At the suggestion of another member I tried expanding my spawn on popcorn kernels. This involved boiling for about 20 minutes, drying, then pressure-canning for 90 minutes at 15psi. After cooling my spawn was divided between the jars, which were allowed to grow for a little over two weeks. On Monday I transferred the contents of a couple jars to fruiting bags with pasteurized straw and started seeing growth by the next day. I thought all was well.
On Wednesday, I started noticing some green stems. As I've been watching, this is actually growth from the popcorn kernels, and it's happening in all four bags! Now I'm quite frankly amazed that the cheapest generic popcorn from the store even has the capability to sprout, but after I put it through all that cooking I would have assumed it simply wasn't possible. I also wonder why it didn't start trying to grow in the original jar, and why it waited until it was in the bags?
So is there anything I should do about this? It will probably be at least another couple weeks before the mycelium is grown out enough to open some air holes for fruiting and I'm worried the corn will be trying to create its own holes before then, but maybe the bag is tougher than it looks. Anyway, any suggestions other than letting it go and waiting to see what happens?
tl;dr: Spawn grown on popcorn kernels, now popcorn is sprouting despite extensive cooking.
[Update] It's NOT the popcorn, there were other seeds in the straw!
In case anyone is still following this post... Today I was working on the patio and noticed there were green stems sprouting out of the remaining pile of straw that I didn't use. I got to digging around and realized there are seeds in the straw itself (which I assume are from whatever type of plant the straw grows from?). These seeds are nearly identical in size and shape to the popcorn kernels, but they don't have the same bright yellow color. I didn't see that when looking through the fruiting bags, but that appears to be what is causing all the growth. Since the straw wasn't boiled, those seeds survived the pasteurization.
Guess I'll have to keep that in mind when I do the next crop!