A: Elizabeth, the formula manufacturers tell parents to throw out leftover formula so you don’t take any chances with your baby’s health. Infants are more susceptible to getting food poisoning from spoiled milk since they are small and their immune systems are just starting to develop.

The foods that commonly cause food poisoning are milk, eggs, mayonnaise, meat, poultry, and fish. These foods are considered “perishable” or prone to spoil because disease-causing bacteria can grow in them rapidly. Bacteria grow most quickly at temperatures between 40 and 140 degrees F—in other words, at room temperature, once the food is out of the refrigerator or off the stove. The longer perishable foods are left at room temperature, the more the bacteria multiply. If someone eats food with high levels of bacteria, they can get sick with food poisoning. Food poisoning usually causes vomiting, diarrhea, and fever, but infants are more prone to complications including dehydration and rarely, even death.

Here are the general rules for handling formula safely:

• Make sure everything is clean before you start.
  Wash your hands before preparing your baby’s bottles. Use clean bottles and nipples.
• If you’re mixing your baby’s formula from powder or concentrate, it’s safe to leave pre-mixed formula in the refrigerator for 48 hours. This is only okay for fresh formula that you haven’t yet fed to your baby.
• Make up your baby’s bottle with the amount she usually drinks, and maybe a little more. When your baby drinks, the bacteria from her mouth get into the formula and can start growing. Don’t let the milk sit out or the feeding last for more than 1 hour, since the milk can spoil.
• When your baby has finished her feeding, discard any leftover milk since it is contaminated with bacteria. It is better to waste a little formula than to take risks with your baby’s health.