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Biomass is already broken down by biochemical processes in nature because biomass consists of natural substances. These processes, which can be listed as aerobic and anaerobic degradation, fermentation, enzymatic hydrolysis, are realized by breaking down the biomass of bacterial enzymes or microorganisms [79].

CH₄ and CO₂ gases are formed together with the solid product in the anaerobic disintegration performed by the bacteria using oxygen in the biomass instead of oxygen in the air. In aerobic decomposition, microorganisms that break down the biomass produce CO₂, heat and solid products by using oxygen in the air. The process in which biomass is converted to sugar and then to ethanol and other chemicals using yeast with acid or enzyme is also called fermentation. The product obtained from fermentation is liquid, unlike anaerobic breakdown. Although the conversion of starch and sugar-based raw materials to ethanol by fermentation is used for commercial purposes, it is difficult and expensive to break down lignocellulosic biomass, such as wood, into sugars. It is not commercially preferable to apply fermentation to lignocellulosic biomass as a hydrolysis pretreatment is required to convert cellulose and hemicellulose into sugar [80].