Not to denigrate the services of Wiley and his Poison Squad, but the reason they had to do all their testing was that capitalism was shifting food from a local economy to a national economy. Honey was no longer something you bought when you could find it, it was something the five and dime always had. Flour wasn't bought from the guy who bought it from the mill, it was bought from the guy who bought it from the distributor who bought it from the packager who bought it from the mill. The states had no enforcement or investigative capabilities beyond their borders which meant that interstate scams were the easiest scams.
Something like the Poison Squad had to evolve at some point or else the food supply chain couldn't have nationalized. It's not that people were willing to eat adulterated food, it's that until the stuff started traveling and anonymizing people had no reason to believe anyone would bother; I mean, if the local dairy farm gets caught mixing calf brains into the milk, they go under. That stuff in cans from who-knows-where? Yeah, the distributor's gonna point fingers and promise it'll never happen again.