| MadSci Network: Chemistry |
I appreciate your question and you may want to ask it again since I can only address it in general (and somewhat speculative) terms. There may be another Mad Scientist who knows much more about this area.
Many materials take on color when they are oxidized. This often has to do with the formation of double bonds between carbons and carbon-oxygen that interact with the light falling upon the object. Many compounds are formed during partial oxidation and the odds of a color forming are good. Complete oxidation would take simple organics to carbon dioxide and water, but unless the condition are just right, you won't have complete oxidation, and the intermediate (partially oxidized) compounds will lend a brown color.
Additives can interfere with the oxidation process by scavenging (tying up) the oxygen (or other oxidizer) [an example of this is the sulfite added to salad bars and wine which is known for putting some sensitive folks at a health risk], shifting the pH (making it more acidic or basic) to discourage bacterialogical oxidation (lemon juice, vingegar), or otherwise acting to lessen biological activity (salt, refrigeration).
Several common vitamins are classed as antioxidants and promoted for breaking the chain reactions caused by free radicals, something similar to the devouring of the ozone layer by Freon breakdown products.