Equipment History and Function
The cream separator is a dairy machine used to separate fresh whole milk into cream and skim milk. Formerly, separation was made by the gravity method, simply waiting for the cream to rise to the top of a pan and then skimming it off. C. G. de Laval of Sweden devised the first mechanical cream separator in 1880, based on the principle of centrifugal force. How it Works
Whole milk is pumped into a bowl, usually through a central tubular shaft. A spindle rotates the bowl at a rate of 6,000 to 9,000 rpm, and a series of identical conical disks separates the milk into vertical layers. The heavier skim milk collects on the outer circumference of the rapidly whirling bowl, while the lighter cream remains in the center. The pressure of the incoming whole-milk supply then forces the cream and skim milk out of the machine and into separate collecting vessels. The cream separator thus makes it possible to control the amount of fat (also called butterfat) remaining in the milk. The gravity method ordinarily leaves 25% of the fat in the milk, while the cream separator leaves only 0.01% to 0.02% of the fat in the skim milk. Since the latter process is also much faster than the gravity method, there is much less chance for harmful bacterial growth.
In 1899 Auguste Gaulin obtained a patent on his homogenizer. The patent consisted of a 3-piston pump in which product was forced through one or more hair-like tubes under pressure. It was discovered that the size of the fat globules produced were 500 to 600 times smaller than the tubes. There have been over 100 patents since, all aiming to produce smaller average particle size while expending as little energy as possible. Today's homogenizer consists of a 3-cylinder positive-piston pump (operating similarly to a car engine) and a homogenizing valve. The pump is turned by an electric motor through connecting rods and a crankshaft.How it Works
If raw milk is left to stand, the fat rises to the top and forms a cream layer. Homogenization is a mechanical treatment of the fat globules in milk brought about by passing milk under high pressure through a tiny orifice. This results in a decrease in the average globule diameter and an increase in the number and surface area. The net result is a much reduced tendency for creaming of fat globules. To understand the mechanism, consider a conventional homogenizing valve processing milk at 2500 psi. As it first enters the valve, liquid velocity is about 4 to 6 meters/second. It then moves into the gap between the valve and the valve seat and its velocity is increased to 120 m/s in about 0.2 milliseconds. The liquid then moves across the face of the valve seat and exits in about 50 microseconds. The whole process occurs between two pieces of stainless steel in a stainless steel valve assembly.
The process of pasteurization was invented by Louis Pasteur. Pasteur’s aim was to destroy bacteria, molds, spores, etc. He discovered that this can be accomplished by exposing them to a certain minimum temperature for a certain minimum time, and the higher the temperature, the shorter the exposure time required. How it Works
There are three forms of pasteurization: batch or vat, HTST, and UHT pasteurization.
- Batch or vat pasteurization was the first form of pasteurization used. A batch of milk is pumped into a vat, heated to 145°F, and then held at this temperature for 30 minutes.
- The second is HTST, which stands for high-temperature, short-time pasteurization. As stated in the name, it is a shorter process that uses higher temperatures. The required temperature to pasteurize milk with this method is typically 161°F. A continuous stream of milk is pumped through a heat exchanger to be heated to the required temperature, and then pumped through a "holding tube." The holding tube is sized so that it takes 15 - 20 seconds for milk to travel all the way through it. Once the milk reaches the end of the tube, the temperature is checked, and if it is still at 161°F, the milk is considered pasteurized.
- The third is UHT, ultra-high temperature pasteurization. This method is used mostly for coffee creamers and boxed juices in the US. The product is brought to 250°F under pressure for only a fraction of a second.