http://apps.americanbar.org/buslaw/blt/2009-05-06/ernst.shtml
Visual Depiction of Food
Advertisers of food products wish to present their products in the most appealing light: they want hamburgers to appear fat and juicy, vegetables to appear crisp and green, and soups to appear robust and chunky. So-called food stylists are commonly employed during commercial filming or photo shoots to ensure that food products look their best for the photographers. However, the law requires that photographs, pictures, or models used in an advertisement accurately reflect the product being represented. Colors should not be enhanced, product consistency should not be modified, and quantity or concentration of ingredients should not be adjusted so as to make the product appear more attractive in the advertisement. So, while it is appropriate to use care and effort to ensure that a product presents its best face to cameras, the product should not be manipulated to misrepresent its actual appearance. One major food manufacturer got into trouble by placing clear marbles in the bottom of a bowl of soup used in an advertisement in order to make the soup appear more chunky. In addition to the legal problems this created, the advertiser suffered a lot of bad publicity.
One exception to this general rule is when a product is modified for purposes unrelated to product appearance or performance. For example, mashed potatoes could be substituted for ice cream in a television advertisement showing the joys of eating ice cream (real ice cream would melt under the hot camera lights). On the other hand, mashed potatoes could not be used in an advertisement emphasizing the creamy texture of a particular brand of ice cream.