Related for Chullin 168:26

What is sourdough? Something that leavens other things. And what is leavened? Something that has been leavened by other things. From when is it called sourdough? As soon as it become unsuitable as dog food. One must not cover blood with ashes which burned [and thus created] during the holiday, because it does not count as something "prepared [before the holiday]". If one brought dust to plaster his roof, lime to whitewash his house, one uses it to cover. Rabbi Yose says: you can on a holiday not slaughter a koi because it is a doubt, and if one has slaughtered him, one does not cover his blood.