The Science Behind Pasteurization

Picture an era when wine and beer were hailed as "sanitary" drinks, believed to shield people from illnesses carried by water. In 19th-century France, wine was a staple at every meal and even regarded as a health-boosting elixir. Beer, packed with nutrients, was considered a safer choice than water. Those were certainly different times, weren't they?

So, what happened when wine and other fermented drinks started to spoil? Alcohol could turn acidic, develop unpleasant odors, become bitter, or lose its taste entirely. It might appear oily or cloudy. Back then, the only food preservation methods were curing, canning, and fermenting. While people knew how to slow down spoilage, they had little understanding of its root causes. It wasn't until scientists uncovered the reasons behind food decay that pasteurization could be invented.

The spontaneous generation theory was widely accepted as the reason life forms like maggots seemed to emerge from decaying matter. However, as scientific knowledge advanced, it became evident that this theory couldn't explain everything. Before the discovery of cell division, experts believed that microorganisms such as bacteria and fungi arose from non-living materials.

How does this relate to pasteurization? It was amidst this backdrop of scientific ambiguity that Louis Pasteur was tasked with investigating wine-related diseases. Continue reading to uncover the distinction between wine and vinegar and how this breakthrough paved the way for widespread pathogen elimination.

History of Pasteurization

The distinction between wine and vinegar is razor-thin. This was Louis Pasteur's revelation in 1856 when a distillery hired him to identify why beet root alcohol was turning sour.

Back then, experts believed fermentation was solely a chemical reaction. Pasteur's studies revealed that yeast, a living microorganism, was responsible for converting beet juice into alcohol. Under microscopic examination, yeast appeared round and healthy. However, spoiled alcohol contained a rod-shaped microbe. Pasteur theorized that this microbe, known as Mycoderma aceti, which is often used in vinegar production, was the culprit behind wine spoilage [source: Feinstein].

These findings laid the foundation for Pasteur's germ theory of fermentation. Later, Pasteur applied these principles to the study of diseases, resulting in some of his most significant advancements in science and medicine.

Meanwhile, Emperor Napoleon III sought Pasteur's expertise to rescue France's wine industry from the so-called "diseases of wine" [source: Lewis]. In earlier experiments, Pasteur found that heating fermented wine eliminated the microbes responsible for spoilage. While he wasn't the first to notice this link—Nicolas Appert, the pioneer of canning, had already demonstrated heat's preservative effects—Pasteur's breakthrough was pinpointing the precise time and temperature needed to destroy harmful microorganisms without altering the wine's flavor. He patented this method, naming it pasteurization, and soon applied it to beer and vinegar as well.

Milk pasteurization wasn't adopted until the late 19th century. At the time, tuberculosis was frequently transmitted through milk. A low-temperature, long-time (LTLT) process, also referred to as batch pasteurization, was initially created to eradicate the tuberculosis pathogen. This led to a significant decline in milk-related tuberculosis cases, to the point where it no longer appears on the CDC's list of foodborne illnesses [source: CDC].

The first commercial milk pasteurizers emerged in 1882, utilizing a high-temperature, short-time (HTST) process. Chicago became the first city to mandate milk pasteurization in 1908 [source: Sun]. Initially, some opposed the idea, echoing the arguments made by today's raw milk advocates [source: Lewis]. We'll delve deeper into the raw milk debate later.

How Does Pasteurization Kill Bacteria?

It's more complex than just heat stroke. To grasp how heat affects bacteria, we must examine their structure. A bacterium is a single-celled organism, akin to a studio apartment where everything needed for survival—food, water, and air—is housed in one space. The walls of this "apartment" contain electrical wiring, gas pipes for energy, and sewage systems for waste disposal. Compared to this tiny organism, even a small animal like a mouse resembles a sprawling city with vast infrastructure to sustain life.

Scientifically, a bacterium consists of the cell envelope, the cytoplasm, and often the flagella. The cell envelope not only encloses the cytoplasm but also facilitates energy-producing processes like photosynthesis and respiration. The cytoplasm contains water, ribosomes, chromosomes, nutrients, and enzymes—essential components for the bacterium's survival. Enzymes, in particular, drive the chemical reactions that power the cell's metabolism. The flagella are small external structures that aid movement, attachment, and defense.

