I followed the world health organisation guidelines for pre-making and storing formula (section 2.1.5 page 10). I have attached the link (hopefully!) to the guidelines. It also explains the reasoning behind why the water should be above 70 degrees, rapid cooling etc.
https://www.who.int/foodsafety/publications/micro/pif_guidelines.pdf
Summarised steps below....
1. Clean and disinfect a surface on which to prepare the feed.
2. Wash hands with soap and water, and dry using a clean cloth or a single-use napkin.
3. Boil a sufficient volume of safe water. If using an automatic kettle, wait until the kettle switches off; otherwise make sure that the water comes to a rolling boil. Note: bottled water is not sterile and must be boiled before use. Microwave ovens should never be used in the preparation of PIF as uneven heating may result in 'hot spots' that can scald the infant's mouth.
4. Taking care to avoid scalds, pour the appropriate amount of boiled water, which has been allowed to cool slightly, but not below 70 °C, into a cleaned and sterilized feeding cup or bottle. The temperature of the water should be checked using a sterile thermometer.
a. If making a batch in a larger container: the container should have been cleaned and sterilized. It should be no larger than 1 litre, be made from food-grade material and be suitable for pouring hot liquids.
5. To the water, add the exact amount of formula as instructed on the label. Adding more or less powder than instructed could make infants ill.
a. If using feeding bottles: assemble the cleaned and sterilized parts of the bottle according to the manufacturer's instructions. Shake or swirl gently until the contents are mixed thoroughly, taking care to avoid scalds.
b. If using feeding cups: mix thoroughly by stirring with a cleaned and sterilized spoon, taking care to avoid scalds.
c. If preparing a batch in a larger container: stir formula using a cleaned and sterilized spoon to ensure even mixing. Immediately pour into individual feeding cups or bottles, taking care to avoid scalds.
6. Cool feeds quickly to feeding temperature by holding under a running tap, or placing in a container of cold water or iced water. Ensure that the level of the cooling water is below the top of the feeding cup or the lid of the bottle
7. Place cooled feeds in a dedicated refrigerator. The temperature of the refrigerator should be no higher than 5 °C and should be monitored daily.
8. Feeds can be stored in the refrigerator for up to 24 hours.
It is not recommended that batches of formula are cooled and stored in large volumes as large volumes can promote inadequate cooling and hence lead to the growth of harmful bacteria.
2.1.6 Re-warming stored feeds
1. Remove feeds from the refrigerator just before they are needed.
2. Re-warm for no more than 15 minutes.
3. To ensure that the feed heats evenly, periodically shake or swirl the feed in its covered container. Note: microwave ovens should never be used for re-warming feeds as uneven heating may result in 'hot spots' that may scald the infant's mouth.
4. Check feeding temperature in order to avoid scalding the infant's mouth.
5. Discard any re-warmed feed that has not been consumed within two hours.
