Pasteurization
Last updated: January 8, 2009.
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: A bottle of pasteurized milk from my local dairy, Craig's Farm Dairy, in Weymouth, Dorset, England.
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 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 turn 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 downmarket restaurants and hotel rooms).
The many other food products that can be pasteurized include almonds, beer, wine, canned foods, cheese, eggs, and fruit juice.
