Some women have higher levels of lipases in their milk, that can lead to milk smelling or tasting sour. Some babies won't like it, but others don't mind. Scalding the milk before freezing can reduce the effect of the lipase.

I thought this was happening with my milk, cos my DD would never drink my ebm. However, my DS was ok with it. So, not sure if it was or just personal preference and DD only liked milk from the source.

There is a little info here, but more if you google.
https://www.breastfeeding.asn.au/bfi...ces-breastmilk