From the food standards agency
https://www.food.gov.uk/multimedia/pdfs/formulaguidance.pdf
When it is not practical to make up feeds just before feeding:
It is best to make up infant formula fresh for each feed but, there are times when this may not be practical and feeds need to be prepared in advance. For example, when taking an infant to a nursery or to the child minder or when leaving the house for a prolonged period of time.
Ready to use liquid feeds are sterile and are the safest option. However, they are a more expensive option and therefore may not suit all parents.
Preparing powdered feeds for later use
It is the length of time for which the reconstituted formula is stored that increases the risk of bacterial growth. Reducing the storage time will therefore reduce the risk. For example, when taking an infant to the nursery it is best to make up the feed(s) on the same morning before leaving home rather than on the night before.
The steps below outline the safest way to prepare and store feed for later use:
• Prepare feeds in separate bottles, not in one large container (e.g. a jug)
• Follows steps 1 to 9 of the section above ‘Preparing a feed using powdered infant formula’
• Store the feed in the fridge at below 5o C. Prepared bottles are best kept in the back of the fridge and not in the door.
• The temperature of the fridge should be checked regularly. A fridge that is opened frequently may need to be set at a lower temperature to ensure that it does not rise above 5 oC during times of frequent access. The thermostat in older fridges without temperature settings may need to be adjusted to ensure that the temperature is below 5o C.
• The risk of infection to a baby will be lower if the feed is only stored for a short time. Feeds should never be stored for longer than 24 hours and this length of time is no longer considered ideal especially for young babies.
Alternatively, you may:
• Put boiling water in a sealed vacuum flask and use this to make up fresh formula milk when needed.
• Care should be taken to avoid scalding when making up the feed
Re-warming stored feeds
• Only remove stored feed from the fridge just before it is needed.
• Re-warm using a bottle warmer, or by placing in a container of warm water.
• Microwaves should never be used for re-warming a feed.
• Never leave a feed warming for more than 15 minutes.
• Shake the bottle to ensure the feed has heated evenly.