Once the cooled wort is brought to the fermenter yeast will be pitched into the tank. Fermentation begins when the yeast is introduced into cooled, aerated wort. The yeast quickly utilize the available oxygen and then switches to the anaerobic phase where the majority of wort sugars are reduced to ethanol, CO2 ,, aroma, and flavor compounds. The liquid in the tank at this point is also known as "Green beer".
After a week of fermentation we gradually reduce the temperature in the tank in two stage all the way down to 3 C. Yeast flocculates to bottom of the tank this process is called the conditioning stage. During this time the yeast continues to reduce various undesirable flavor compounds, flocculate and settle. At this point in the process, oxygen is one of the enemies to beer!