Protein powder image. You should also be careful about storing protein (protein supplements) taken after pre-workout during the summer. This is because stored food mites may breed if opened protein powder is left in a place with high temperature and humidity for a long time. Experts explain that it is not just a problem with protein, but the same risk can occur in most powdered foods such as flour, grain powder, powdered milk, and baking mix. U.S. Food and Drug Administration (···