Formula preparation guidance in the UK has caused confusion for years, partly because the instructions given by formula companies, those from the NHS, and the habits passed down from previous generations are not always consistent — and partly because the 70°C rule was not introduced until 2007, meaning many grandparents and experienced parents prepared formula differently and saw no obvious harm.
The 70°C rule exists because Cronobacter sakazakii (formerly Enterobacter sakazakii), a bacterium found in some batches of powdered infant formula, is capable of causing meningitis, septicaemia, and necrotising enterocolitis in very young infants, and is uniformly killed by water at or above 70°C. The risk is low in absolute terms — but the consequences when infection occurs in a newborn are severe enough to take the precaution seriously.
Healthbooq (healthbooq.com) covers infant feeding safety and formula preparation.
Why Powder Formula Is Not Sterile
Powdered infant formula is manufactured to a high standard but cannot be guaranteed sterile. The drying process does not kill all bacterial spores, and Cronobacter sakazakii in particular can survive the manufacturing process. Contamination rates in commercially tested samples are low but not zero.
This is distinct from ready-made liquid formula, which is heat-treated during manufacturing and is genuinely sterile. Ready-made formula is more expensive per feed but requires no preparation beyond opening the carton and, if warming is desired, gentle warming.
The Correct Method
The NHS preparation guidance is:
Fill the kettle with at least one litre of fresh tap water. Do not use water that has been boiled before (it concentrates minerals). Boil and allow to cool for no more than 30 minutes. The water should still be at least 70°C at the point of use.
Measure the correct volume of water into a sterilised bottle. Add the exact number of scoops of formula powder — never pack the scoop, always level it. Add powder to water, not water to powder. Seal and mix. Cool the feed to feeding temperature by holding the bottle under cold running water or in a bowl of cold water, testing on the inside of the wrist before feeding.
Common errors:
Using water cooled below 70°C: the most dangerous mistake, because bacteria in the powder may survive. "Leave it to cool for a while" can mean reaching temperatures that are inadequate for bacterial killing.
Adding too much or too little powder: too little provides insufficient nutrition; too much is harmful to infant kidneys and can cause hypernatraemic dehydration. Always use the scoop provided with the specific formula tin — scoop sizes are not standardised across brands.
Pre-making bottles in advance and storing in the fridge: this is acceptable as a fallback (the NHS guidance acknowledges it as an option for when feeds need to be made in advance, storing for no more than 24 hours at the back of the fridge at below 5°C and reheating quickly when needed) but introduces more risk than fresh preparation and should not be the routine method.
Reheating in a microwave: microwaves heat unevenly and can create hot spots that burn the baby's mouth. Use a bottle warmer or warm running water.
Sterilisation
All bottles, teats, rings, and caps used for feeding an infant under twelve months should be sterilised after washing. Cold water chemical sterilisation (using Milton-type tablets), steam sterilisation (electric or microwave), and boiling in water for ten minutes are all effective. Sterilisation kills bacteria but does not sterilise — the equipment will recontaminate with air on opening. This is fine for normal use; the 70°C water preparation kills any bacteria introduced after sterilisation.
Equipment should be washed in hot soapy water before sterilising to remove milk residue, which can harbour bacteria and protect them from sterilisation agents.
Water Source
For the first year, formula should be made with tap water (UK tap water meets safety standards for infant feeding) that has been boiled. Well water, spring water, and mineral water are not recommended because mineral content can vary and may be too high in sodium or other minerals for infant kidneys.
If using bottled water in an emergency: choose still (not sparkling) and check the label for sodium content below 200mg/L and sulphate content below 250mg/L.
Pre-Made Liquid Formula
Ready-to-feed cartons are sterile and require no preparation. They are especially useful for the first few weeks at home when the demands of newborn care make precise formula preparation difficult. The main barrier is cost — they are significantly more expensive per feed than powdered formula. Some brands offer small ready-made cartons that make overnight feeds straightforward.
Key Takeaways
Powdered infant formula is not sterile. It can contain harmful bacteria including Cronobacter sakazakii and Salmonella, which can cause serious illness in young infants. The NHS recommends making up formula fresh for each feed using water that has been boiled and cooled to no less than 70°C, which kills any bacteria in the powder. The most common mistakes in formula preparation are: using water cooled below 70°C, making feeds in advance and storing at unsafe temperatures, adding powder before water, and inadequate sterilisation of equipment. Pre-made liquid formula, while more expensive, is sterile and does not require these precautions.
