I have a bird who gets medicine twice a day. I hide it in a mashed potato fruit mixture. I make enough that the potato mixture lasts several days and add the medicine as I need it. I take a small amount and microwave it for about 8 seconds (just enough to soften it). Then I normally add a couple blueberries to the mixture for a little more moisture and flavor. At that point, if I feel like I've made too much for that "serving" I'll put some back in the container for next time. When I reheat the saved-for-later mixture, it always sparks and catches fire (about the size of a lighter flame) in the microwave (it always goes out as soon as I stop the microwave). Of course, I never use the caught fire serving and always start over. This only happens with the saved-for-later bits and not the original mixture. I read the "Why did my banana catch fire in the microwave?" (Why did my banana catch fire in the microwave?) thread and that might answer it if it were all of the mixtures that caught fire, but it's only the parts that I tried not to waste and save for the next medicine-hiding dose. Any ideas what causes just the left-over bits to catch fire?
Try putting a mug of water next to the food in the microwave.
I've had this same problem with baker's chocolate and that did the trick. I think it's because there isn't enough water to absorb the microwaves so the excess energy causes sparking.
I'm not sure that makes sense. Sparking happens when you have a conductor. Microwaves are just EM waves so you need a conductor to get elections to move and spark. And without water, microwaves will not heat anything up. You need free water molecules (as opposed to ice which won't work). In this case, if there is no high metal content, the mix has dehydrated and surface got hot enough to catch light. You can try this with bread. I used to microwave bread about 1 minute on each side and this would dehydrate the bread enough to be crunchy!– MegasaurJul 27, 2014 at 6:23
I actually have been added a mug of water and haven't had any problems since. I started doing this after one of the servings that wasn't a "saved-for-later" overage caught fire. There have been no more fires! :)– BrookeAug 10, 2014 at 20:31