i have a resistive heat mat sold for warming up lizard enclosures, it works perfectly somewhere around the middle of the setting... i did a batch too hot the other day and i'm pretty sure most of it got killed
the other thing is it takes a lot of time and agitation to get powdered milk to fully mix through... the too hot batch didn't mix up properly and had a lot of granular powder clumps in it still, this batch i just did is smooth, like i said, gelled up