I can't really speak from my experience but in Lilies by Mcrae, they discuss mentor pollen in it. I can't quote from memory but he says something to the fact of that they will freeze and thaw the mentor pollen to make it not viable so that when they apply the mentor and the intended pollens, they will hopefully only get pollination from the intended pollen, although somehow the "dead" mentor pollen will help. Now I don't recall seeing a statement that talked about how many times it was frozen and thawed to make it "dead" but I would imagine it would have to be more than a couple.
