Pasteurization can be done as a batch or a continuous process. A vat pasteurizer consists of a temperature-controlled, closed vat. The milk is pumped into the vat, the milk is heated to the appropriate temperature and held at that temperature for the appropriate time and then cooled. The cooled milk is then pumped out of the vat to the rest of the processing line, for example to the bottling station or cheese vat. The most common process used for fluid milk is the continuous process. The milk is pumped from the raw milk silo to a holding tank that feeds into the continous pasteurization system. The milk continuously flows from the tank through a series of thin plates that heat up the milk to the appropriate temperature. The milk flow system is set up to make sure that the milk stays at the pasteurization temperature for the appropriate time before it flows through the cooling area of the pasteurizer. The cooled milk then flows to the rest of the processing line, for example to the Gable Top Carton Milk Filling Line, or Bag in Box Bag Filling Machine.
Older pasteurization methods used temperatures below boiling, since at very high temperatures, casein micelles will irreversibly aggregate, or “curdle”. Newer methods use higher temperature, but shorten the time. The two main types of pasteurization used today are high-temperature, short-time (HTST, also known as “flash”) and extended shelf life (ESL):
HTST milk is forced between metal plates or through pipes heated on the outside by hot water, and the milk is heated to 72 °C (161 °F) for 15 seconds. Milk simply labeled “pasteurized” is usually treated with the HTST method. HTST is widely applicable for: dairy products, fruit juice, beverage, syrup, soy sauce, vinegar, fluid containing particles and various thick sauce etc.
UHT, also known as ultra-heat-treating, processing holds the milk at a temperature of 140 °C (284 °F) for four seconds. During UHT processing milk is sterilized and not pasteurized. This process allows milk or juice to be stored several months without refrigeration. The process is achieved by spraying the milk or juice through a nozzle into a chamber that is filled with high-temperature steam under pressure. After the temperature reaches 140 °C the fluid is cooled instantly in a vacuum chamber, and packed in a presterilized airtight container. Milk labeled “ultra-pasteurized” or simply “UHT” has been treated with the UHT method.
ESL milk has a microbial filtration step and lower temperatures than UHT milk. Since 2007, it is no longer a legal requirement in European countries to declare ESL milk as ultra-heated; consequently, it is now often labeled as “fresh milk” and just advertised as having an “extended shelf life”, making it increasingly difficult to distinguish ESL milk from traditionally pasteurized fresh milk.
A process similar to pasteurization is thermization, which uses lower temperatures to kill bacteria in milk. It allows a milk product, such as cheese, to retain more of the original taste.