Fementation is, in a nutshell, very simple. 


Yeast eats sugar, and craps out ethanol alcohol and carbon dioxide. 


In reality a whole host of other things go on to produce a myriad of flavour molecules such as acids. In effect, a quick, efficient, clean fermentation results in a high ABV, less flavour product. Accordingly, a long, drawn out, messy, fermentation creates crazy flavours, often known as esters.

Alcoholic fermentation is a biochemical process in which sugars such as glucose, fructose, and sucrose are converted into small amounts of ATP, producing ethanol and carbon dioxide during the process. … In this form of anaerobic respiration, pyruvate is broken down into ethyl alcohol (C2H6O) and carbon dioxide.