With the stage set, here's the pivotal moment: when temperatures rise sufficiently, the bacterium's enzymes undergo denaturation, altering their shape. This renders them ineffective, halting their functions and causing the cell to shut down.

Heat also compromises the bacterium's cell envelope. Proteins and fatty acids within the envelope lose their structure, weakening it. Simultaneously, the cell's internal fluid expands as temperatures increase, raising pressure. This expanding fluid presses against the weakened envelope, eventually causing it to rupture and spill the cell's contents.

Thermoduric bacteria are more resilient to heat. Using the apartment analogy, these bacteria have reinforced walls, double-paned windows, insulated pipes, and emergency supplies. To control these heat-resistant bacteria, refrigeration is essential to prevent their multiplication. [source: Todar]

Thermal Processing and Pasteurization

Thermal processing encompasses various heat-based techniques used in food preparation. Its primary goal is to eliminate pathogens and deactivate enzymes that could spoil food during storage. Even simple actions like reheating leftovers in the microwave count as thermal processing, making it a common practice in everyday life.

Pasteurization is a gentler form of thermal processing. Unlike ultra-high temperature or sterilization methods, which eradicate all microorganisms, pasteurization only targets specific pathogens. Why not use higher temperatures for greater safety? Because excessive heat alters the food's properties. Since milk is most commonly associated with pasteurization, we'll use it as an example to explain the process.

At higher temperatures, such as those used in UHT processing, milk undergoes changes that make it less appealing to consumers:

The sidebar reveals that each thermal processing method has a specific time requirement. For instance, HTST pasteurization takes 15 seconds. This duration is based on thermal death kinetics, which describe the conditions needed to eliminate bacteria. The D-value represents the time required to kill 90% of a specific bacteria type at a given temperature. Higher temperatures reduce the D-value, and lower temperatures increase it [source: Lewis].

Pasteurization eliminates the most heat-sensitive pathogens while preserving the qualities consumers expect in milk: a creamy texture, fresh taste, and white color.

Methods of Pasteurization

Batch (or "vat") pasteurization is the oldest and simplest method for treating milk. The milk is heated to 154.4 degrees Fahrenheit (63 degrees Celsius) in a large container and maintained at that temperature for 30 minutes. This can be done at home using a stovetop and a large pot or, in small dairies, with steam-heated kettles and precise temperature controls. Constant stirring ensures every part of the milk is evenly heated [sources: Lewis, Sun, Goff].

High-temperature short-time (HTST) pasteurization, also known as flash pasteurization, is the most widely used method today, especially for large-scale operations. It is faster and more energy-efficient than batch pasteurization. Although the higher temperature may impart a slightly cooked taste, consumers have grown accustomed to this flavor over time [source: McGee].

Here’s how HTST pasteurization works:

Milk Contamination

Why doesn’t pasteurization guarantee milk safety? Despite pasteurization, milk can still cause foodborne illnesses. In this section, we’ll explore how milk can become contaminated from the farm to your table.

Food Safety and Raw Milk

The debate over raw milk versus pasteurized milk is highly contentious. Beyond public health concerns, it’s a politically and emotionally charged issue. In the U.S., raw milk sales are legal in 28 states but cannot cross state lines [source: The Wall Street Journal]. Here’s a summary of both sides of the argument.

Proponents of pasteurization argue it safeguards public health by preventing foodborne illnesses. It also extends milk’s shelf life while preserving its taste, texture, and nutrients. The CDC and FDA advocate for mandatory pasteurization due to its role in reducing foodborne illness risks. Nutrition expert Marion Nestle, in her food politics blog, supports the right to consume raw milk but warns of its inherent dangers.

The Weston A. Price Foundation is a leading advocate for raw milk. They present a comprehensive case, arguing that pasteurization deactivates natural enzymes and components in milk that protect against spoilage and aid digestion. Their research highlights that heat treatment alters milk’s nutritional profile, particularly reducing vitamin C, certain B vitamins, and minerals like calcium and magnesium. They also critique conventional dairy practices, asserting that raw milk producers prioritize better care for cows, land, and milk quality. Additionally, they point out that pasteurization doesn’t eliminate disease outbreaks linked to pasteurized milk.

Regardless of where you stand on this issue, the choice between raw and pasteurized milk remains personal, provided you reside in a state permitting raw milk sales. If you’re undecided, check out the links on the next page for further insights into pasteurization and the raw milk debate.