Student Article High School Science Journal Author: Olga Slobodyanyuk

Pasteurization

August 05, 2007

Pasteurization is the process of heating milk to a specific temperature for a specific period of time to destroy harmful organisms such as bacteria, protozoa, molds, yeast and viruses.  The name comes from Louis Pasteur, a French scientist who invented the process in the middle of the nineteenth century.  Pasteurization is different from sterilization because sterilization removes all life forms from food, while pasteurization does not.  It is the most important procedure in processing of milk.  Nowadays there are two common types of pasteurization, High Temperature Short Time (HTST) and Ultra High Temperature (UHT)

      Of these two types, the High Temperature Short Time is the more commonly used one.  There are two methods for this type, batch and continuous flow.  The batch method is used for milk by-products, especially ice-cream.  The milk is placed into a jacketed vat at 63°C (145°F) for 30 minutes, which is surrounded by hot water or steam, and then cooled to about 4°C (39°F) very quickly, either in the vat or after being removed from it.  The continuous method saves more time and energy.  It uses a plate heat exchanger, which is metal plates put together into a frame, through which the milk goes, and which is heated by surrounding water or steam.  All milk particles must be heated for the same time and at the same temperature.  The High Temperature Short Time type of pasteurization is more common in the United States.  Milk after this method has a shelf life of two to three weeks and is labeled as “pasteurized.”

      The Ultra High Temperature type of milk pasteurization is more common in Europe.  Its shelf life is three to six months, but if it gets contaminated, it spoils faster then HTST milk.  Also the milk can be stored into larger packages, which is better for bulk buyers such as large restaurant industries, and making the package costs less.  But equipment used is more complex and expensive.  This method heats milk for only one-two seconds, but to over 135°C (275°F).  This high temperature kills the spores in milk and reduces the spoiling of nutrients.  It’s not as popular in the U.S.A. because people are afraid to buy warm milk.  In U.S. this milk is labeled as “ultra-pasteurized.”  The two methods for this type are direct and indirect heating.  Direct heating damages milk less, because it holds it under high temperature for less time then indirect heating.  The milk is pumped into a chamber of high pressure steam, and then quickly cooled down in another vacuum chamber.  In the indirect heating method, the milk is not brought into contact with the steam.  Plate Heat Exchangers, like those used in HSTS are used, but pressure of the liquid going through the tubes is limited by gaskets.   
 

      Sources:

1. Pasteurization- http://en.wikipedia.org/wiki/Pasteurization
2. UHT Processing- http://www.foodsci.uoguelph.ca/dairyedu/uht.html
3. Pasteurization- http://www.foodsci.uoguelph.ca/dairyedu/pasteurization.html
4. Pasteurization: Definition and Methods- http://www.idfa.org/facts/milk/pasteur.cfm
5. Ultra-high-temperature Processing- http://en.wikipedia.org/wiki/Ultra-high_temperature_processing

 
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