Louis Pasteur, a French scientist, discovered the pasteurization process in the 1850s as a way to prevent organisms in wine from spoiling. Pasteurization was later applied to milk and is now the process commonly used on milk found in grocery stores. Because of pasteurization, milk now lasts much longer before spoiling and is safer for human consumption. There are two common ways of pasteurizing milk. They differ in containing methods and the length of time exposed to high temperatures.
Beginning of Batch Pasteurization
To start the batch pasteurization process, unpasteurized milk is placed into a vat pasteurizer that "consists of a jacketed vat surrounded by either circulating water, steam or heating coils of water or steam," according to the University of Guelph. The vat begins to heat the milk and keeps it heated as the milk is moved around the vat. Some pasteurization processes heat the milk partially in a plate or tubular heater before entering the vat, but this modification is usually only used for milk byproducts. The vat process is especially common for ice cream companies, as it helps the mix quality later in the process.
End of Batch Pasteurization
The milk must stay in the vat pasteurizer until every particle inside is completely heated. Milk can either be left to cool in the vat or removed while still hot. After the milk has cooled, it is released to either the bottling station or the cheese vat, depending on the destination for the pasteurized milk.
Beginning of Continuous Pasteurization
Companies typically choose to use the continuous method of pasteurization over the vat method because it saves time and energy. Most continuous processing of milk uses "a high temperature short time (HTST) pasteurizer," according to the University of Guelph. The milk constantly runs through a series of plates and tubes that keep it at a specific, high temperature. The plate heat exchanger is made up of a stack of corrugated stainless steel plates held together by a frame.
End of Continuous Flow Pasteurization
Milk enters the HTST pasteurizer and is heated by running through metal plates or pipes heated by hot water on the outside. The milk is kept at 161 degrees Fahrenheit for at least 16 seconds. It is then cooled back to 39 degrees Fahrenheit and moves through a heat exchanger to "pre-warm cold milk just entering the system," according to Raw Milk Facts. Continuous methods are also used for egg nog and frozen dessert mixes. Depending on the destination for the milk, time kept in the heated plates will vary.