by Chris Woodford. Last updated: July 8, 2012.
Go back in time a century and the idea of keeping fresh food for more than a day or so would have seemed not just ludicrous but positively dangerous: there's no surer way to make yourself ill than by eating food that's turning bad. Before electric refrigerators were developed in the early 20th century, people bought their food fresh each day and used it pretty much straight away. These days, with better technology for food preservation, some kinds of milk can be kept in a refrigerator for the best part of a month!
It's not just the cool box in your kitchen that makes this possible but the way the milk (and other foods) are specially treated before they reach your home. The key is a process called pasteurization, where fresh foods are heated briefly to high temperatures, to kill off bacteria, then cooled rapidly before being shipped out to grocery stores. By greatly increasing the shelf life of packaged foods, pasteurization has proved itself to be one of the most important food-preservation technologies ever developed. Let's take a closer look at how it works!
Photo: Economies of scale: This bottle of pasteurized milk came from my local dairy, Craig's Farm, in Weymouth, Dorset, England, about 16km (10 miles) or so from my home. That's relatively local compared to milk you buy in most grocery stores, which may have traveled hundreds of miles to your home or even been shipped in from another country or continent. Pasteurization means milk keeps longer, but it also means it can be stored longer and shipped further. Good news for the super stores perhaps, and the giant farming corporations, but not so good for small farmers who struggle to compete against mighty corporations and vast economies of scale.
How pasteurization was invented
If you think pasteurization is an odd name to give to heating food, you're right; the process is named for its discoverer, French biologist Louis Pasteur (1822–1895), who stumbled on the idea in the mid-19th century while trying to discover exactly what made food go bad. Pasteur had trained in science and was working at the University of Lille, France when some winemakers invited him to solve a problem they were having: they couldn't figure out why certain of their wines were turning bad more quickly than others.
With the help of a microscope, Pasteur discovered that the yeast used in making wine and beer contains different bacteria. Some of the bacteria helps to produce the alcohol in the drink from sugar (by a process called fermentation), while other bacteria turns the drink bad after it's made. Pasteur's simple solution was to heat the wine briefly to kill off the harmful bacteria so the drink wouldn't go bad so quickly. This idea, which became known as pasteurization, proved hugely successful and was later adopted to help preserve a wide range of other food and drinks.
Building on his ideas, Pasteur turned his attention to medicine. With other scientists such as Robert Koch (1843–1910), he made a number of important contributions to the germ theory of disease: the idea that some diseases are passed between humans when bacteria carry them through the air.
Photo: Louis Pasteur. Engraving courtesy of US Library of Congress.
How foods are pasteurized
Different foods are pasteurized in different ways. Even milk, virtually all of which is pasteurized, can be preserved by several different processes.
Traditionally, ordinary milk was pasteurized in large batches by heating it to around 60°C (140°F) for 30 minutes or so. It's also possible to use a faster process and a hotter temperature for less time: typically heating to 72°C (161°F) for just 30 seconds or so (or two separate periods of 15 seconds), which means the milk will then keep (in cool conditions) for a further 7-12 days (starting from when it leaves the processing plant, not when it arrives in your home).
The hotter the pasteurization temperature, the longer the milk will keep. In a slightly different process, milk can be pasteurized at a much higher temperature of about 140°C (290°F) for just 2-3 seconds, producing what's called UHT (ultra-high temperature) milk that keeps for months (that's the stuff you get in little plastic containers with foil lids in restaurants and hotel rooms).
The many other food products that can be pasteurized include almonds, beer, wine, canned foods, cheese, eggs, and fruit juice.
Photo: A milk production line in a diary. The milk is pasteurized before being loaded into cartons so it keeps much longer. Photo by Donald S. McMichael courtesy of Defense Imagery.