Odor of Human Milk
Under most circumstances, fresh human milk has a mild, slightly sweet scent. Occasionally, human milk that has been frozen and thawed may smell soapy and may be rejected by the baby. In Breastfeeding: A Guide for the Medical Profession, Ruth Lawrence, MD, postulates that for some mothers, milk stored in a self- defrosting freezer may have had changes in its lipid structure due to the freeze-thaw cycles that occur in such freezers.
In a few cases, mothers have reported that their milk began to smell soapy as soon as it cooled, regardless of whether it had been frozen. "When these mothers heated their milk to a scald (not boiling) and then quickly cooled and froze it," writes Lawrence, "the effect was not apparent and their infants accepted the heat treated milk. That process inactivated the lipase (fat-digesting enzyme) and halted the process of fat digestion." However, high heating may lower some nutrient levels, including ascorbic acid (vitamin C). If the milk already smells sour, heating will have no effect on flavor or smell. Milk that smells rancid likely is, and should be discarded.