108:4 If a bag of flour got wet in only one spot, whether it’s still wet or has already dried, one takes hold of that part (with the sack and the surrounding flour – Mishnah Brurah 466:11) in his hand and removes the rest, which may still be used; only the part that got wet is prohibited. (It’s appropriate to sift the remaining flour before using it – MB 466:12.) If it got wet in several spots, rendering this course of action impossible, then if it is still wet, he should sift the flour. What remains as crumbs on the sieve is chometz and the rest is permitted. Similarly, if mice ate from the flour, he should sift it. If the flour has already dried, sifting it accomplishes nothing and all the flour is prohibited.
108:5 One may not bake the flour on the same day that it was ground because the flour is still warm and will leaven quickly when water is added. Therefore, we must leave it alone for at least 24 hours after grinding. (If one violates this and works the dough, it is permitted after the fact, but he must then be extra careful in his preparations to ensure that it does not leaven – MB 453:43.)