How to prepare formula safely: the 70°C rule matters

At 3 a.m., with a screaming baby and half a brain cell operational, the idea of boiling a kettle, waiting, measuring, and cooling a bottle sounds unreasonable. It is also the correct approach. Here is why it matters, and some ways to make it less painful.

Why preparation method is a safety issue

Powdered infant formula is not a sterile product. Unlike ready-to-feed (RTF) formula, which is manufactured under sterile conditions, the powder can contain Cronobacter sakazakii and Salmonella — bacteria that are rare but capable of causing serious illness in newborns, including meningitis and sepsis.¹

The only reliable way to eliminate these bacteria during home preparation is to use water that is at or above 70°C when it makes contact with the powder. Water that has simply been boiled and left to cool below that temperature does not reliably kill Cronobacter.²

Step-by-step: the NHS method

This is the method recommended by the NHS and consistent with WHO guidance.¹²

Before you start:

1. Wash your hands thoroughly with soap and water.
2. Clean and sterilise the bottle, teat, cap, and any utensils you'll use. Sterilisation methods — steam, microwave steam bag, cold water with sterilising solution, or boiling — are all acceptable. Allow equipment to air-dry or reassemble immediately; rinsing with unboiled tap water after sterilising defeats the purpose.

Making the feed:

3. Boil fresh tap water. Do not re-boil water that has already been boiled — repeated boiling concentrates mineral content. Do not use bottled water without boiling it first (it is not sterile and may have high mineral content unsuitable for infants).
4. Leave the boiled water to stand for no more than 30 minutes. It will remain above 70°C during this window.
5. Pour the required amount of hot water into the sterilised bottle first.
6. Using the scoop provided in the tin, measure the correct number of levelled scoops — level with a clean, dry knife or the built-in leveller on the tin. Do not pack scoops or use heaped scoops. Do not use a different brand's scoop in a different brand's tin; scoop volumes vary.
7. Add the powder to the water (not the other way around).
8. Fit the cap and ring; swirl or gently shake until dissolved. Avoid vigorous shaking if you can — it introduces air bubbles and can lead to a gassier feed.
9. Cool the bottle quickly by holding it under cold running water or placing it in a bowl of cold water, turning it to cool evenly.
10. Test the temperature on the inside of your wrist — it should feel warm or neutral, not hot.
11. Feed immediately.
12. Discard any formula left in the bottle after the feed. Do not save it for later. Bacteria multiply rapidly in warm formula.

What not to do

Microwave — never heat formula in a microwave. Microwaves heat unevenly and create hot spots that can scald a baby's mouth even when the bottle feels cool on the outside.

Pre-cooling water below 70°C before adding powder — sometimes recommended in older guides as a way to speed cooling. This defeats the purpose of the hot water step. Water below 70°C does not reliably inactivate Cronobacter.

Making up several bottles in advance — the NHS previously advised against this because bacteria can multiply in stored formula. If you must make bottles in advance, do so correctly (full hot-water method), cool immediately, store in the coldest part of the fridge (below 5°C), and use within 24 hours.¹ See the storing-prepared-formula article for the full storage rules.

Using bottled water without boiling — bottled water is not sterile and the mineral content of some brands is inappropriate for infants. Always boil, whether tap or bottled.

Shaking vigorously or using a formula mixer that aerates the feed — this adds air. A gassy, uncomfortable baby will have you questioning the formula when the preparation method is the actual issue.

Night feed strategies

The full method takes time, which is brutal at night. A few approaches that work within safe limits:

Ready-to-feed for nights — keep a small supply of RTF formula at room temperature. No kettle, no cooling, no waiting. Decant into a sterilised bottle and it's ready in under a minute. The cost is the main downside.

Thermos flask method — boil a kettle before bed, pour the hot water (above 70°C) into a clean thermos flask. When you need it, the water in a well-insulated flask should still be above 70°C for several hours. Pour into the sterilised bottle, add powder, cool quickly. Check by measuring the water temperature before use if you are unsure — a thermometer costs less than a tin of formula.

Pre-sterilised bottle sets — prepare sterilised bottles with measured powder in the scoops ready to go (in a small container, not in the bottle), and keep the hot water in a flask. Combine when needed. This separates the two elements without compromising either.

Do not prepare the full bottle in advance and leave it at room temperature overnight. This is one of the higher-risk things you can do.

Sterilisation: how long, what method

All bottles and feeding equipment should be sterilised until a baby is 12 months old.¹ After that, thorough washing in hot soapy water (or a dishwasher) is generally considered sufficient for healthy babies, though the NHS recommends continuing sterilisation if you are uncertain.

Sterilising methods that work:

• Electric steam steriliser — fast, no chemicals, requires electricity
• Microwave steam bag — fast, cheap, requires a microwave-safe container
• Cold water sterilising solution — no heat required; useful for travel; items must be fully submerged with no air bubbles; change solution every 24 hours
• Boiling — submerge in a large pan of boiling water for 10 minutes; effective but degrades teats faster

All methods work. The best one is whichever you'll actually do consistently.

When to call your pediatrician

Formula preparation mistakes are common and most are not emergencies. Contact your GP or call NHS 111 if:

• Your baby develops a fever (38°C or higher in a baby under 3 months) within a few days of a feed you are concerned about — same day
• Your baby is unusually lethargic, difficult to rouse, or not feeding — same day or 999 depending on severity
• Your baby shows signs of a gastrointestinal illness (vomiting, diarrhoea) and is under 3 months old — same day
• You have reason to believe the formula itself was contaminated or subject to a product recall — contact the manufacturer and your GP

For routine questions about preparation method, your health visitor is a good first call.

Tracking tip

Logging each feed in PooPeeMilk — including which batch of formula was used — gives you a timestamped record if you ever need to piece together whether a preparation issue or product problem coincided with a change in your baby's behaviour or health.

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Also in this cluster: Storing prepared formula · How to choose a formula · How much formula by age

Sources

• NHS. Making up infant formula. https://www.nhs.uk/conditions/baby/breastfeeding-and-bottle-feeding/bottle-feeding/making-up-infant-formula/
• World Health Organization. How to prepare formula for bottle-feeding at home. 2007. https://www.who.int/publications/i/item/9789241595414