Defrosted breastmilk

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Why do I have to use thawed breastmilk straight away? I have to be away from ds (10 months) and due to circumstances beyond my control the milk is frozen and will need to come out the freezer tonight but not used until Friday evening. What is the reasoning why it has to be used straight away? Any help gratefully received

24hours after being defrosted.

Will it be in the fridge? If so then it won't defrost fully for ages so you'll have a bit of leeway.

Guidelines where I live (outside the UK) are that it can stay in fridge for 48 hours total (e.g. If it was refrigerated for a day before freezing it can stay in the fridge a day after defrosting. If it was frozen immediately it can stay in fridge 48 hours after defrosting).