Cheese aging, also called ripening or maturation, is the process of holding cheese under controlled temperature and humidity for weeks, months, or years so that enzymes, bacteria, and molds can break down its proteins and fats, changing its texture and flavor. A fresh, rubbery curd becomes progressively softer or firmer and more pungent as aging continues, depending on the cheese; fresh cheeses like mozzarella or ricotta are eaten within days, while cheddar, Gruyère, or Parmigiano-Reggiano may be aged for months to years. Aging conditions, along with the milk and cultures used, are what give many regional cheeses their distinct character.