During much of the first volume, the book discusses specified cooking times and temperatures in order to kill bacteria and other pathogens. When it says for example that a certain bacteria dies at 130 degrees Fahrenheit for 20 minutes, what does that mean? Like for a steak, does it mean the internal temperature of the steak needs to be 130 degrees Fahrenheit for 20 minutes, or just exposed to that temperature for that amount of time? Thank you!