Though yeast is a microbe so small we can barely see them it is one of the most important ingredients in beer.
When grain is being mashed enzymes convert starch into sugar. When yeast is added to the wort during fermentation it begins to consume this sugar and multiply. As it goes through this process it metabolizes and produces alcohol and carbon dioxide (which provides carbonation). Once there is no more sugar to be eaten the yeast goes dormant.
Brewer’s use two main families of yeast; saccharomyces cerevisiae lagers for brewing ale and saccharomyces uvarum for brewing lagers.
There are many different strains of these yeasts that can be used to create additional flavours and aromas to beer; esters which create fruitiness like banana, apricot and apples or phenols that create spiciness like pepper and clove.