The following is excerpted from: "Butchering, Processing and Preservation of Meat" by Frank G. Ashbrook.

"Saltpeter is also an important ingredient of the curing mixture. It has two functions-fixing color and checking the growth of certain bacteria. Meat owes its red color to hemoglobin, an unstable pigment which, say the meat specialists of the U.S. Department of Agriculture, oxidizes to brown methemoglobin and combines with nitric oxide to form red nitrosohemoglobin. Nitric oxide is formed through the reduction of nitrate to nitrite. Certain bacteria which occur normally on fresh meat are responsible for bringing about this reaction. A combination of nitrite and nitrate in the ratio of 1 to 10 makes a superior product. Saltpeter (nitrate of Potassium) preserves and dries the meat but is used almost entirely because it effectively fixes the bright red color of the lean meat. Nitrate of soda (Chile saltpeter) is a little stronger, and 1.7 ounces of nitrate of soda will replace 2 ounces of saltpeter. The exact quantity of saltpeter of nitrate of soda to be used should be weighed and mixed thoroughly in the curing mixture. It is undesirable and quite unnecessary to use more of either saltpeter or nitrate of soda than the amount recommended. "