Take your old growler back in to the Beer Exchange for a refill, and they won’t just put your old bottle up to the tap. They rinse the bottle with water, clean it with brewery-grade acidic sterilizer, and rinse it again. They also inspect the cap’s rubber gasket to make sure it isn’t acting as a petri dish, and they get their draft lines professionally cleaned every two weeks and replaced once a year. Taste the beer, not the mold.