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Q: Why did my beer mysteriously darken(turned brown, black, or purple)? What causes that?

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 beer, this browning is 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.