Liquid milk for consumption is mostly either pasteurized or sterilized. Pasteurization is a milk process, designed to inactive the major pathogenic and spoilage bacteria in a raw milk. Further improvements in shelf-life can be obtained by careful control of postpasteurization contamination (PPC), by use of good quality raw milk and carefully manipulations of the processing conditions. However due to survival and growth of thermouric bacteria or any post-pasteurization contaminants milk will spoil. To keep milk longer then few days at ambient temperature, it needs to be sterilized. The traditional process involves heated milk in the temperature range 114 – 120 Celsius degree for 20 – 30 minutes. More recently UHT process has been introduced. These are continuous sterilization process and involve temperatures in excess of 150 Celsius degree. One of the main purposes of heat treatment is to reduce the microbial population in raw milk. Also, chemical reactions take place and there are changes in physical properties. Some important changes are decrease in pH, precipitation of calcium phosphate, penetration of whey proteins and interaction with casein, Mallard brewing and modifications to the casein micelle. The two most important kinetic parameters are the rate of reaction of inactivation at a constant temperature and the effect of temperature change on reaction rate. The heat resistance of vegetative bacteria and microbial spores at a constant temperatures is characterized by their decimal reduction time (D), this is the time required to reduce the population of 90% or one decimal reduction.