The instructions for preparing infant formula are specific and, to a new parent managing the 3am feed on four hours of sleep, can feel unnecessarily complicated. They are not. Each step exists because powdered infant formula carries a real, if small, microbiological risk, and the preparation method is the only available control. Understanding why the rules exist makes them easier to follow consistently.
The risk is not to alarm parents who use formula – it is manageable and manageable simply. But the risk is real, and the deaths and serious illnesses caused by improperly prepared formula, while uncommon, have shaped the guidance for a reason.
Healthbooq (healthbooq.com/apps/healthbooq-kids) covers infant feeding and formula preparation.
The Microbiological Background
Powdered infant formula is manufactured under stringent quality controls but is not sterile. It can be contaminated with Cronobacter sakazakii (formerly Enterobacter sakazakii), a Gram-negative bacterium that is ubiquitous in the environment and occasionally present in powdered formula. In healthy adults, Cronobacter is harmless. In infants under 12 months, particularly preterm infants and those under 2 months, it can cause life-threatening meningitis, bacteraemia, and necrotising enterocolitis.
Salmonella species can also occasionally contaminate powdered formula. The same preparation precautions prevent Salmonella infection.
Cronobacter infection in infants is rare but serious. The CDC and WHO have documented clusters linked to powdered formula preparation, most occurring because formula was made with inadequate water temperatures or allowed to stand at room temperature. Deaths have occurred.
The 70°C Rule
WHO and UK NHS guidance specifies that formula should be made with water that has reached at least 70°C. This temperature kills Cronobacter and Salmonella. Water must be freshly boiled and then allowed to cool – not to room temperature, but to the point where it is still above 70°C. Boil the kettle, then wait no more than 30 minutes before using the water. A thermostat flask that maintains 70°C is an alternative.
Using water that has cooled to room temperature before adding formula does not provide the same bacterial kill. The instruction to "cool the kettle to room temperature" that some parents follow (perhaps to reduce the burn risk) is not safe practice for powdered formula preparation.
After the formula is mixed with the hot water (shaking thoroughly), it needs to be cooled rapidly before feeding: run the sealed bottle under cold water or place in a bowl of iced water. Test the temperature on the inside of the wrist before feeding.
Step-by-Step Preparation
Wash hands thoroughly. Use a clean, sterilised bottle (sterilised by steam, microwave steam bag, cold water chemical method, or boiling). Boil fresh tap water (filtered water and mineral water are not recommended for infants; bottled water is not necessarily sterile or suitable). Allow no more than 30 minutes for the water to cool in the kettle. Measure the required volume of water into the bottle first, then add the correct number of level scoops of formula using the scoop provided. Do not pack the scoop; level it off with a knife. Seal the bottle and shake to dissolve. Cool rapidly and test temperature. Feed immediately.
Storing Made-Up Formula
Prepared formula should ideally be used immediately. If it must be stored, it must go straight into the refrigerator (at or below 4°C) and be used within 24 hours. Never reheat formula that has already been partially drunk (saliva introduced to a bottle promotes bacterial growth). Never keep a warmed bottle for more than one hour before discarding.
A common practice – making up a batch of bottles and storing in the fridge – is acceptable if the above rules are followed. A more common problematic practice: making formula with lukewarm water and leaving it on the counter is not acceptable.
Ready-to-Feed (RTF) Formula
Ready-to-feed liquid formula is commercially sterile. It does not carry the Cronobacter risk of powdered formula and does not require preparation with hot water. It is appropriate for premature infants, infants under 2 months, and whenever the convenience or safety of powdered formula preparation is a concern. The WHO recommends RTF formula for infants at highest risk (particularly immunocompromised infants and those in intensive care).
The significant limitation of RTF formula is cost: it is substantially more expensive than powdered formula for equivalent amounts.
Formula Temperature
Formula does not need to be warmed – babies can feed from bottles at room temperature or even cool. The cultural expectation that formula should be body temperature is not a nutritional requirement. If a baby accepts room-temperature formula, there is no reason to warm it, and warming is one less step to manage. If warming is preferred, stand the bottle in a jug of hot water; do not microwave (microwaves heat unevenly and can create hot spots that burn the baby's mouth).
Key Takeaways
Infant formula preparation has specific safety requirements that exist for important microbiological reasons, not bureaucratic ones. Powdered infant formula is not sterile: it can contain Cronobacter sakazakii and Salmonella bacteria, which are harmless to most adults but can cause severe illness in infants. Using water at 70°C or above kills these bacteria and is the single most important safety step. Formula must be prepared fresh for each feed or stored correctly (refrigerator, used within 24 hours). Ready-to-feed liquid formula is sterile and does not carry the same risk, but is considerably more expensive.
