Beef tallow sparking in microwave?

I buy the 35 pound buckets of beef tallow from US Wellness meats. I vacuum seal it after scooping it out and then freeze it. When I take one out of the freezer I put it into a quart mason jar and then microwave it so it melts to conform to the jar.

But just this night when I opened a vacuum sealed bag, I put it in the microwave, and after about a minute it started sparking/arcing. The microwave flashed and made loud noises. So I turned it off and took the tallow out and there wasn't too much a bad smell. I turned on an exhaust and ceiling fan just in case.

Then I smelled the tallow and even tasted a bit, and it seemed normal. I did a muscle test (basically leaning forward is yes and backward is no) and asked myself if it was safe to eat. I leaned forward.

I've bought this tallow for years. And it's from the same bucket batch that I've been pulling out of the freezer. It's the same mason jar I always heat up too. Could this particular bag of tallow have a really high iron or other heavy metal content? Could the stainless steel spoon I used have leeched enough metal to conduct in the microwave? The bag had some lead contamination?

The lid and spoon were not in the microwave. I boiled the rest to melt in the quart mason jar in a pan of water. I saw no bits of metal in there. Also I cleaned the microwave out. And then we stuck in a mug of water and tested it for 30 seconds and it was fine. But then I stuck my tallow quart jar in there and not 5 seconds into microwaving it sparked! So it's gotta be the tallow, or maybe the spoon leaching. But I always use a stainless steel spoon.

Anyways, any thoughts on this? I don't think it's too big a deal. I've heard that grapes and even green beans can spark in the microwave. But thought I'd put it out there. I have some ghee and olive oil worst case scenario if I must eat some other fat for a few days. Thanks for reading.

3DStudent,

I don't have an answer why it sparked in the microwave, but how I have filled jars with tallow before is heating it up in a crock-pot/slow-cooker and then pouring into jars.

Microwaves work by heating up the water in the food. Properly rendered fat should have no water content at all - so the microwave energy has nowhere to go. So it will tend to concentrate in whatever few absorptive spots there are, such as the container you use or a speck of metal or perhaps a tiny bit of water at the bottom, which could boil and blow out the jar bottom since the rest of the tallow forms a solid plug. Did you see sparks actually in the tallow or did you just see flashes in the microwave and assume it was the tallow?

I had something weird happen where I was microwaving some stew and there was a bright pink light coming out of the middle of the stew. I have seen graphite glow that color when used as an electrode to a plasma arc, so I thought maybe whatever it is had graphite in it. What color was it?

I'm not sure where the sparks were, as it was a bit alarming and I went to quickly turn off the microwave. But due to my reasoning, I'd say it was the tallow. Because it's the same jar, and I always use a metal spoon to scoop it out, and I had cleaned the microwave. And putting other stuff in it produces no sparks. I want to say the spark was a white color when heating the tallow. I suppose this could have just been so well rendered that there was very little water and the microwave registered its contents as "empty".