I thought I'd find out how they do it properly.
I turned to the Adnams Brewery, and enlisted the help of their Quality Manager, Belinda Jennings, who I first met in a field in Suffolk...
Step 1: Raw materials
Water, malt, hops, yeast. That's it, for a proper beer.
What affects the final flavour is the way these things are treated.
The softness of the water, how dark the malt is roast, the species of hop, the strain of yeast.
Malt adds sweetness, and provides the sugar for fermentation. Hops add bitterness, especially to balance the sweetness of the malt, and the yeast, of course, turns the sugar into alcohol.
Most brewers (Adnams included) will happily give their recipes, but they won't give their yeast - established brewers have strains that are slightly different to other brewers' strains, and so affect the flavours. Adnams have been using the same strain of yeast since 1945.
Step 2: Preparation and control.
The remains of the milled malt go to be animal bedding - very little of anything is actually thrown away by Adnams.
In 2008, Adnams installed a whole new brew-house. As well as being highly controllable and automated (in-line systems weigh out ingredients instead of sacks having to hefted in by hand), it also recycles the waste steam from the brewing process - it saves 30% on energy costs, but it also means that the brewery doesn't smell of brewing, which is a shame IMO.
Step 3: Wort.
The heating pattern - how hot, how long, when the temperature is changed etc - affects how much of what flavours are dissolved out of the malt, and will affect the final flavour.
When the wort is transferred to the next tun, the solids are left behind, eventually to end up as cattle feed.