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?

You are right! Frozen milk with excess lipase would smell sour too! Like what Idza said, scald the express milk first before freezing can help. Found this blog post on how to scald the milk: http://simplyrebekah.com/2010/05/03/excess-lipase-scalding-breast-milk/ Like what the ladies here have mentioned, breast milk that has turned bad will taste somewhat like cow's milk that has turned bad. Otherwise, there is a chance that your baby may still accept the slight change in taste (due to the freezing process). Apart from the change in taste, the nutritional value of the breast milk is unaffected by the excess lipase in the milk. I think the most important thing to note is to keep to the storage guideline. Avoid feeding your baby any milk that is kept beyond the recommended storage duration. And you can also read through this article that mentioned a few different types of "smell" a breast milk may have: http://breastfeeding.support/my-breastmilk-smells/