Salt is perhaps most well known for its roles as a food preservative and flavoring agent. It has been used to preserve food for many thousands of years and is the most common seasoning, but salt also plays other, lesser known roles in the food we eat: It is an essential nutrient, it provides flavor and texture, and it enhances color. For these reasons, salt is used in food production, which is why the sodium content of processed foods is so high.
Salt as a Food Preservative
Salt-curing meat and other foods is the oldest method of food preservation, and it was heavily used before refrigeration was available. Salt acts as a preservative by drawing moisture out of food, because microbes that can spoil food need moisture to grow. Many disease-causing microbes are also simply unable to grow in the presence of salt.
When salt is mixed with water it is called a brine. Brining is soaking food in this heavily salted water and is used to preserve and flavor the food (pickling is a form of brining).
Salt as a Texture Enhancer
Most people don't realize that salt plays a large role in creating texture in food. When making yeast breads, the amount of salt greatly affects the rate of yeast fermentation and gluten formation, both of which will significantly affect the bread's final texture. Salt also has a profound effect on the gelatinization of proteins, which occurs in cheese production and many processed types of meat such as sausage, bologna, and ham. In processed meat products, salt helps retain moisture, and so less saturated fat is needed. Large salt crystals are also often used to add a crunchy texture, as with pretzels.
Salt as a Flavor Enhancer
Salt acts in multiple ways to enhance the flavor of food. Not only is the "salty" flavor element one of the most desired by humans, but salt can also affect other flavor elements, such as sweet and bitter. In small amounts, salt will intensify sweetness and is therefore often sprinkled on fresh fruit or added to candies like caramel. Salt can also counteract bitter flavors in food—salt is often used to "de-bitter" cruciferous vegetables and olives. Salt will also help release certain molecules in the food, bringing out some of the ingredients' flavors and making the food more aromatic.
Salt as a Nutrient Source
Although most Americans consume far too much sodium, it is a nutrient that is essential for survival. Pure table salt is comprised of approximately 40 percent sodium and 60 percent chlorine. Most table salts in the United States also have iodine added to them to prevent iodine deficiencies. Iodine deficiencies can cause disorders of the thyroid, including goiters.
Salt as a Binder
Because salt helps form protein gels, it can be used as a binding agent. When salt is added to foods such as sausage or other processed meats, it causes gelatinization of proteins which then hold the product together.
Salt as a Color Enhancer
The vibrant color of many processed meats, such as ham or hot dogs, is partially due to salt. The presence of salt helps promote and maintain color, whether artificial or natural, and prevents it from turning gray or muddy. Salt also increases caramelization in bread crust, helping create that golden color.