that is odd .
As I read your post , the schmutts comes from the darker
( ie stained ) area
does anything come off lighter , non-stained periphery ?
how do you sanitize ? dishwasher ? ( having pretty hot water )
dilute bleach ?
fascinating to see what the schummts looks like under a microscope
the Bacti- report , and the augur cultures would also be interesting
I imply no Health issues by being interested in the above .
after all , you and S.E. haven't keeled over
just yet .