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How to prepare milk for baby
Check the expiration date.
If you're using any kind of packaged formula, check the package for the expiration date or use by date. If the date has passed, discard the formula.
Babies' immune systems are not as robust as adults', so they are more susceptible to food-borne illnesses that may be present in expired formula. If you have an unopened, expired can of baby formula at home, many stores will allow you to return it for a free replacement.