Freezing food or other items does not kill all the bacteria or other microorganisms present. Freezing temperatures may kill some bacteria, and the cold prevents bacteria from growing and multiplying as long as the temperature remains below freezing; however, bringing the food back to room temperature allows the bacteria to wake up and begin multiplying once more.
Cooking food is one method of wiping out bacteria. Different heat levels kill different microorganisms, which is why chicken, pork and beef must be cooked to specific temperatures to render the food safe to eat. Cooking does not destroy toxins left by bacterial growth, so simply cooking food that has been allowed to sit at room temperature too long does not render it safe to eat.
Another method of killing bacteria on food is removal of the water necessary for the bacteria to survive. Salting food can draw out moisture, rendering the food inhospitable to bacteria. Dehydration and freeze drying are other methods of removing moisture and preserving foods.
Bacteria also dislike acidic environments. Pickling foods involves preserving them in an acidic bath, and if the proper acidity is maintained, it prevents bacteria from entering the food or growing as long as the environment remains controlled.