Breastmilk usually smells either sweet or like nothing at all, but for some mothers pumped milk has a funny smell - either an "off" smell or a soapy odor.
If your milk smells like this, you may have higher levels of a normal enzyme called lipase in your milk. Lipase breaks down the fat in milk, and the more you have of it the faster this process occurs.
Here are a few things to know about excess lipase:
1) Since mothers have differing amounts of lipase in their milk, some find that the milk starts to smell funny immediately after it's expressed, while others find that it can retain a normal smell for a number of hours or days before it starts to smell bad. To figure out what your "window" is, pump some milk and smell and/or taste it at regular intervals.
2) The milk is not harmful. Some babies, however, object to the smell or taste, and won't drink it. If your baby doesn't care, there's nothing wrong with feeding it to your baby. It's sometimes the reason why a baby refuses to take a bottle.
3) You can inactivate the process of lipase fat-breakdown by scalding your milk before the smell develops. To scald it, heat it on the stove until you see little bubbles around the edges of the pot (180 degrees F). Then cool and store the milk. Don't bring it to a boil. Some mothers use a bottle warmer and thermometer to do this. According to kellymom.com, "scalding the milk will destroy some of the antiinfective properties of the milk and may lower some nutrient levels, but this is not likely to be an issue unless all of the milk that baby is receiving has been heat-treated." I know of one mother who pumped at work and brought in an electric burner and scalded the milk in her office!