Is it dangerous to feed powder-based infant formula that has been left at room temperature for more than two hours?

Question

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.)

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Example 1:

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.

Example 2:

Any formula left in the bottle an hour after your baby has started feeding should be discarded[.]

Example 3:

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.

Answer 1

According to the US Department of Energy's Pacific Northwest National Lab webpage Just How Fast Can Bacteria Grow? It Depends.

Bacteria are among the fastest reproducing organisms in the world, doubling every 4 to 20 minutes. Some fast-growing bacteria such as pathogenic strains of E. coli can sicken and kill us

See also the presentation Bacterial Growth

The bacteria E. coli often causes illness among people who eat the infected food. Suppose a single E. coli bacterium in a batch of ground beef begins doubling every 10 minutes.

And explains that there will be 4,096 times as much E. Coli after 2 hours.

According toMedical Microbiology. 4th edition.:

Virulence is the measure of the pathogenicity of an organism. The degree of virulence is related directly to the ability of the organism to cause disease despite host resistance mechanisms; it is affected by numerous variables such as the number of infecting bacteria, route of entry into the body, specific and nonspecific host defense mechanisms, and virulence factors of the bacterium. Virulence can be measured experimentally by determining the number of bacteria required to cause animal death, illness, or lesions in a defined period after the bacteria are administered by a designated route.

In other words, the number of bacteria present are a key factor concerning whether or not illness or death occurs.

Answer 2

The source seems to be this document FAO/WHO. 2006. Enterobacter sakazakii and Salmonella in powdered infant formula. Meeting Report and the document preceeding it from 2004 there are many references in them.

All I could find about storage time is:

Increased holding times at room temperature result in large increases in risk, as a result of the growth that occurs. This effect is exaggerated for warmer room temperatures.

We miss the jump from a collection of numbers and risk analysis to a definite number, I suppose that choosing the 2 hours number is taking into account non-optimal preparation conditions, and other real-life possibilities.

More source are:

From the journal of pediatric gastroenterology and nutrition

parents should strictly adhere to the 2 widely accepted recommendations by FAO/WHO: to prepare the feed just before consumption

A booklet for parents from the World Health Organization How to Prepare Powdered Infant Formula

Throw away any feed that has not been consumed within two hours.

And

Powdered infant formula is not a sterile product1. It may contain bacteria that can cause serious illness in infants, such as Enterobacter sakazakii. Although infections caused by E. sakazakii in formula are rare, they can be serious and sometimes fatal.

From this booklet for professionals from the World Health Organization Safe preparation, storage and handling of powdered infant formula

According to the FAO/WHO risk assessment (FAO/WHO, 2006), risk is dramatically reduced when PIF is reconstituted with water that is no less than 70 °C, as this temperature will kill any E. sakazakii in the powder. This level of risk reduction holds even if feeding times are extended (i.e. up to two hours), and even if ambient room temperature reaches 35 °C. Consequently, reconstituting PIF with water no less than 70 °C dramatically reduces the risk to all infants, even slow feeding infants and infants in warm climates where refrigeration for the prepared formula may not be readily available (e.g. developing countries).

And

According to the FAO/WHO risk assessment for E. sakazakii in PIF, increased feeding durations are generally associated with increased risk due to possible bacterial growth. This risk is increased for warmer ambient temperatures (30 °C and 35 °C). However, when PIF is reconstituted with ≥70 °C water, risk is dramatically reduced, and this risk reduction remains valid for feeding times of two hours. This finding has practical implications for the reduction of risk of E. sakazakii infection for slow-feeding infants and for infants in warm climates where ambient room temperature may be around 35 °C. It is recommended that formula is not held at room temperature for more than two hours, even if water at no less than 70 °C is used to reconstitute PIF. This is because the feed may have become contaminated during preparation, or harmful bacteria may have been introduced into the cup or feeding bottle from the infant's mouth. Also, hot water (70 °C) may have activated bacterial spores of harmful bacteria in the formula. Holding prepared feeds above refrigeration temperature for extended periods provides the opportunity for such bacteria to grow