Rare under either spelling, that is. Interestingly, my source for this, the Worthless Word of the Day, spelled it percribrate when posting it worhtlessly as a word of the day it but as percrebrate in the compiled glossary. Given the etymology and the few other citations I could find, I believe it really is i: from Latin percribrare, to sift thoroughly, from cribrum, sieve. But those are very limited citations, and you probably get to chose which you prefer without fear of correction.
After pasta is cooked it must be percribrated.