Batch (or "vat") pasteurization is the simplest and oldest method for pasteurizing milk. Milk is heated to 154.4 degrees Fahrenheit (63 degrees Celsius) in a large container and held at that temperature for 30 minutes. This process can be carried out at home on the stovetop using a large pot or, for small-scale dairies, with steam-heated kettles and fancy temperature control equipment. In batch processing, the milk has to be stirred constantly to make sure that each particle of milk is heated [sources: Lewis, Sun, Goff].
High-temperature short-time (HTST) pasteurization, or flash pasteurization, is the most common method these days, especially for higher volume processing. This method is faster and more energy efficient than batch pasteurization. Though the higher temperature may give the milk a slightly cooked flavor, HTST pasteurization has been used for so long that people are used to the flavor [source: McGee].
Here are the basics of HTST:
- Cold raw milk (39.2 degrees Fahrenheit and 4 degrees Celsius) is fed into the pasteurization plant.
- The milk passes into the regenerative heating section of the plate heat exchanger. The plate heat exchanger is basically a series of stainless steel plates stacked together with some space in between, forming chambers to hold the milk as it passes through. Let's call the odd-numbered chambers "A" chambers, and the even-numbered chambers, "B" chambers. In the regenerating section, cold milk is pumped through the A chambers, while milk that has already been heated and pasteurized is pumped through the B chambers. The heat from the hot milk passes to the cold milk through the steel plates. This warms the milk to 134.6 to 154.4 degrees Fahrenheit (57 to 68 degrees Celsius).
- Next, the milk passes into the heating section of the plate heat exchanger. Here, hot water in the B chambers heats the milk to at least 161.6 degrees Fahrenheit (72 degrees Celsius). This is the goal temperature for HTST pasteurization.
- The hot milk is then passed through a holding tube. It takes the milk about 15 seconds to pass through the tube, fulfilling the time requirement for this method of pasteurization (remember the D-values?). The milk has been officially pasteurized once it passes through the holding tube.
- Now the pasteurized milk is sent back through the re-generative section, where it warms the incoming cold milk. This cools the pasteurized milk to about 89.6 degrees Fahrenheit (32 degrees Celsius).
- In the last part of the process, the cooling section of the plate heat exchanger uses coolant or cold water to bring the milk to 39.2 degrees Fahrenheit (4 degrees Celsius).