I don’t know what kind of water you use to soak the yuba. It takes 4-6 hours for cold water and 2-3 hours for warm water to fully soak it. Remember not to use boiling water for soaking. It will easily appear. The outside is rotten, but the inside is still hard. If it is soaked in cold or warm water and the outside is rotten but the inside is hard, then it should be considered as a quality problem.