What are the most important chemicals for the flavour of beer, and what effect do they have? I mean chemicals as components in the beer not regarding if they are naturally or artificially added.
When yeast ferment the sugar into alcohol it produces a lot of different by-products that give beer most of it's flavour. The brewer can influence the by-products produces by controlling the temperature at which the fermentation takes place.
Hops also add a lot in particular aroma. Since so much of tasting depends on the nose aroma comes of as flavour too. By boiling hops most of the aroma disappears and your left with only bitterness. By boiling hops for a shorter time, or not at all you can add more aroma. Brewers often talks about bittering vs aroma hops. Hops bitterness and aroma contribution depends on the variety of hops used. Hops growers are constantly coming up with new types of hops.
Malt, grain having been processed, also add flavour. Though most malt come from the same type of grain, barley, it can be processed/malted in many different ways giving a different flavour. When mashing (getting the sugar out of the malt and into the liquid) you can adjust the flavour by temperature. At high temperature (68c) you get almost only unfermentable sugar, which will give you a sweet result, while mashing at 64 will give you fermentable sugar and therefore a dry product.
Water does not add so much to the flavour in it self as it helps other parts of the recipe add a certain flavour. PH and content of minerals have a huge impact on mashing and fermentation.
I don't want chemicals (as in not natural grown plants) in my beer.
The standard natural ingredients are:
To be a craft brewery in Texas you are limited to those ingredients
A good link
There are different types of hops and barley. The type and amount affect the flavor. Naturally the brew process, water, and yeast also affect the flavor.
A hoppy beer will have more hops. Link to hops. An IPA is heavy on the hops.
Link to barley / malt
There are also wheat and rye beers but I don't know much about them.
Brewers may use corn syrup and number of other ingredients.
Low calorie beer will naturally not use as much high calorie ingredients.