Careful preparation and handling of infant formulas are important to ensure their safety. Despite label instructions covering their use, some parents fail to follow basic hygienic practices.⁵⁷ Parents should be instructed to use proper hand-washing techniques whenever preparing infant formula or feeding their infant, and to use a clean surface for formula preparation. They also should be given guidance on (1) proper storage of formula product remaining in the original container that will be used or mixed later and (2) proper storage of formula that has been prepared, if it is not to be fed immediately. All formulas should be prepared in clean containers and fed from clean bottles with clean nipples. In most cases, it is not necessary to sterilize bottles (or nipples) before mixing formula in them, especially if they have been washed in a dishwasher.⁵⁸ Detailed pictorial information on the preparation of standard infant formulas is available on the WIC website.⁵⁹

Once opened, cans of ready-to-feed and concentrated liquid product can generally be stored covered (with a plastic overcap or aluminum foil) in the refrigerator for no longer than 48 hours. Powdered formula (both unopened and opened cans) should be stored in a cool, dry place, not in the refrigerator. Once opened, cans of powder should be covered with the overcap. Opened powder products can be used for up to 4 weeks with no loss of quality, if proper precautions are taken to avoid microbiological contamination.

Ready-to-feed formula should be shaken before use to resuspend any mineral sediment and can be poured directly into the bottle and fed immediately. Formula from concentrated liquid or powder can be prepared in individual bottles just before each feeding or in a larger clean container before transferring the desired amount to individual bottles. For mixing, use of a blender is specifically advised against because of the risk of bacterial contamination. If multiple feeds are prepared at a time, bottles for later use should be refrigerated immediately. This is especially important for powder products prepared with hot water (see discussion later in this section), because they take longer to cool to reach a safe storage temperature. All prepared bottles should be used within 24 hours. Unopened bottles of prepared formula should be taken out of the refrigerator no more than 2 hours before being fed. After this time, any remaining contents should be discarded. Any remaining formula not ingested by the infant during the feed should be discarded.⁵⁸

Concentrated liquids should also be shaken before use. The normal preparation of formula from concentrated liquid products requires dilution with an equal volume of water. Most concentrated liquid products contain 38 to 40 kcal/fl oz and are diluted with an equal volume of water for feeding. Mixing instructions are shown in Appendix C: Increasing Caloric Density of Infant Formulas.

In preparing formulas from powder products, it is important to adhere closely to the manufacturer's instructions on the label; most powders of standard formulas are mixed using 1 level, unpacked scoop of powder per 2 fl oz of water. It is important to use the scoop provided by the manufacturer with the specific product and not to use standard measuring spoons or scoops from other products, because powders from different manufacturers provide slightly different amounts of nutrients per unit of volume, and scoop sizes vary accordingly.

For special feeding situations, both powders and concentrated liquids can be reconstituted to provide formulas with more than the standard energy (calorie) concentration, which is 19 kcal/fl oz or 20 kcal/fl oz (Appendix C). Instructions for preparation of more concentrated formulas from powder should be obtained from the manufacturer for the specific product in question. In some instances, instructions may be available on the manufacturers' websites listed at the end of this chapter.

In the early months after birth, especially, infants prefer warm infant formula. This warming can be accomplished by putting the unopened bottle in a bowl of warm water for 5 to 10 minutes prior to feeding. Bottles of infant formula should not be warmed in a microwave oven. Microwave ovens can create "hot spots" in the formula in the bottle, and burns to the infant's mouth can occur, despite the formula seeming to be at the right temperature when tested by the caregiver before feeding.⁶⁰

Ready-to-feed and concentrated liquid products are commercially sterile-that is, they contain no pathogenic organisms. Liquid products may contain small numbers of nonpathogenic spores that are capable of growing only at very high temperatures, so-called thermophiles. These organisms may spoil the formula if it has not been stored properly.

