While some people use salt as a way of enhancing the flavor of their food, salt has long been used as a preservative as well. Food preservation is not as big of an issue today with the use of refrigeration and other preservation techniques, but old-fashioned methods are sometimes still used. To preserve the food, salt works in several ways to change the food in ways that make it last longer.
Bacteria requires moisture to grow. If food is robbed of some of its moisture, the bacteria has a more difficult time growing, thus making the food last longer before it spoils. Salt is dehydrating; it's absorbent and immediately begins reducing the moisture in food. Bacteria requires water that's "free," which means the molecules aren't tied with other molecules. When the salt absorbs the water, the water molecules are no longer "free," so bacteria can't grow as effectively.
The acidity of different types of food varies depending on its genetic makeup. Salt also has its own level of acidity. Foods that have lower acidity levels are more likely to spoil quickly. Salt alters the pH balance in the food and can slow down the process of spoiling. This is why foods, such as meats and vegetables, are more commonly preserved with salt. They have higher pH levels, which equate to lower acidity levels. Salt increases the acidity and slows down the spoilage process.
When salt is introduced to food, its molecules combine with those natural to the food and the actual genetic makeup of the food changes. The natural structure of the food's molecules is weakened, which can change the speed at which bacteria grows and changes. These molecular changes that occur due to the presence of salt don't change foods' taste, except to add the taste of salt.
Salt can be added to foods as a preservative in a couple different ways. If the food is in a liquid or semi-liquid state, a specific formula of salt and water is typically used. This formula varies depending on the food and its need for salt. Foods in solid form, such as meats, can use salt in its solid form or in a mixture of salt and water, known as brine.