In the previous kitchen chemistry post, we discussed the metabolic process by which glucose and oxygen is converted to carbon dioxide plus water, together with a great deal of energy:
In contrast, without oxygen, only one eighteenth of the energy can be obtained, through the breakdown (glycolysis) of glucose into pyruvic acid and hydrogen atoms (which are temporarily attached to the coenzyme NAD, and must be recycled in a later stage of metabolism):
In the absence of oxygen, there are two ways of taking this reaction further, and recycling the hydrogen atoms (neither alternative produces any extra energy). In our muscles (when short bursts of energy are required, as in sprinting), the pyruvic acid is converted into lactic acid. The bacteria that convert milk into yoghurt do the same thing (and it is the lactic acid which gives yoghurt its sour taste):
Alternatively, in yeast, the pyruvic acid is converted into acetaldehyde and carbon dioxide. In a second step, the acetaldehyde in turn is converted into alcohol (ethanol, C2H5OH):
This fermentation process is used in the making of beer and wine (where the alcohol is the desired product – in ancient times, the germ-killing properties of alcohol made beer or diluted wine two of the few safe thirst-quenchers). Such fermentation is also used in bread-making (where the bubbles of carbon dioxide gas are the desired product, making the bread dough rise).
Because glycolysis produces so little energy, sourdough cultures are forever hungry, and constantly need to be fed with flour.