Powdered formula products are heat-treated during manufacture and must meet strict standards regarding the allowable amounts and types of bacteria they may contain. However, they are not completely sterile and, in rare cases, may contain pathogenic organisms. Of ongoing concern has been the occasional presence of Cronobacter sakazakii (formerly named Enterobacter sakazakii) in some powdered infant formulas and in the environment, as well as in other foods, water, and kitchen surfaces.^61,62,63,64 As of 2014, the FDA mandated prerelease testing of all batches of powdered infant formula specifically for Cronobacter and Salmonella species.⁶⁵ Despite these steps, apparent C sakazakii contamination of infant formula led to a manufacturing plant shut down and exacerbated an infant formula shortage in 2022. This opportunistic organism has been the sporadic cause of rare, severe infections (sepsis and meningitis) and even death, usually in preterm infants in the early months after birth and in other immunocompromised infants. For this reason, powdered infant formulas generally are not recommended for these high-risk infants.

Safe, potable water should be used to prepare infant formula. This means that the water is both free of microorganisms capable of causing disease and low in minerals and other contaminants that may be detrimental. In some instances, the use of bottled water may be the best choice. Municipal water supplies are generally free of pathogenic microorganisms but may contain variable concentrations of minerals, including fluoride. Well water needs to be tested for pathogens regularly and may contain high concentrations of fluoride as well as other minerals, such as copper or arsenic. High levels of water copper have been reported to cause gastrointestinal symptoms and possible hepatotoxicity. Arsenic is a carcinogen.

Fluoride in the infant diet is largely dependent on the water supply. The advisable range of fluoride intake for infants is targeted to minimize dental caries risk and avoid all but mild fluorosis (see Chapter 43: Nutrition and Oral Health in Children). Beneficial effects of fluoride are believed to come from topical, rather than systemic, effects. Regardless of the fluoride content of the water supply, no supplementation is advised in the first 6 months after birth, because the teeth have not erupted. After 6 months of age, the need for additional fluoride depends principally on the fluoride content of the water (for recommendations, see Chapter 43: Diet, Nutrition, and Oral Health). Human milk fluoride content tends to be low, and reports on the relationship of human milk fluoride to local water fluoride content are inconsistent. Infant formula is manufactured with defluoridated water, and the fluoride content of water used to dilute concentrated liquid or powder formula will largely determine the formula fluoride content. Many home water filtration systems, including those using ion-exchange resins, activated alumina, and reverse osmosis, reduce water fluoride content. Filtered water or low-fluoride bottled water can be used for mixing infant formula when there is concern about possible fluorosis, especially during the first 6 months after birth. Because of the inherent fluoride content of the ingredients, soy formulas have higher fluoride content than cow milk formulas.

If there is any doubt about bacterial contamination, water used for formula preparation should be brought to a rolling boil for 1 minute; longer boiling may concentrate minerals to an undesirable degree. Instructions from most manufacturers are to cool the water to at least 38°C (approximately 100°F) and using this lukewarm water to prepare formula. Water for diluting concentrated liquid formula can be allowed to cool before mixing formula. Varying temperature recommendations have been made for water used to reconstitute infant formula powder. Historically, the recommendation was for water to be allowed to cool, as for concentrated liquid. In 2004, an expert group convened by the Food and Agriculture Organization (FAO) of the United Nations and the WHO recommended that powder formula be prepared with water that is at least 70°C (approximately 158°F) to decrease the risk of infection with C sakazakii.⁶⁶ Their data suggested that this approach could result in as much as a 4-log decrease in the concentrations of C sakazakii.⁶⁷ A temperature of 70°C implies that after boiling, water is allowed to cool at room temperature for no more than 30 minutes before it is used. This recommendation has since been adopted and promulgated by some authorities, at least for high risk infants,⁶⁸ but not by others.⁶⁰ US manufacturers do not recommend powder formula reconstitution with 70°C water in part because of concerns about the potential risks of burns, labile nutrient loss (notably vitamin C), and formula clumping.⁶⁹