Never keep formula more than 2 hours at room temperature. Never reuse formula once a baby has consumed some of it.
http://www.who.int/foodsafety/publications/micro/pif_guidelines.pdf
Minimizing the time from preparation to consumption is an effective measure for controlling the risk of
infection with E. sakazakii. Prepared feed should be discarded after two hours, unless it has been stored in the
refrigerator since preparation (see Section 3.1.3). Leftover feed should never be saved for later, or added to a
freshly prepared feed, as harmful bacteria may have had the chance to grow during the feeding period.
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.
[...]
3.2.3 Temperature of water for reconstitution
According to the FAO/WHO risk assessment, 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 may not be readily available
(e.g. developing countries).
When PIF is prepared with water cooler than 70 °C, it does not reach a high enough temperature to completely
inactivate E. sakazakii present in the powder. This is a concern for two reasons: a) a small number of cells
may cause illness, therefore it is important that cells present in the PIF are destroyed; and b) the potential for
surviving cells to multiply in the reconstituted formula. This risk is increased when the reconstituted formula is
held for extended periods above refrigeration temperature.
Concerns have been raised over the use of very hot water for reconstituting PIF, but risk of E. sakazakii is only
dramatically reduced when water at a temperature of no less than 70 °C is used. Currently, the instructions
on many PIF products lead to PIF being reconstituted with water that is around 50 °C. But, according to the
FAO/WHO risk assessment, reconstitution with 50 °C water generally results in the greatest increase in risk,
unless the reconstituted formula is consumed immediately. Under no circumstances is risk reduced when PIF is
reconstituted with 50 °C water. Manufacturer's instructions should be reviewed in the light of the findings of
the risk assessment.
3.2.4 Storage of prepared feeds
Because PIF may contain harmful bacteria, it is best to prepare it fresh for each feed. However, in practical terms
this is not always possible. Feeds may need to be prepared in advance, e.g. for the crèche, babysitter, or if you
are going out for the day. In these circumstances, feeds should be prepared using water no less than 70 °C,
cooled quickly immediately after preparation, and stored in the refrigerator (at 5 °C or colder) for no more than
24 hours.
Feed stored in the refrigerator should be used within 24 hours of preparation. Even if water no less than 70 °C
was used to reconstitute PIF, spoilage bacteria may have survived that can grow at refrigeration temperatures
and can cause feeds to spoil. The quality of reconstituted PIF may also deteriorate on prolonged storage.
Feeds should be cooled quickly before placing into the refrigerator, as hot feeds will increase the refrigerator's
temperature. Feeds can be rapidly cooled by placing under cold running water or in a bowl of cold water.
3.2.5 Re-warming and use of stored feeds
Because of the possibility of growth of harmful bacteria at temperatures above 5 °C, stored formula should not
be removed from the refrigerator and re-warmed until immediately before feeding. Feeds should not be left
warming for more than 15 minutes as re-warming for extended periods means that the feed will be held at a
temperature that is ideal for the growth of harmful bacteria. Holding feed in bottle warmers for lengthy periods
was reported as one of the probable causes of an outbreak of E. sakazakii infection (Gurtler, Kornacki and
Beuchat, 2005).