In 1906, two laws were passed that laid the groundwork for what would become the Food & Drug Administration. There was a book written by Upton Sinclair a progressive/socialist muckraker that was intended to show the plight of poor immigrants in the US called "The Jungle". The book became quite popular, but much to Sinclair's surprise the public reaction to it pretty much ignored his economic points. Instead, the focus was on the vivid descriptions of unsanitary conditions in meat packing houses.

President Roosevelt was reportedly suspicious of Sinclair but decided to dispatch two people he trusted, Charles P. Neil and James Reynolds, to inspect meat packing plants in Chicago. The Neil-Reynolds report found that the worst of Sinclair's accusations were complete falsehoods (Sinclair had claimed that people fell into vats and were ground up as burger while no one did anything, rat infestations etc.). However, they did report on various unsanitary practices such as failure to regularly clean certain rooms. A transcript of their testimony to congress is available here.

Their description of problems range from the absurd, for example Dr. Neil criticizes the meat packing plants for bad working conditions because they did not having enough natural light, which is intentional because sunlight exposure can taint raw meat, and they were too cold ?!? which again, when leaving raw meat hanging around it is supposed to be cold. But they also described areas of real concern such as build up of dirt and grime and failure to regularly clean working surfaces.

The report sparked enough concern in President Roosevelt that he took out his "big stick" and put pressure on congress to pass a law. Like most legislation that he took a personal interest in, the Federal Meat Inspection Act was quickly passed into law.

Another related law was passed in 1906 called the Pure Food and Drug Act a law which was

Initially it put an emphasis on correctly labeling what was in a food product or medicine. At that time, many medicines had "secret ingredients", often opiates. The law required that products with cocaine, alcohol, heroine, morphine or cannabis list the amounts and recommended dosages.

However, it wasn't long before Harvey Wiley, the first FDA commissioner, ordered the seizure of 40 barrels and 20 kegs of Coca-Cola and attempted to use the Pure Food and Drug Act to ban Coca-Cola. He used the argument that Coca-Cola was adulterated and misbranded under the law.

It was adulterated because it contained a "poisonous or added deleterious ingredient", what was this terrible ingredient? Caffeine.  

It was misbranded because it no longer contained and coca leaves and contained very little kola nuts.

In U.S. vs. Forty Barrels and Twenty Kegs of Coca Cola 241 US 265 (1916) The Court ruled that caffeine was an added ingredient in context of the law and that there was evidence that caffeine may be a "poisonous or deleterious" ingredient and evidence that it might not be. The Court remanded the case to a lower court to be heard by a jury to determine whether caffeine was poisonous or deleterious. However, the case was not tried by jury because by this time Harvey Wiley had resigned. Coca Cola agreed to a settlement that led to them reducing the amount of caffeine but not eliminating it. 

In addition to going after Coca Cola, our fearless defenders in the FDA also went after cider vinegar. In U.S. v. NINETY-FIVE BARRELS, MORE OR LESS, ALLEGED APPLE CIDER, 265 U.S. 438 (1924) The Court ruled that apple cider vinegar made from dehydrated apples, instead of fresh apples, was in fact misbranded when it was called "Apple Cider Vinegar" even though chemically it was found to be identical. 

In 1951, SCOTUS ruled that the word "imitation" could be used to protect a company from being prosecuted for misbranding. In 62 CASES OF JAM v. UNITED STATES, 340 U.S. 593 (1951) it was determined that a product labeled "imitation jam" could not be penalized because it did not have the proper ratio of sugar to fruit that the FDA required to be listed as "jam" because by using the word "imitation" the company was not claiming the product was jam.