I've been brewing for a couple years now, and lately my bottle collection has been more and more difficult to keep clean. I've always been thorough about rinsing the bottles with hot water immediately after pouring, and then turning them upside down to try in my dish rack. Lately, 3/4 or more of my bottles have been left with a hazy residue in the bottom even after that rinse.
I've tried soaking them in OxyClean, StarSan, vinegar, and dish soap + water. My bottle jet doesn't seem to do anything to it, even after any of those soaks. The only thing that's been successful is using a bottle brush and lots of elbow grease. That was fine when I only had to scrub a few bottles per batch, but scrubbing almost all of them every time I bottle is getting tiring.
My brews have been extract + steeping grains, and partial boil (except a couple split boils). My water is moderately hard, about 5 gpg.
What is this residue, can I prevent it, and is there an easier way to clean it?