Breast milk has all the necessary nutrients such as sugar, protein, fat, vitamins, and minerals in the right proportions for the child's absorption and development. Therefore, even though there is no time to breastfeed directly, many mothers still try to pump and preserve milk so that their children can use it gradually. However, during the preservation process, for many reasons, it is inevitable that there will be times when thawed breast milk has a sour smell, strange taste... So when seeing unusual signs, how can mothers know whether the milk can be used or not?