This is a very informative excerpt about the whole hot/cold water debate. (Bit long sorry but does explain it properly and you can then make your own choice)
It’s all very confusing. Is it bacteria living in the milk powder or the water that’s the issue? Or is it that freshly made formula is ok, but stored formula is problematic because the bacteria might have time to grow?
To try and get to the bottom of this I contacted the World Health Organisation (WHO), which published new guidelines on preparing infant formula in 2007.
The WHO says their main concern is a bacterium called Enterobacter sakazakii, which is relatively rare (it is estimated to affect one in 100,000 babies), but when it does strike, it is deadly in 20-50% of cases. During an outbreak in France in 2004, nine babies fell ill (most of them premature babies), and two died. Powdered baby formula was thought to be the cause. Contamination with Salmonella enterica (a bacterium that causes serious food poisoning) is also a worry.
Here’s what Peter Karim Ben Embarek of the WHO told me: “What is not commonly known is that formula powder is not a sterile product. Harmful bacteria like C.sakazakii could be present in the powder and multiply in the prepared bottled if left for too long. The use of hot water is to kill cells of C. sakazakii that maybe present in the infant formula powder.”
Although bacteria in dried milk powder won’t grow, they can survive for up to a year. And once water is added to the powder they will start to grow quickly.
Ben Embarek and his colleagues have done experiments to replicate pretty much every way of making up a bottle of formula milk (adding cold water; lukewarm water; hot water; storing bottles in the fridge; outside a fridge, etc).
They found that the two best ways of reducing the risk of infection are:
Preparing formula with water hotter than 70 ?C (this kills the bugs), and then cooling it by standing the bottle in cold water.
Once the formula has been prepared, drinking it ASAP
That doesn’t mean you can’t prepare a batch of formula in advance, but you do need to refrigerate it (and it would be safer to make a fresh bottle every time). Refrigerated bottles can be stored for up to 24 hours. You can also transport formula in a cool bag, but it should be used within 2 hours.