Claim: If you mix infant formula powder with boiled water and leave the resulting mixture at room temperature, it should be considered spoiled after 1 or 2 hours.
Claimed by: Infant formula powder producers (Example 1), various parenting websites (Example 2) and various anonymous forum participants (just google for
infant formula spoiling).
Reason to doubt: None of the sources making this claim cites any kind of scientific evidence. There are numerous reports of anonymous forum individuals who have fed their children with infant formula left at room temperature for more than two hours multiple times without (noticeable) negative effect (Example 3). At the same time, I have not been able to find a single report online of someone who fed (correctly prepared) formula left at room temperature in the 2-4 hour range and reported an illness caused by that.
Research already done: A bacterium called Enterobacter sakazakii can be lethal for infants under certain conditions and grows at room temperature. However, as I understood it, for that to happen the formula must be contaminated with that bacterium first. In addition, I could not find hard data on the likely-hood of off-the-shelf infant formula powder being contaminated in that way. (Link to secondary source, since the link to the primary source seems to be broken.)
HiPP USA says:
November 16, 2015 at 5:43 am
[...] Just keep in mind you do not want to feed a bottle that has been mixed and at room temperature longer than 2 hours.
Any formula left in the bottle an hour after your baby has started feeding should be discarded[.]
That will sometimes put the bottle in the 4-6 hour range at room temperature. I use similac advance and my son has never had any problems from this.