Changes

A: We can science this, I promise! Food darkens in two ways: enzymatically with oxygen and phenols, or from maillard reactions from cooking. Unless you boiled your beerDarkening can occur from scorching the wort during wort production, this or using malt extract (which is darker from oxidation, or from production of the extract). If the darkening is not from the hot side, then the browning is was caused by oxidation. Check out this [https://en.wikipedia.org/wiki/Browning_(food_process) Wikipedia page on food browning] for an overview of enzymatic browning.