How Pasteurization Works

Milk Contamination

Why doesn't pasteurization make our milk completely safe? Pasteurized milk still causes outbreaks of foodborne illness. In this section, we'll look at the many ways milk can become contaminated on its journey from the cow to the table.

The Cow: Before the cow is even milked, pathogens in the surrounding environment can get into the cow's feed or water. During milking, bacteria on the inside or outside of the cow's udder can get into the milk. If the milking device (human or mechanical) hasn't been properly sanitized it may contaminate the raw milk.
Storage and Transfer of Raw Milk: Any time the milk is transferred or stored, all equipment and containers must be sterile to prevent contamination. The storage temperature must be low enough (usually 4 degrees Celsius) to keep any bacteria remaining in the milk from growing.
Pasteurization: We know that pasteurization doesn't kill all the bacteria in milk, but it won't even kill the ones it's supposed to if the guidelines for time and temperature aren't met. One way the dairy industry checks milk to make sure it has been properly pasteurized is by testing for alkaline phosphatase. This enzyme has the same D-value as the tuberculosis bacterium, so if it's found in pasteurized milk, that means that time and temperature requirements were not met [source: Sun].
Equipment: Postpasteurization contamination (PPC) because of flaws in equipment or poor sanitation practices is the most common reason for pasteurization failures [source: Lewis]. Equipment has to be properly maintained and tested, and cleaned and sterilized between uses.
The plate heat exchanger is one potential source of PPC, since cold raw milk and hot pasteurized milk pass each other on opposite sides of the heat exchange plates. If the plates have leaks or cracks, the raw milk can contaminate the pasteurized milk.
Storage and Transfer After Pasteurization: Milk is vulnerable to what the industry calls time-temperature abuse whenever the milk is transferred or stored. This includes all points at or between the processing plant, the warehouse, the store and your home. The weak link in the overall cold chain is usually that indeterminate period after [the milk] leaves the retail outlet and reaches the consumer's refrigerator. [source: Lewis]
Now that it's been brought to your attention, the pressure is on to get the milk home and into the fridge as quickly as possible. Check the temperature of your refrigerator regularly, too. It should always be less than 41 degrees Fahrenheit [source: USDA Food Safety and Inspection Service].