USDA would be the main agency that monitors food processing plants. I have seen a wide range of how closely the USDA monitors operations. Some are a real pain in the butt nit picking about everything while others are practically non-existent, see them a couple times a day but mostly never around. I would say the majority are good people that are focused on making sure facilities are following the proper protocols and testing to produce safe food. I have had very little direct interaction with FDA so I can't really comment much on that agency other than I know that work at an absolute snail's pace. Better than the EPA though, they are more of a glacial pace. My FDA experience has been mainly in pet food production facilities.