Many pet foods advertised as preservative-free do not contain preservatives. Almost all rendered meats have synthetic preservatives added as stabilizer, but manufacturers aren't required to list preservatives they themselves haven't added. Premixed vitamin additives can also contain preservatives. In the 1003 Journal of the American Veterinary Medical Association, veterinarian Philip Roudebush reported finding low concentrations of synthetic antioxidant preservatives in all analyzed samples of products labeled as chemical free or all-natural. Other types of additives depend on whether the pet food is semi-moist, dry or canned. Because semi-moist food contains 25-50 percent water, antimicrobial preservatives must be used. Propylene glycol was frequently used in cat food until it was pulled in 1992 for causing a variety of health problems. Processing greatly alters the nutritional value of the food ingredients. Veterinarian R. L. Wysong states in Rationale for Animal Nutrition: Processing is the wild card in nutritional value that is, by and large, simply ignored. Heating, freezing, dehydrating, canning, extruding, pelleting, baking and so forth, are so commonplace that they are simply thought of as synonymous with food itself. Because the ingredients that pet food companies use are not wholesome, and harsh manufacturing practices destroy what little nutritional value the food may have had in the first place, the final product must be fortified with vitamins and minerals.