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What is Pasteurization?

Purpose, methods, and origin.


Pasteurized Cheese
Photo by Chris Hufnagel [flickr]

Pasteurization is a method of reducing pathogenic microorganisms in food through heat. The concept of heat treating food to prevent spoilage began hundreds of years ago, before refrigeration, when people were looking for ways to extend the shelf life of beer and wine. It wasn't until French scientist Louis Pasteur began experimenting with the time and temperatures needed to effectively reduce microorganism that an effective method was developed that adequately also preserved the flavor and texture qualities in food.

Previous methods for heat treating food used excessive temperatures and exposure times, which greatly affected flavor and often caused evaporation of the alcohol that the process was meant to treat. Louis Pasteur found that briefly heating liquids to 50-60 degrees Celcius (120-140 degrees Fahrenheit) was sufficient for preventing spoilage and reducing acidity in wine. This quick heat method was discovered in 1864 and eventually became known as Pasteurization.

Effects of Pasteurization

While the method was originally designed to treat wine and beer, scientist eventually realized it's wide spread applications. Milk naturally contains several microorganisms that are not only responsible for spoilage, but can cause serious disease or death, such as tuberculosis, scarlet fever, brucellosis, and more. Although these microorganisms may be in small quantities when the milk is fresh, they quickly proliferate in the nutrient rich fluid. Pasteurization not only greatly reduces the likelihood of these food borne illnesses, but it delays spoilage. Although other foods, like juice, eggs, wine, and beer, are often pasteurized, milk is the most commonly pasteurized food product.

Opponents to pasteurization often cite the reduction of nutrients in the heat treated milk. Heat does, in fact, destroy some vitamins and minerals, but the small reduction in nutrient content has been shown to be small. Calcium and phosphorus are reduced by approximately 5%, while a less heat stable Vitamin C is reduced by approximately 20% through pasteurization. Thiamin and B12, which milk is often a major dietary source of, is reduced by approximately 15% after pasteurization.

Pasteurization Methods

Flash Pasteurization (Ultra High Short Time - UHST) - This method is used for juices, alcoholic beverages, milk, and some cheeses. Liquids are passed through heated pipes or metal plates that maintain a consistent temperature of 160-165 degrees Fahrenheit, for about 15-30 seconds. Once heated, the product is quickly cooled and placed in sterile packaging. This method is shown to cause a five fold (or 99.9%) reduction in harmful bacteria. While this method preserves color and flavor better than some other methods, some cheeses do not hold up particularly well to this treatment.

Ultra-Pasteurization - This method involves heating food to ultra high temperatures (about 275 degrees Fahrenheit) for a brief one to two seconds. Foods like juice, milk, soy milk, honey, and wine are often treated with ultra-pasteurization. Milk treated with this method has an extended shelf life of six to nine months when unopened, but the high heat may cause a slight browning of the milk due to the maillard reaction. These high temperatures not only kill pathogenic bacteria, but it also effective at destroying bacterial spores that are not destroyed through UHST pasteurization.

Cold Pasteurization - This term is used to describe a variety of methods for reducing microorganisms in food without heat. These methods include irradiation and Pascalization (pressure treating). Because no heat is used, these methods are not technically Pasteurization, but they achieve the same affect.

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