I typed in "food styling" and FTC into Google, and it was in every result on the first page. (I was not using personalized search but I do have cookies enabled, so your results may vary.) If you do not want to pay for the digital copy (I think it is $10 on Lexis for the case, not including search charges), you can go to your local bar association law library. Larger law libraries may have a copy of appropriate FTC rulings. Alternatively, a law school library will have a paperpound copy (but may charge non-students and non-alumni for entry).
In a nutshell, in Campbell, the company used marbles to stand-in for peas and other various vegetable ingredients. Someone complained that the ads were deceptive. The FTC pursued an enforcement action against the company. In its ruling, the FTC noted that a food product (and any ingredients) advertised must actually be the food product sold. Otherwise, the ad is deceptive.
The application to burger commercials: The advertised product must be an actual sample of the sold product. All of the ingredients in the sold product must be in the advertised product.
The ruling is not explicitly limited to the food products being advertised and sold, however it does not address any other ingredients except for those advertised and sold. Consequently, if a foodstuff is not part of the final sold product, the advertisement can use a stand-in. Consequently, the milk in cereal commercials can be glue (or paint), while the cereal in milk commercials can be plastic.