Man has a long history of adding chemical substances to foods for various reasons, including improving shelf life, flavor, appearance or texture. Two preservatives, smoke and salt, which were also used to give food a good taste, are probably the oldest food additives. Like spices, they have been used since early times to improve the taste of foods obtained from plant and animal sources. With rapid urbanization in the 19th century, additives became increasingly necessary, especially to protect food against spoilage. In the 20th century, as the industrial production of food developed further, the importance of additives increased — for example, during this time, emulsifiers were developed that improved and simplified the production of margarine. Additives, like trace elements, vitamins and essential amino acids, have also become important. More recently, the importance of flavor enhancers as well as flavoring compounds has increased substantially.