to produce much more energy per unit of glucose.  But, the wort should be held in an anaerobic state for the rest of fermentation so the yeast will produce alcohol.

[Information for this post comes mainly from the Oklahoma University “Chemistry of Beer” MOOC, Fermentation unit, posted 4 Apr 2014.  Additional information can also be found in Dr. Fix’s Principles of Brewing Science, chapter 3.  Image from http://en.wikipedia.org/wiki/File:S_cerevisiae_under_DIC_microscopy.jpg, slightly modified.]

Advertisement