As the dairy industry advanced in the 20th century, it became desirable to offer a more standardised milk product. Larger dairies blended milk from various herds. To even out the concentration and texture of the lipid content of milk, it became commonplace to break milk down into two components, cream and skimmed milk. These were then reunited via “emulsification” to the desired semi-skimmed or full-fat milk. (Lipids are a large group of organic substances that don’t appreciably interact with water; they include fats, oils, hormones, and certain components of membranes.)
In large processing plants, the cream and the skimmed milk go through separate paths. If one or the other is in excess at the end, it gets sent back to the start. Each cycle through the process deteriorates the proteins in the skimmed milk, so discerning buyers stipulate they will accept only first-pass milk.
Homogenisation of the fat and the water that has been separated from the whole milk can be achieved through the process of emulsion. To do this, it is necessary to break the fat globules into such small particles that they behave like colloids (microscopic and permanently suspended solids). This is achieved by squeezing the milk through very small holes under high pressure. This practice is commonly used in the production of food products. For example, Coca-Cola uses pressure chambers that produce up to 35,000 pounds of pressure per square inch (psi) to achieve a stable and homogenous mix of ingredients.
The most common system for homogenisation is the use of a high-pressure blending valve. To homogenise milk and cream, it is necessary to apply pressure of between 100–250 bar. The gap through which the liquid is forced is much larger than the resultant globule size of the fat after it passes through the valve. The valve exerts churning forces that break fat globules down to the small size desired for stable, homogenized milk.
Returning to the salad dressing analogy …