Microorganisms Used in the Fermentation
Biochemical conversion of sugars to ethanol or fermentation can be carried out by fungi or bacterial microorganisms. The use of yeast, commonly known as Baker s yeast, is the oldest traditional method of fermenting sugars into alcohol. Yeasts are eukaryotic microorganisms classified in the kingdom Fungi, and are mostly unicellular organisms, although some species with yeast forms may become multicellular through the formation of a string of connected budding cells known as pseudohyphae. Yeast size can vary greatly depending on the species, typically measuring from 5-10 micrometers (gm). Most yeasts reproduce asexually by mitosis and many do so by an asymmetric division process called budding. The common Baker’s yeast is known as the native or wild type species Saccharomyces cerevisiae. In addition to this, several wild yeast species Pachysolen tannophilus, Candida. shehatae, Pichia stipitis, Candida brassicae, Mucor indicus, etc., are also known to produce ethanol from sugars. Certain types of bacteria can also be used in the fermentation of sugars to ethanol. Bacteria constitute a very large domain of prokaryotic microorganisms, typically a few micrometers in length, and have a wide range of shapes, ranging from spheres to rods and spirals. Some of the widely studied bacteria species for the conversion of sugars to ethanol include Zymomonas mobilis, Clostridium ljungdahlii, Clostridium thermocellum, and Thermoanaerobacterium.