The main ingredient used in the fermentation of beer is malted barley, which is rich in enzymes that convert starches into sugars. The brewing process begins with mashing the malted barley to extract these fermentable sugars. Yeast is then added to the liquid (wort), where it plays a crucial role in converting the sugars into alcohol and carbon dioxide, leading to fermentation. Additional ingredients like hops might be added for flavor and aroma. Overall, malted barley is essential for creating the base of beer, contributing to its flavor, color, and alcohol content.