The short extract below is lifted from Thornbridge Brewery (UK) website - -
I mentioned oxygen earlier. This is used initially by the yeast to grow. We add a certain amount of yeast to our wort. Pretty much, the stronger the beer will be, the more yeast we add initially. Let's take Jaipur, for example, where we add around 7 – 8 million cells of yeast per millilitre of finished wort. A lot of yeast! The yeast is floating around in a super rich environment of nutrients, mostly in the form of sugars, amino acids and peptides that the cells start voraciously feeding on (well absorbing through their cell walls at any rate) and undergoing a process called budding. This is where the yeast cell (referred to as the mother cell) creates another yeast cell (referred to as the daughter cell). Something called Yeast Viability is important here. This is one of the first laboratory checks we do on the yeast. We take a sample of the yeast that is going to be added, or pitched, into the wort, we count and calculate how many cells are present in one gram of yeast (not all by eye of course... it's usually between 500 and 2500 million cells in a gram!) and we also stain the sample with a special dye called Methylene Blue. If the yeast cells are healthy, an enzyme present inside of the cell will change the dye from blue to colourless. We calculate the percentage of cells that are able to do this and express it as Yeast Viability. We really want this to be greater than 90% otherwise we're just adding dead yeast to the wort and this can potentially cause off-flavours.
So we have yeast, oxygen and wort. The yeast is increasing in biomass (usually by anything up to 5 times) by a process called budding and using up nutrients to get its own nitrogen and carbohydrates and fatty acids and anything else it needs to grow. But what happens when the oxygen is all used up? This is when the yeast switches its mode of metabolism from aerobic to anaerobic. Anaerobic means in the absence of oxygen and it is here that the yeast begins the actual fermentation and sugars are converted to carbon dioxide and alcohol. Without boring you all to tears (I'm sure I can see some already... right at the corner of your eyes...) effectively, sugar is absorbed into the cell and broken down by a series of enzymes into the aforementioned carbon dioxide and alcohol. As well as this, there are a bunch of side reactions which both use and yield energy for the yeast cell and also give an interesting array of flavours to the finished beer. Fruity esters or flavour active ketones (such as the rich,intensely buttery diacetyl) or sulphury compounds or even heady, harsh, fruity or bitter fusel alcohols. They all blend together to provide the final beer flavour. Some of these are good and sought after, others not so much and a brewer puts all of his or her training and understanding into keeping some of these metabolic by-products out of the finished brew.
The rest of the process is described just as well.