Water is removed from your breast milk using a process called freeze-drying (or lyophilization). This process occurs in a freeze-drying chamber, and involves freezing the milk to very low temperatures and removing the air in the chamber to create a vacuum. Over time, this causes the ice crystals in your milk to turn into water vapor. Freeze-drying is a well-known process in the food and medical industries as it an excellent method for preserving a wide variety of heat-sensitive materials such as proteins, probiotics, & pharmaceuticals. Freeze-drying was discovered in the 13th century by the Inca people in the Andes mountains of present-day Peru, who used the technique to preserve potatoes into a dish known as chuño. It was used by the military in WWII to transport blood plasma, and by NASA in the 1960s to make the first astronaut foods. Learn more about the freeze-drying process here.