Fermentation is a metabolic process that converts sugar to acids, gases or alcohol. It occurs in yeast and bacteria, but also in oxygen-starved muscle cells, as in the case of lactic acid fermentation. Fermentation is also used more broadly to refer to the bulk growth of microorganisms on a growth medium, often with the goal of producing a specific chemical product which in this case is beer. Fermentation takes place in the lack of oxygen. The number & capacity of fermenter tanks derives the production capacity of beer in a microbrewery.