The Role of Yeast in Alcohol Fermentation – The Catalysts Group

The Role of Yeast in Alcohol Fermentation

What is Yeast?

Yeast is a unicellular eukaryotic microorganism, belonging to members of fungal species found in soil and plant surfaces such as fruits, Flowers etc. Yeast is abundant in nature and approximately 1500 varieties are known to mankind. It is a facultative anaerobe that can grow both in presence & absence of oxygen with varying sizes from 3 microns to 40 microns depending on the species and reproduce asexually by process of budding or fission.

Yeast is used for various purposes and few of them are listed below:-

  • Alcoholic Beverages – Beer & Wine
  • Baking in Food Industry
  • Bioremediation – process of clearing pollutants using microorganisms.
  • Industrial Ethanol Production
  • Non-alcoholic beverages
  • Nutritional supplements
  • Probiotics
  • As a model organism in Scientific research
  • Genetically engineered bio factories
  • Generation of electricity in microbial fuel cells

Yeast & Alcohol Fermentation 

Role of yeast in fermentation of alcohol has a rich & interesting history. Yeast etymologically means boil, bubble or foam. This is the earliest known microorganism which was domesticated by mankind, its evidence dating back to 5000 years ago. Archaeological digging found evidence of yeast-raised bread and designs of breweries about 4000 years old. They found vessels which were believed to have contained alcoholic beer and mead, containing yeast colonies that survived civilizations, providing the first ever biological use of yeast in the pre-modern era.  Countries like early Egypt knew how the alcoholic beverages were brewed, but back then it was considered to be a mystic process, rather than a comprehensible one. Between the 17th century to 19th century, the role of yeast was not clear in the fermentation process though it was considered essential. It was only after Louis Pasteur’s scientific breakthrough called Pasteur Effect, in 1857, that the role of yeast in fermentation of alcohol was understood and established. This in a way opened floodgates for commercial use of yeast in fermentation, when in the late 18th century two yeast strains were discovered and identified for commercial use namely – Saccharomyces cerevisiae and S. carlsbergensis. The yeast species Saccharomyces cerevisiae (top fermenting yeast) & S. carlsbergensis (bottom fermenting yeast) are majorly used in the alcohol fermentation process where the latter one mostly in case of beer fermentation, while the former in other alcoholic beverages around the world.

The ethanol  industry uses various raw materials to produce alcohol by yeast fermentation. The process involves grating a feedstock, like sugarcane, field corn and other cereals, and subjecting it to enzyme action (alpha-amylase, glucoamylase) to convert complex carbohydrates into simple sugars. Which in turn is taken up by yeast to convert into ethanol.

Yeast behave differently in presence (propagation) & absence (fermentation) of oxygen and reproduces asexually by budding or Fission. During industrial ethanol fermentation, Yeast converts carbohydrates, such as starch and sugar into carbon-di-oxide and alcohol. While CO2 is used for production of dry ice, alcohol is used in the beverages industry across the globe and as fuel

Yeast is inseparable to the process of alcohol fermentation, even though there are other microorganisms that can generate glucose in absence of oxygen and produce alcohol & other organic acids and all of those capable of anaerobic metabolism do produce ethanol, however they are inept at producing them in sufficient quantities. The main issue with fermentation with organisms other than yeast is that they cannot tolerate the impact of alcohol even at lower levels. Yeast is one such organism which can continue to grow and metabolize sugar & starch even in the presence of 14 to 18 percent alcohol where the other microbes fails to do so.Such levels of alcohol tolerance make yeast one of the most viable microorganisms for alcohol fermentation around the world.
Alcoholic fermentation is the most common type of fermentation, and it is characteristic for S. cerevisiae. Wine, sake, cider, beer, whiskey, rum, vodka, liquor and many other distilled beverages manufactured using the ‘yeast fermentation’. Absence of oxygen is essential to achieve best results in alcohol preparation using yeast.. `Fermentation is an energy-releasing form of metabolism in which both the substrate (initial electron donor) and by-product {final electron acceptor) are organic compounds’ (Jackson, 2009). During fermentation ethanol acts as the final electron acceptor (though for yeast it is a by-product), whereas glucose is the preferred electron donor (substrate) (Jackson, 2008).

Biotechnology industry is focusing on developing Genetically engineered yeasts that can  use the sugars present in cellulosic biomass such as agricultural wastes, paper wastes, wood chips etc. to convert them into value added products.