Heating up food jars

[emetophobe12]

How do you heat up the food in the jars?
If I make food and put it in the fridge I reheat in the microwave to piping hot and then let cool. But the jars say to warm in hot water. Surely this isn't going to be hot enough to kill any bacteria when heating up food?

You could tip the contents into a bowl and microwave that
The jars have been heat treated so they won't have bacteria in them so you can just warm them