Yesterday I took a pork shoulder, removed some of the fat and sinew and skin, cut it up into smaller parts, and put them into the freezer in freezer bags. It took me just under half an hour to remove the bits I didn't want.
This morning I took a piece out to defrost to cook later in the week and I noticed that some of the fat that I had left on, had discoloured into a greyish-green colour.
Out of the 5 pieces I put in the fridge, 2 have fat that looks like this. The other 3 piece look alright.
Only the fat and sinew covered area was effected.
It was pretty warm yesterday when I did this. Maybe 32 degrees Celsius. The meat was still a bit cool when it went into the freezer.