Fermentation: the inefficient trick behind bread, beer and sprinting muscles
Pasteur called it life without air. Fermentation lets cells squeeze energy from sugar with no oxygen at all, and people have exploited it to make and preserve food for some 13,000 years. The catch is thrift: it yields only 2 to 5 units of ATP per glucose, against about 32 when oxygen is available.
Chemically, fermentation is a way of breaking down organic molecules in which another organic compound receives the released electrons, rather than oxygen or some inorganic substance. Glucose is split, usually via glycolysis, into pyruvate, and from there pathways branch toward a range of end products. The variety is remarkable: nearly 300 combinations of products are known, and the discovery of new ones suggests the full diversity is still unmapped.
Because it needs no outside electron acceptor, fermentation works almost anywhere, which is why more than a quarter of bacteria and archaea use it. It is especially common among the Bacillota and rare among the Actinomycetota, and fermenting microbes cluster in places like animal guts, sediments and food. Typical products include lactate, acetate, ethanol, carbon dioxide, hydrogen and butyrate. Oddly, it was probably not a useful energy source for the earliest life, because the ancient ocean held little of the complex organic fuel it requires.
Our bodies ferment too. During hard exercise, and in emergencies such as sepsis or severe blood loss, human cells fall back on it to top up aerobic energy for somewhere between 10 seconds and 2 minutes. Lactate builds up and eventually forces a rest. Gut microbes, meanwhile, ferment food and release products that we and our livestock absorb for energy.
In the kitchen, fermentation preserves cereals, dairy, rice, honey and bread while adding flavour and texture, and modern brewing relies on specially engineered yeasts and substrates. Industry uses it on a vast scale to make commodity chemicals: ethanol for beer, wine and spirits, and lactate, which becomes lactic acid for curing, flavouring and keeping food from spoiling. Louis Pasteur's famous phrase dates from 1876, before anaerobic respiration had even been discovered.
Source: Fermentation
