The fragments of the hydrolysis are metabolized by facultative and fastidious anaerobes to carboxylic acids
like acetic acid, propionic acid, butyric acid, valeric acid and hexane acid. In addition, they are metabolized
to alcohol, hydrogen (H2) and carbon dioxide (CO2). There is a poor change in the energy content during
the acidification, the so called COD (chemical oxygen demand). Acidogenic bacteria have a high pH-tolerance,
so the milieu can get very acidic. At the acidification of carbohydrates the pH-value can decline fast and far
(as far as pH < 4,0), because in contrast to protein acidification there are no pH-buffering ions which are
setting free to keep the pH-value between 7,0 and 7,5.
The partial pressure of hydrogen plays a decisive role at the acidification, because of the effect on the
hydrogen transporter NAD^^. The lowest effect
on the agitation takes the loaded NAD-molecules (NADH + H^+^) if their regeneration is carried out by a H2-Production.
Figure 4: Oxidation of NADH + H^+^
This oxidation will just happen, if the partial pressure of the hydrogen is low. If it is too great, the loaded
NAD-molecules enter into the agitation path and cause the generating of alcohols instead of acids. Depending on
a high partial pressure of hydrogen there is an increasing existence of H^+^-Ions and so the pH-value is
getting lower. Also the generating of propionic acid and butyric acid is increased and in series the generating
of methane by propionic acid is inhibited. The partial pressure of hydrogen can just hold down by hydrogen using
bacteria, for example methanogenic bacterials.