How Pasteurization Works

Inside an industrial pasteurization plant that processes of diverse variety of foods.
Inside an industrial pasteurization plant that processes of diverse variety of foods.

Imagine a time when people thought of wine and beer as "hygienic" beverages that protected them from water-borne diseases. In early 19th-century France, wine accompanied every meal, and was considered a medicinal tonic. Beer was thought to be healthier than water because it contained nutrients. Those were the days, right?

What were people to do, then, when wine and other fermented beverages began to develop their own diseases? Alcohol could turn sour, smelly, bitter or even lose its flavor altogether. It might take on an oily sheen or become cloudy. The only food preservation practices at the time included curing, canning and fermenting. And we knew how to delay food spoilage, but knew little about what caused it. It would take a deeper understanding of what made foods go bad before the method of pasteurization could be developed.

The theory of spontaneous generation served as a popular explanation at that time for why certain forms of life would suddenly appear out of decaying matter -- think maggots growing out of rotting flesh. But as scientists began to understand better how reproduction worked, it became clear that spontaneous generation didn't explain everything. They hadn't yet discovered reproduction by cell division, so scholars still believed that smaller organisms like bacteria and fungi grew from inanimate matter.

So what does all of this have to do with pasteurization? It was in this environment of scientific uncertainty that Louis Pasteur was called upon to study the diseases of wine. Read on to learn the difference between wine and vinegar and how that discovery led to mass pathogenocide.

History of Pasteurization

There's a fine line between wine and vinegar. That's what Louis Pasteur discovered in 1856 when an alcohol manufacturer commissioned him to determine what was causing beet root alcohol to sour.

At that time, scientists thought that fermentation was a purely chemical process. Pasteur's research into fermentation led him to the discovery that it was yeast, a living organism, that turned the beet juice into alcohol. Under the microscope, yeast was round and plump. But when the alcohol spoiled, it contained a different microbe that was rod-shaped. Pasteur speculated that this rod-shaped microbe called Mycoderma aceti, which is commonly used to make vinegar, caused the wine to spoil [source: Feinstein].

These discoveries formed the "germ" of Pasteur's germ theory of fermentation. Years later, Pasteur would apply the same concepts to the origins of disease, leading to some of his greatest contributions to science and medicine.

In the meantime, Emperor Napoleon III enlisted Pasteur to save France's wine industry from the "diseases of wine" [source: Lewis]. In previous experiments, Pasteur had discovered that heating the fermented wine would kill the microbes that caused it to spoil. He wasn't the first to see that connection. Nicolas Appert, the inventor of in-container sterilization, also known as canning, had already shown that treating food with heat could preserve it. Pasteur's contribution was to determine the exact time and temperature that would kill the harmful microorganisms in the wine without changing its taste. He patented the process and called it pasteurization. Before long, the process was also used for beer and vinegar.

The pasteurization of milk didn't come into practice until the late 1800s. Back then, tuberculosis was commonly carried by milk. A low-temperature, long-time (LTLT) process, also known as batch pasteurization, was first developed to kill the tuberculosis pathogen. The incidence of tuberculosis contracted from milk fell dramatically, and in fact it no longer makes the Centers for Disease Control and Prevention's list of foodborne illnesses [source: CDC ].

The first commercial milk pasteurizers were produced in 1882, using a high-temperature, short-time (HTST) process. The first law to require the pasteurization of milk was passed in Chicago in 1908 [source: Sun]. Some people didn't like the idea of pasteurizing milk in the beginning, for many of the same reasons that today's raw milk advocates cite [source: Lewis]. We'll talk more later about raw milk and why some people love it and some people hate it.

How Does Pasteurization Kill Bacteria?

It's not just a simple case of heat stroke. To understand what heat does to a bacterium, we need to know about its structure. A bacterium is a single-celled organism. Think of it like a studio apartment, one room containing all the things a person needs to live: food, water, air. The walls of the apartment enclose the electrical wiring and gas pipes that deliver energy, along with the sewage pipes that get rid of waste products. In contrast to the size of this single-celled organism, even an animal as small as a mouse would be like a huge city with thousands of buildings and extensive infrastructure to keep it "alive."

In more scientific terms, a bacterium is made up of the cell envelope, the cytoplasm and, often, the flagella. Besides holding in the cytoplasm, the cell envelope is where energy-generating functions like photosynthesis and respiration happen. The cytoplasm refers to everything inside the cell envelope, a mixture of water, ribosomes, chromosomes, nutrients and enzymes -- all the things that keep the bacterium alive and kicking. Enzymes are especially important because they cause the chemical reactions that make up the cell's metabolism. The flagella are tiny appendages on the outside of the bacterium that help it move around, attach to surfaces or fend off enemies.

