Caregivers often prepare infant formula only to have the baby refuse it or drink a small amount. This leads to a dilemma: can the unused portion be warmed up again later, or must it be thrown away? Public health and food safety guidelines state the answer is a definitive “no,” as rewarming formula introduces significant health risks for the infant.
Why Rewarming Formula is Unsafe
The primary danger in rewarming formula is the rapid multiplication of bacteria that occurs during the cooling and re-heating cycle. Infant formula, especially the powdered variety, is not sterile and may contain bacteria such as Cronobacter or Salmonella before preparation. The formula itself is a nutrient-rich environment, providing an ideal food source for any bacteria that may be present.
Once formula is prepared and warmed, it enters the “temperature danger zone” (between 40°F and 140°F), which is the range where bacteria thrive and multiply quickly. Rewarming it a second time keeps it in this risky temperature range for an extended period. This repeated exposure allows bacterial colonies to reach levels that can cause serious illness in an infant, whose immune system is still developing.
The act of rewarming often does not reach a high enough temperature to sterilize the liquid completely, only making it warm enough for a feeding. Repeated exposure to heat can degrade specific nutrients incorporated into the formula to support infant development. Essential micronutrients like Vitamin C and Thiamine, which are sensitive to heat, can be destroyed with repeated warming, reducing the overall nutritional quality of the feed.
Formula Storage and Disposal Guidelines
To avoid the temptation of rewarming, follow strict guidelines for the storage and disposal of prepared formula. Prepared formula that has not been warmed should be used immediately or stored in the refrigerator, where it can be safely kept for up to 24 hours. Storing it at a consistent refrigerated temperature of 40°F or below significantly slows bacterial growth.
Once prepared formula is taken out of the refrigerator and warmed for the first time, it must be used within one hour or discarded. If the baby begins a feeding but does not finish the bottle, any leftover formula must be thrown out after one hour from the start of the feeding. This is because the baby’s saliva introduces bacteria into the bottle, which can contaminate the formula and lead to rapid bacterial proliferation.
It is never recommended to refrigerate a bottle that has already been warmed or partially consumed to save it for a later feeding. The combination of the mouth bacteria and the time spent in the temperature danger zone makes the formula unsafe to consume, regardless of subsequent refrigeration.
Safe Initial Warming Methods
Warming formula is a preference for some infants, but it is not a requirement for nutrition or safety, as most babies can drink formula at room temperature or even cold. If a warm bottle is desired, there are safe methods for the initial—and only—warming process. The recommended technique involves placing the bottle in a bowl of warm water or holding it under running warm tap water for a few minutes.
A commercially available bottle warmer can also be used, provided it is used according to the manufacturer’s instructions to prevent overheating. Before offering the bottle, caregivers should test the temperature by placing a few drops on the wrist; the liquid should feel only lukewarm. Never use a microwave oven to warm formula. Microwaves heat liquids unevenly, creating “hot spots” that can severely burn an infant’s mouth and throat. Using a warm water bath or bottle warmer ensures the formula is heated gently and consistently for its single, safe use.