Breast milk sour after warming

I feed my baby expresses breast milk in the day and direct latch at night. However, when I heat up my expressed milk, they occasionally smell sour, even though it was expressed the day before. What’s the reason? Is it spoiled?

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did you refrigerate the milk? because milk only lasts 24h in the fridge

2y ago

Yup. And it’s only after heating. When it’s still cold, it smells and tastes fine