The term CURING SUBSTANCE can be solved by the four-letter word SALT. SALT is a mineral composed primarily of sodium chloride, a chemical compound belonging to the larger class of salts. It is used in many different ways, including as a preservative, flavor enhancer, and curing agent. In food preservation, SALT is used to draw moisture out of food, preventing the growth of bacteria and other microorganisms. SALT is also used to cure meats, such as bacon, ham, and salami.