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Acidogenesis (sometimes referred to as fermentation) is the biological process that results in further breakdown of the remaining components by acidogenic (fermentative) bacteria. In this process, volatile fatty acids are produced, along with (depending upon the feedstock) ammonia, carbon dioxide, and hydrogen sulfide, as well as other byproducts. Thus:

Whereas the production of volatile fatty acids (VFAs) is increased when the process pH is in excess of 5, the production of ethanol (C₂H₅OH) is characterized by a pH lower than 5 with reaction process coming to a halt at a pH less than 4.

In a balanced bacterial process approximately 50% of the monomers (glucose, xylose, amino acids) and long-chain fatty acids (LCFA) are broken down to acetic acid (CH₃COOH). Twenty percent is converted to carbon dioxide (CO₂) and hydrogen (H₂), while the remaining 30% is broken down into short-chain volatile fatty acids (VFAs). Fatty acids are monocarboxylic acids that are found in fats and have fewer than six carbon atoms whereas long-chain fatty acids. If there is an imbalance in the digester process, the relative level of volatile fatty acids will increase with the risk of accumulation, since the bacteria that degrade the volatile fatty acids have a slow growth rate and cause an imbalance between the various phases of the process. A steady degradation of the volatile chain fatty acids is therefore crucial and often a limiting factor for the biogas process.

Hydrolysis of simple fats results in 1 mol glycerol and 3 mol long-chain fatty acids and, therefore, high proportions of fat in the digester feedstock will result in large amounts of long-chain fatty acids, while large amounts of protein, which contain nitrogen in amino groups (-NH₂), will produce large amounts of ammonium/ammonia (NH⁴⁺/NH₃). In both cases this can lead to inhibition of the subsequent decomposition phase, particularly if the composition of the biomass feedstock varies.

See also: Acetogenesis, Acidogenesis, Acidogenic Digestate, Anaerobic Digestion, Methanogenesis.