Salt MInes6500 B.C. salt mines were discovered in Salzburg, Austria. It was also discovered that when salt was added to food it allowed the food to be eaten longer (even though it was very salty). The scientific reason is that the salt removes the water in the food. The bacteria cannot survive without water so the growth of bateria is slowed down.
CanningMany pre-cooked foods like soup, sauces, and vegetables are still canned today. Some uncooked foods are also canned today such as canned fruit. The corn syrup or fruit juice canned with the fruits stops oxidation of the fruit. See this science experiment stop oxidation in fruit.