You can leave the breastmilk at room temperature for a few hours to know how spoilt milk taste or smells like. It will be sour. When breastmilk are frozen, it will have lipase which is the strong metallic taste.
Some babies are okay with frozen breastmilk and some are not. I have friends who slightly scald the milk before freezing to reduce the lipase. You can also try to thaw it quickly under running water instead of thawing it overnight in the fridge. This way can reduce the strong taste as well.
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