Why does my frozen breastmilk smell off when I defrost it?

If it smells sour it’s gone off, don’t offer it to baby. Hard to say why, maybe something wasn’t sterilised properly or milk was kept in the fridge too long before freezing. But sour smell is a definite sign of spoilage.

Yes, it’s still fine to add to baths :)

Might be high lipase

Ours does too. Drives me mad. Can you mix it with fresh milk?

It depends on the "off"!
Sour/rancid then don't use it
Sometimes it'll have a stronger "soapy/metallic smell" because of lipase, some women have a higher concentration of it and freezing can make it more apparent (not being in a sealed container or a bag with the air squeezed out can also do this) in this case it's safe, it just depends on baby and if they'll accept it

Just to add, while high lipase can cause sour smell, it won’t make the milk taste sour. So if it tastes soapy or metallic it’s ok, if it tastes sour - not ok

My frozen milk smelled kinda soapy. Pediatrician said there was nothing wrong with it but that it just happens sometimes. My LO wouldn't drink it straight but i found if I mixed it with a fresh (unfrozen) bottle in small amounts, it wasn't wasted, she would drink it, and it reduced pressure on me for fresh production.