I found bottles in the garage from a year ago. Got them from F-in-law and they were from Germany. Caked with crap in the bottom. I seriously contemplated throwing them away.
Instead I tired bleach. A little in each bottle plus warm tap water. Let them sit for 2 days, b/c I didn't get back to them. Lets face it, I left them in the garage for a year without attending to them, what was an extra day right? Anyway, completely clean when I dumped them out.