Yeast is a living organism that is responsible for fermenting sugars into alcohol and carbon dioxide during beer production. It also determines the final flavor of the beer. Hops add bitterness to beer and are crucial to balance out the sweetness from malt. Malted barley provides beer's color, sweet malty flavor, body from dextrins, and a good head from proteins. Clean source water requires less treatment by breweries and comprises 90% of beer, greatly impacting its quality.