of warm water. breast milk should never be microwaved as this can produce dangerous hot areas and may also destroy the milk's antibodies. many experts recommend discarding thawed milk that isn't used within 24 hours. breast milk changes to meet a baby's needs so that breast milk expressed when a baby is a newborn won't as completely meet the same baby's needs when he or she is a few months older. also, storage guidelines might differ for preterm, sick or hospitalized infants. expressed milk may be donated to milk banks, which provide human breast milk to premature infants and other children whose mothers cannot provide for them.