Continue to incubate up to 48 hours: Depending on your conditions, the tempeh may take up to 48 hours total. The mycelium will continue to thicken, forming a white layer around the beans and binding them into a dense, firm cake. The tempeh is done when the entire surface is covered with dense, white mycelium (some black or gray spots are okay), as well as the spaces between the beans. The beans should be bound together firmly as a cake. You may want to slice a small piece off the edge to make sure the cake is firm all the way through. The tempeh should smell pleasantly nutty and mushroomy. It may also have a light ammonia smell.