I would first clean any visible food particles from the thermometer, then I would get some
Star San. This is a foaming sanitizer that I think we all agree works very well In order for this to work, the thermometer must be in contact with the sanitizer for about 3 min.
What I do, is before and after each batch, i take a 5 gallon bucket and create a about 5 gallons of this and soak all of my tools in it. Including my thermometers.