I've lost count of the times this has been a thread.
It's to do with the fact that dogs aren't given real bones to chew anymore and more sophisicated dog foods, which in the past contained more ground up bones.
It's calcium that made them white.
Also it was only after the water had evaporated, and the more organic matter had disapeared after a couple of days that the turds turned white.
Due to restrictions in letting you dog crap anywhere it liked, it's less likely than it's been hanging around for a few days.
There are exceptions of course, Gillingham being one.
Clapham Gull, I know we're not supposed to discuss people's professions on here (see the phone in thread) - but your indepth knowledge of the white dog poo leads me to believe you are a dog warden?