The original functions for fast food were to make food more digestible and to preserve it for use during times when food isn't available, but now factory and industrial processing diminishes the quality of food. Chopping, soaking, fermenting, grinding, and drying are all forms of food processing. What affects the food during its processing is the products used in the process. Some fast food is made by mechanically separating meat, which is where the bones and carcass of a leftover chicken are mixed together in a food processor. McDonald's claims they do not process their chicken like that, they say their chicken is boneless white meat. Some do not believe that though because it stays fresh for a very long time.
These are fast food french fries after being in these jars for three years. The fries on the left are McDonald's and the ones on the right are from KFC. As you can see the McDonald's fries have not aged much, which isn't natural considering they have been there for three years.