OK, please read. I said it was the SIGN of corruption, not that it's actual corruption. Short of having the ability to conduct chemical tests, that is the criterion the Church generally set for the time until which conditional Last Rites might be administered. At some point shortly after the rigor mortis, the corruption begins, but, as stated, short of a priest carrying a lab kit with him, it's considered the sign that the soul has left the body, where there's no life force left, and as a sign the corruption is beginning. It's not corruption itself.