My info came from a mate who's a PhD in chemistry. Bio's not his branch of chemistry, but he seems to know what he's talking about. Just on logic, what my mate says seems to make sense to my simplistic brain, in the the acid we add ie sulfuric is a water based substance.
Assuming the bio holds water during washing (noticeable by the colour and clarity), then it can hold some of the sulfuric acid. Further washing will dilute the acid and once the bio is completely dried it can no longer exist.
This issue has always confused me. I thought I had it straight having had the discussion with my mate, but now from what you report Paul has stated, I'm back in the realms of confusion.
I'll try dropping Paul a mail and see what he says.