It's a combination of temp & time -- 131F for four hours will pasteurize (and, conveniently, is below the coagulation temperature for both the white & yolk of eggs, so you can home-pasteurize eggs for mayo/etc with a water bath). Contrariwise, 140F wouldn't be safe if it only reached it for a few seconds.
yeah, the difference being the time...hence my original question. At 135 degrees you need at least 35 minutes of hold time once you have reached the target temperature.
correct. that's why it's beautiful to control the entire process with software -- we know your time/temp curve. And why the USDA still gives one temperature which assumes an instantaneous point of contact, but higher than you need if you can simply hold the temp. (As a heuristic, staying lower gives you better quality food.)
