Milk has been the one of the main nutrient sources of human diet for centuries. Microbial studies on milk date back to the seventeenth century, when Kircher used a microscope, and observed the minute worms in milk. Two centuries later, in the1850s, Pasteur proved that the spoilage of milk resulting the sour taste was caused by microorganisms. Pasteur's discoveries on the effect of heat on undesirable microorganisms in beer and wine opened a new era in food science. Therefore, the process was named "pasteurization". In the following years of his invention, pasteurization was conducted in Germany and the U.S.A. (Jay, Modern food microbiology. Aspen Publishing, Gaithersburg, MD, 2000). Another breakthrough in milk safety was refrigeration, which became popular after the 1950s. With the advances in heat treatment and low temperature storage, shelf life of pasteurized milk had been increased significantly. Today, pasteurization, partial sterilization, refrigeration, dehydration, and fermentation are commonly used to increase the shelf life of milk and dairy products. Besides these traditional methods, there are also novel methods used to prevent dairy products from spoilage and pathogen contamination such as high pressure processing an UV light.