Tempeh has an assertive flavor. It obtains much of its nutty, subtle, tangy flavor from the fermentation process. When the temperature changes, the bacteria present in tempeh may cause small patches of gray or black spores to bloom on the surface of the cake. These spores are harmless and only add to tempeh’s unique yeasty flavor. You can store opened tempeh, tightly wrapped, in the refrigerator for up to five days.