Yeast is the most important ingredient in beer brewing. Yeast is a living creature, metabolizing, reproducing, and living off the ingredients in the beer. It is responsible for the converting of sugar to alcohol and carbon dioxide in the fermentation stage.
Yeast is also the final component that determines the flavor of the beer.
There are thousands of varieties and strains of yeast. Even in the air, wild yeast is floating around ready to contaminate. Only cultivated strains of yeast should be utilized in the brewing of beer. If other yeast contaminate the beer, the results can be over carbonation, strange flavors, and all kinds of fermentation peculiarities. Therefore, picking the right yeast for the desired beer is an absolutely critical.