The chemical reactions in the different phases of the brewing process are very complex, with interactions between very many and various components, which also differ according to the style of beer and according to the ingredients used to make it. It’s quite simple: when you heat up malt enzymes, especially amylase and protease, you convert the starch into sugars and amino acids. To convert the sugars into alcohol (ethanol) and carbon dioxide you add yeast as a catalyst. The amino acids then produce the nitrogen necessary for the growth of the yeast cells. Metabolites of these processes, perhaps micro-organisms and colloidal particles, are removed by filtering the batch. 

From Beer & Health