Milk does evaporate, but the process differs significantly from the simple disappearance of pure water, resulting in a solid substance left behind. This difference is due to milk’s complex nature as a solution, a colloid, and an emulsion, rather than a single chemical compound. Understanding how the liquid component escapes reveals why the remaining non-liquid matter must be addressed separately.
The Physics of Evaporation
Evaporation is a surface phenomenon where liquid molecules transition into the gaseous phase without reaching the boiling point. This phase change occurs when individual water molecules acquire sufficient kinetic energy to overcome the intermolecular forces holding them within the liquid. Molecules at the surface with this higher energy escape into the air, contributing to the vapor pressure above the liquid.
The rate of evaporation is directly related to the temperature, as greater heat increases the average kinetic energy of all molecules. In an open container, escaped vapor molecules diffuse away, preventing a balance with the rate of condensation. Thus, the liquid continues to vaporize until it is gone. When milk evaporates, it is primarily the water component that undergoes this transition, acting as the volatile solvent.
Milk’s Complex Composition
Milk is overwhelmingly water, typically making up about 87% of its mass, similar to a dilute aqueous solution. The remaining 13% consists of non-volatile solids, which is why milk does not evaporate completely like pure water. These solids include carbohydrates, proteins, and fats, each existing in a different physical state within the liquid.
Lactose, the primary carbohydrate, is dissolved in the water, forming a true solution, while minerals also dissolve to form salts. Proteins, such as casein and whey proteins, are colloidally dispersed as tiny particles or micelles suspended throughout the liquid. Milk fat is present as an emulsion, existing as microscopic globules surrounded by a protective membrane.
The presence of these non-volatile solutes and suspended particles influences the physical properties of the milk. The solutes increase the boiling point and decrease the vapor pressure compared to pure water, which slightly slows the rate at which water molecules escape. As water evaporates, the concentration of all these solids increases steadily in the remaining liquid.
The Residue: What Evaporation Leaves Behind
The result of milk evaporation is the concentration of substances that cannot become a gas under typical conditions. As water is preferentially removed, the percentage of total solids—including lactose, fat, proteins, and minerals—rises significantly. This process is utilized commercially to create products like evaporated milk, where about 60% of the water is removed, or milk powder, where nearly all the water is eliminated.
When milk is heated, the proteins, particularly casein, undergo denaturation and aggregation due to the increased concentration and heat exposure. This change in protein structure causes them to coagulate and form a visible film or “skin” on the surface of heated milk. This film is a dense layer of denatured protein and concentrated fat globules. The remaining mass, if all the water is removed, is a brittle, yellowish-white solid composed of milk solids.
This final residue is a mixture of the non-volatile components. Lactose makes up nearly 40% of the solids-not-fat, and proteins make up about 35%. While the water component of milk evaporates completely, the other nutritional and structural components remain, leaving a concentrated mass.