776. Washing the Flour Sacks

108:6 It is advisable to make new sacks to hold the flour, or at least to open the seams of the existing sacks and to wash them thoroughly with hot water, detergent, abrasion and beating. (After the fact, if one didn’t open the seams before washing the sacks, the flour is still permitted – Mishnah Brurah 453:32.)

108:7 One may not place a sack of flour on an animal's back unless there is a thick leather buffer in between. Otherwise, the flour will get hot and moist from the animal's sweat. If at all possible, one should make sure not to put many sacks on top of each other, which will also cause the flour to heat up and leaven when kneaded. (If one did not listen to these laws and put the flour on the animal’s back, or piled many bags together, the flour is still permitted, though he must wait 24 hours before using it – MB 453:34-35.)