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