CoachA fermentation uses no external acceptor: the organic substrate is split into a more oxidised and a more reduced product, ATP is made only by substrate-level phosphorylation, and the NADH of glycolysis is re-oxidised by reducing a metabolite. Yeasts make ethanol and ; lactic bacteria make lactate (yoghurt, sauerkraut, sore muscles); E. coli makes a mixture of acids, ethanol and gases; clostridia make butyrate, butanol and acetone; propionibacteria make the propionate and of Swiss cheese. The yield is two ATP per glucose against about thirty by respiration, so a fermenter must process fifteen times more sugar for the same growth, and its products — acids, alcohol — poison its own medium: fermentations preserve food because the fermenter makes the food uninhabitable, including for itself.
Examples
Example 2.8 (Syntrophy: living on the rest)
In an anoxic sediment the fermenters leave behind acetate, propionate, butyrate and . Oxidising propionate to acetate and has a positive standard free energy () and is impossible — unless a methanogen next door consumes the hydrogen as fast as it forms, keeping its partial pressure below , at which the reaction becomes exergonic. Neither partner can grow alone on propionate; together they turn it into methane. This interspecies hydrogen transfer is why anaerobic digestion is always the work of a consortium, and why the archaeon and its bacterial partner are often found pressed against each other.