Why does my milk separate in the fridge?

Human breast milk is a complex substance with a general composition of 87% water, 3.8% fat, 1.0% protein, and 7% lactose. Fat is the most highly variable macronutrient of milk. The fat concentration of breastmilk is largely determined by the degree of breast fullness. The fuller the breast (from going a long time between feeding or pumping), the lower the fat concentration. The emptier the breast (the more often baby feeds or you pump), the higher the fat concentration. The length of the feed is irrelevant to the fat content when the baby is latched– some babies take a full feed in 4 minutes per side while others take 40 minutes to get the same amount. You can’t tell anything about fat content from the length of the feed. Breast milk fat also varies by the time of day, how efficient the breast was emptied, and where in you are in the stages of lactation. Fat content in breast milk peaks mid-morning, and is at its lowest overnight. Colostrum is low in fat and carbohydrates while mature milk has higher fat

Breast milk separates when store in the fridge because of the temperature change. As the milk cools, the fat (which is lighter than water) in the milk rises and the more water based components stay at the bottom of the container. This is normal, just like water separates from oil in salad dressing. There was an old wives tale circling for many years that shaking breast milk to blend it back together after this separation occurred could some how damage the milk. We have long ago disproven that myth. It is totally fine to shake your breast milk prior to giving it to baby so they get all of that yummy milk. Both the fatty and watery layer are crucial for baby’s growth and you would never see this separation if baby were directly feeding it from the breast. The research tells us that baby’s milk intake (the volume of milk – not the amount of fat in that milk) is the only thing that has been correlated with infant growth in exclusively breastfed babies. All milk is good milk.