by Paul Winalski » Tue Apr 28, 2015 10:42 am
Good article, but the biochemistry isn't quite right. All sugar metabolism starts with a process called glycolysis, which starts with glucose (other sugars are converted to glucose) and ends by producing two molecules of pyruvic acid, two molecules of water, and two free hydrogen atoms. The energy released by this process is captured by the formation of two molecules of adenosine triphosphate (ATP). This process is anerobic--it does not require oxygen to take place. Just about all cells do glycolysis.
The problem then is what to do about those two free hydrogen atoms and the pyruvate.
The first option is respiration, where the hydrogens atoms are used to reduce molecular oxygen to water, releasing quite a lot of energy that again is captured as ATP. The pyruvate is converted to acetate and CO2. The acetate enters a process called the Krebs cycle where it is converted to two molecules of CO2 and several more pairs of hydrogens, which undergo respiration and generate more energy.
Respiration can only take place where there is molecular oxygen available. In the absence of oxygen, cells use one of several pathways called fermentation. There are two main ones:
In homolactic fermentation, the two hydrogens are used to reduce the pyruvate to lactate. This is the process that takes place in human cells when oxygen levels are low. For example, in strenuous exercise muscle tissue is consuming energy faster than respiration can produce it, so it uses fermentation instead. The lactic acid is carried to the liver where it is rebuilt into glucose. Buildup of lactic acid in muscle tissue is responsible for muscle fatigue.
Ethanol fermentation is a two-step process. First pyruvate is converted to acetaldehyde and CO2, then the acetaldehyde is reduced to ethanol, consuming the spare hydrogens. This is the pathway yeasts use when oxygen isn't available. Releasing ethanol instead of lactic acid avoids acidifying the environment that the yeasts are living in.
Respiration produces far more energy than fermentation does, so cells that are capable of respiration prefer to use it when they can. For example, there is oxygen in bread dough and the yeasts produce only CO2 and water. Yeasts only resort to fermentation when there isn't any oxygen present, as rapidly becomes the case in a wine fermentation vat.
Carbonic maceration forces the cells in grape tissue to use ethanol fermentation rather than respiration for their energy needs. As noted in the article, they can't keep this up for long and soon die.
-Paul W.
