Ash, also known as food-grade activated charcoal is used on some soft cheeses to neutralize the surface pH of the cheese to create a friendly environment for the growth of the beneficial Penicillium Candidum mold while inhibiting unwanted mold growth.
The history of using ash on cheese is traced back to the small farmhouse cheese makers in France, who preserved their autumn cheeses through the winter months by coating them with "sel noir" (black salt): a mixture of salt and ash made from vine cuttings or charcoal from the fireplace. This provided an instant natural rind for protection and neutralized surface acidity, allowing natural molds to grow. Ash draws moisture out and allows the cheese curd to mature without the rind becoming too rancid or sticky. Cheese that are coated with ash may be matured for longer periods of time. These tend to develop a blue-gray, spotted rind, a condensed, nutty texture and a strong creamy flavor.
Activated charcoal specifically refers to food and medical grade charcoal that has gone through rigorous testing for sulfur and other contaminants.