What is Yeast?
Yeast is a microscopic unicellular fungus that is used to make bread, wine and beer.
There are many different types of yeast in our environment, however the species we are interested in as brewers, distillers and wine makers are members of the genus Saccharomyces cerevisiae. The name Saccharomyces cerevisiae is a combination of three root words; "saccharo", derived from the Latin word “saccharum” meaning sugar, the Greek word "Myces" for fungus and "cerevisiae" the Latin word for brewery. Saccharomyces cerevisiae is more commonly known as “Brewer’s Yeast”.
A Brief History of Yeast
According to historians, the first beers were accidentally brewed in Mesopotamia and Egypt and their fermentation was spontaneous. Air-borne wild yeasts fermented the must/wort left in the sun and beer was born.
Yeast acquired the nickname “gift of god” because there didn’t appear to be any ingredient added to the brew that could start the fermentation process. It seemed to just “happen”!
Styles of beer varied from region to region as a result of different air-borne wild yeast strains that were to be found in a region. The first scientific research on brewer's yeast was conducted by Louis Pasteur in 1876 (Les études sur la Bière) and allowed the selection and storage of the most appropriate brewing strains as well as the development of beer pasteurisation.
Nowadays, some beers are still spontaneously fermented (lambics and Gueuze) but most are fermented with specially selected and isolated strains of yeast.
What is Fermentation?
All organisms need to “eat” to survive; yeast likes to “eat” sugar to get its sustenance. When yeast metabolises sugar in the absence of air, ethanol (alcohol), carbon dioxide and various other substances are produced; this process is known as fermentation.
Does Yeast determine the flavour of beer?
Yeast plays a major aromatic role in brewing and is often a brewers' secret ingredient.
Some strains of yeast produce prolific quantities of esters during fermentation. Esters are aromatic organic chemicals that we perceive as flavours. Some esters taste like banana, others may taste like cloves while still others can taste like vanilla and many other flavours, some are agreeable, some are not so agreeable depending on your taste
The same strain of yeast can produce different flavours just by fermenting at different temperatures!
Different strains of yeast have specific features that may be better suited to certain substrates. Some strains are better suited to fermenting grapes while others may be better suited to fermenting malt and the like.
Some strains of yeast have a high tolerance for alcohol while others may not. Some strains of yeast are very efficient at metabolising sugar (high attenuation rate) while others are not. Some yeast strains thrive in cooler temperatures (lager yeast has an optimum range of 7 °C to 15 °C) while others prefer warmer temperatures (ale yeast has an optimum range of 15 °C to 25 °C). Yeast can still function outside these ranges but may either give a slow/sluggish ferment or produce an overabundance of estery flavours and ferment too rapidly.
It is important to choose the most appropriate strain of yeast for the style of brew that you wish to produce.