A lot of them goes into how you prepare the f/t as well. If you ever watch any of my feeding videos then you'll notice mine will try to eat my hand if I open that tub and don't have a rat present.

I thaw them out naturally for a 4-5 hours. Then I fill a sterilite container up with hot water from the tap and then I put them in the hot water(still inside the ziploc bag) and I just open a bit of the ziploc bag so I can make sure all the air gets out. Then I reseal and leave that part out of the container and I put the top on. This helps to retain the heat. After that I let the water get cool and then I put everything on paper towels which is on a plate I never use and I put them under a heat lamp for 15-20 minutes. After that I hit them with a hair dryer to scent the room of rats then they're ready to go.