Milk is primarily composed of water, fats, and proteins, but the short answer is yes, it does contain cells. The fluid we drink is a complex biological secretion that originates from specialized glands and naturally includes microscopic cellular material. Understanding milk’s composition requires looking at its chemical building blocks and the biological process by which it is produced.
The Chemical Building Blocks of Milk
Milk is a complex fluid, consisting mostly of water, which makes up about 87% of its volume in cow’s milk. The remaining 13% is a mixture of various organic and inorganic solids dissolved or suspended in the water.
The primary carbohydrate is lactose, a disaccharide sugar composed of glucose and galactose. Lactose is less sweet than table sugar, dissolved in the watery phase of milk, and provides a source of energy.
Milk fat is present as an oil-in-water emulsion, where tiny droplets are dispersed throughout the water. Each fat droplet, known as a milk fat globule, is surrounded by a membrane that keeps it suspended and prevents coalescence. Milk fat is largely a mixture of triglycerides and carries the fat-soluble vitamins A, D, E, and K.
Milk proteins are divided into two main categories: caseins and whey proteins. Casein makes up about 80% of the total protein and is dispersed as large structures called micelles. Whey proteins stay dissolved in the liquid after the caseins have been separated.
How the Mammary Gland Produces Milk
Milk production occurs within the alveoli of the mammary gland, which are sacs lined with secretory epithelial cells. These cells synthesize and release milk components through two distinct biological mechanisms.
The aqueous components, including proteins, lactose, and minerals, are released via merocrine secretion. Vesicles containing the synthesized materials fuse with the cell membrane, releasing their contents into the central cavity of the alveoli. Lactose, created inside the cell, draws water into the alveoli through osmosis, giving milk its high water content.
Milk fat is secreted through apocrine secretion. The fat droplet accumulates within the epithelial cell and is then pinched off, taking a portion of the cell’s outer membrane with it as the milk fat globule membrane. This physical pinching-off is why milk naturally contains small fragments of secretory cell cytoplasm.
Addressing the Cell Question: Somatic Cells in Milk
The cells found in milk are collectively known as somatic cells, a natural component of the biological secretion. Their concentration is measured as the Somatic Cell Count (SCC), a widely used indicator of milk quality and animal health.
Somatic cells are primarily composed of two types: epithelial cells and immune cells. Epithelial cells line the mammary gland ducts and alveoli, and they are regularly shed during the normal renewal cycle and apocrine secretion. Immune cells, or leukocytes (white blood cells), enter the milk to protect the gland from infection.
A low SCC, typically below 100,000 cells per milliliter, indicates a healthy, uninfected mammary gland. When an infection like mastitis is present, the immune response dramatically increases white blood cells, causing the SCC to rise significantly, often exceeding 300,000 cells per milliliter.
The presence of somatic cells, especially in high numbers, impacts product quality because the cells contain enzymes that degrade fats and proteins. These enzymes, such as lipases and proteases, remain active even after pasteurization. This leads to accelerated spoilage and off-flavors like bitterness and rancidity during storage. Pasteurization kills the somatic cells but does not remove them, meaning their enzymes and cellular debris remain in the fluid.