How to identify if the frozen breastmilk has turn bad? if there is excess lipase, the milk also has a sour smell, is that right?

Like what everyone says, smelling and tasting the milk is the best way to determine if the milk has gone bad. If it smell/taste sour, it is best to throw it out. Here are some useful Do's and Don'ts when it comes to storing milk: http://www.mayoclinic.org/healthy-lifestyle/infant-and-toddler-health/in-depth/breast-milk-storage/art-20046350 To sum up, when storing your breast milk, use glass or hard plastic, BPA-free container or milk storage freezer bags specially designed for freezing breast milk. Label the containers with the date that the milk was expressed and place them at the back of the freezer when the temperature will be the lowest. Follow the storage guidelines and always thaw the oldest milk first. Lastly, if your baby rejects the milk (likely because it smell/taste sour), prepare another bottle for him/her. Your baby is likely the next best judge (apart from you) on whether the milk is fit for his/her consumption.