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?

Just like spoiled cow's milk, spoiled breast milk smells sour and should be disposed of. Smell is a much better indicator compared to its appearance, as breast milk can look lumpy or clumpy -- it is normal for breast milk to separate, and the look of normal breast milk can alter depending on your baby's age and/or what you eat. If your breast milk smells sour or rancid, then there is NO way to salvage it. However, if you find that your frozen breast milk smells soapy and you know it should be good, scald your breast milk before freezing it. Breast milk high in lipase causes the fat to break down and results in a soapy smell and your baby may refuse the milk. Scalding the breast milk before freezing it deactivates the lipase.