Now that we've set the scene and introduced the characters, here comes the dramatic climax. When the temperature gets hot enough, the enzymes in the bacterium are denatured, meaning they change shape. This change renders them useless, and they're no longer able to do their work. The cell simply ceases to function.

Heat can also damage the bacterium's cell envelope. Proteins and fatty acids making up the envelope lose their shape, weakening it. At the same time, fluid inside the cell expands as the temperature rises, increasing the internal pressure. The expanding fluid pushes against the weakened wall and causes it to burst, spilling out the guts of the bacterium.

Thermoduric bacteria are more heat-resistant and harder to kill. In terms of our apartment analogy, thermoduric bacteria have reinforced walls, double-paned windows, insulated pipes and an emergency supply of water and food. These heat-defying bacteria have to be kept under control by refrigeration, which keeps them from multiplying. [source: Todar]

Thermal Processing and Pasteurization

The term thermal processing applies to a range of heat treatments used for food processing. In general, the point of thermal processing is to kill pathogens and inactivate enzymes that cause negative changes to the food during storage. The most common type of thermal processing is the kind that happens in the kitchen at mealtime. Even the most domestically challenged among us have heated something in the microwave and have therefore "thermally processed" something.

Pasteurization constitutes one of the milder forms of thermal processing. Ultra-high temperature and sterilization methods kill all microorganisms in the food, while milder heat treatments like thermization and pasteurization only kill some of them. Why not use a higher temperature if it will kill more pathogens? The answer is that higher temperatures change the characteristics of the food. Since milk is what most people think of in relation to pasteurization, we'll use the pasteurization of milk throughout the rest of this article to show how pasteurization works.

At higher temperatures, as with UHT, several things happen to milk that make it less desirable to consumers:

  • The proteins in milk are altered, changing how the milk acts when used to make other foods like cheese.
  • Protective enzymes in milk are inactivated, making it more susceptible to spoilage.
  • The Maillard reaction, a chemical reaction between proteins and sugars, occurs at higher heats and causes browning, discoloring the milk.
  • The milk may taste "cooked."

If you look at the sidebar, you'll notice that each method of thermal processing requires a certain length of time. For example, HTST pasteurization takes 15 seconds. This minimum time requirement is based on the thermal death kinetics of the bacteria. No, that's not the name of a death metal band; it's a way to describe the conditions needed to kill bacteria. The D-value is the amount of time it takes to kill 90 percent of one type of bacteria at a particular temperature. The higher the temperature is, the lower the D-value, and vice versa [source: Lewis].

The pasteurization of milk kills off the most heat-sensitive pathogens but retains the qualities of milk that consumers expect: creamy texture, fresh flavor and milky-white color.

Methods of Pasteurization

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).

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].

Food Safety and Raw Milk

The debate over which is better -- raw milk or pasteurized -- is a hot topic right now. Besides being a matter of public health, it's a politically and emotionally charged issue for many people. In the United States, the sale of raw milk is currently legal in 28 states though it can't be transported over state lines [source: The Wall Street Journal]. Here are the highlights of both sides of the argument.

The main argument in support of the pasteurization of milk is that it protects the public from foodborne illness. It's also believed to extend the shelf life of milk while maintaining its flavor, texture and nutritional content. The U.S Centers for Disease Control and Prevention, and the U.S. Food and Drug Administration take the position that pasteurization should be mandatory for all milk products due to its potential for causing foodborne illness. In her food politics blog, nutrition expert Marion Nestle writes that while she supports the right to drink raw milk, she also believes that raw milk carries inherent dangers of which we should all be aware.

The Weston A. Price Foundation is the most outspoken proponent of raw milk. This organization makes a very in-depth argument for raw milk. It claims that enzymes and other milk components that naturally protect the milk from spoilage and help humans digest milk are deactivated by pasteurization. The group presents research that shows that heat treatment causes significant changes in the nutritional content of milk -- especially vitamin C, some B vitamins and several minerals, such as calcium and magnesium. It also objects to conventional dairy practices and believes that producers of raw milk are much better caretakers of the cows, the land and the milk. The organization also emphasizes the fact that pasteurizing milk does not prevent outbreaks of disease from pasteurized milk.

Whichever side of this debate you take, the type of milk you drink is still a matter of personal choice as long as you live in a state that allows the sale of raw milk. If you haven't made up your mind yet, explore the list of links on the next page for more information on pasteurization and the debate over raw milk.

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More Great Links


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  • Centers for Disease Control MMWR Weekly. "Preliminary Foodnet Data on the Incidence of Infection with Pathogens Transmitted Commonly Through Food." April 10, 2009. (Aug. 5, 2010)
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