How Pasteurization Works

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.

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]

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:

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.

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:

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 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.