Skim, Cream, Butterfat, and Whey: A Process Overview of Milk Centrifugal Separation
High-speed disc stack centrifuges play three key roles in standardized milk production:
- Clarification: removing foreign matter—like hair, stray cow cells, and dirt
- Purification: pulling the butterfat from the raw milk in order to produce skim milk and cream
- Standardization: regulating the fat content of your finished milk by blending some cream back in, so that you can have 1% milk, 2% milk, half-and-half, and so on.
These centrifugal separation operations are among the first steps in milk processing, prior to the milk moving on to the pasteurizer and homogenization. They give milk producers an enormous amount of control, allowing them to ensure the safety of their products while increasing yields and tightly controlling for quality.
The Role of the Centrifuge in Milk Processing
Raw milk starts out somewhere around 4% butterfat (by weight). If you just let that milk sit at room temperature for 24 hours, a good portion of that would rise to the top on its own, forming cream and leaving behind skimmed milk.
Obviously, letting milk sit at 70º for a full day is a terrible way to run your dairy. Not only would it substantially reduce shelf-life, but it’s also not even the ideal separating conditions for raw milk: Your “skim milk” would still be very fatty, and you wouldn’t get much cream. The ideal milk/cream separation temperature is much closer to 100ºF—which is an absolutely awful temperature to let milk sit at for any meaningful amount of time. On top of that, simply separating the milk and cream isn’t all you want to do. This is a natural product, and subsequently is going to enter your process with many possible impurities: straw, hairs, epithelial cells from the udders, white blood corpuscles, red blood corpuscles, bacteria, and all manner of sediment and sludge.
In other words, your milk feed is a mixture of tiny globules of oil (i.e., butterfat), a water-based fluid (your ideal skim milk), and particulates (cells, dirt, sediment, and so on). This is an ideal situation for centrifugation.
A centrifugal separator is a machine that separates any mixture into its components by spinning it. That spinning creates the “centrifugal force” that separates components of different densities (like oil and water, or bacterial spores and milk). Once you have those streams separated, you can direct them into new channels for further processing.
Centrifugal Dairy Processing: Step-by-Step
Centrifugal dairy processing begins with that raw milk feed, which has a butterfat content around 4%, and an unknown load of particulate impurities. That raw milk is fed into a disc stack centrifuge with solid discs, running at a relatively slow speed. The slower speed keeps the milk and fat intact, while the solid discs speed the separation of particulate impurities from the fluid.
This “clean” milk now moves on to purification, where the butterfat and milk are actually separated. This separator runs much more than twice as fast as the clarifier—perhaps as high as 6,000 to 10,000 rpm. It also has a slightly different design: the discs in its stack are perforated with “distribution holes” around their periphery. The result is an incredibly efficient separation: the butterfat content in the skim milk is usually reduced to just 0.01% and 0.05%, whereas the cream is nearly 35% to 40% butterfat. It’s really as close to complete separation as is physically possible.
Finally, the milk moves on to standardization: cream is blended back into the 0.01% skim milk in order to create whole milk (generally greater than 3.5% milk fat), 1/2%, 2%, and so on. From there, the milk is pasteurized and homogenized.