I call bullshit. I've worked a number of fast food jobs in high school and never have I seen anyone have the time to jerk off into the food. Its really not practical.
The worst I've seen is someone spit into a customer's burger (which i feel is really gross and unnecessary but hey whatever makes you feel better, the customer was a huge asshole anyway)
The worst I've done is accidentally sneeze on a stack of frozen burgers and I cooked them anyway
But all the other outlandish shit people claim they do liek beating off into the mayonnaise and stuff like that is complete bull because there are always managers and cameras and snitches around.