Analysis of ancient pottery has shown that humans have been consuming cheese and other dairy products for more than 7000 years. Recently, scientists discovered an actual chunk of 3300 year old cheese from an Egyptian tomb. And in it, they found traces of the bacteria that causes the nasty disease brucellosis.
And this is where Louis Pasteur comes in…
Louis Pasteur, a French chemist and microbiologist, was one of the first scientists to work with bacteria. He demonstrated that it was bacteria, not “bad air”, that caused milk and other liquids to go off. In the 1860s, Pasteur worked out how to heat liquids to kill these bacteria, without changing how the liquids tasted. This process was called pasteurisation.
Pasteurisation: a game changer
Pasteurisation changed life as we know it for two reasons. First, it reduced the number of acid-producing bacteria (which cause milk to go sour). As a result, milk could be kept fresh for longer periods of time.
Secondly, pasteurisation killed the disease-causing bacteria that could be present in milk. This included diseases like tuberculosis and brucellosis; at the time they made many, many people sick, and were the cause of many deaths. In addition to improved animal health, pasteurisation is an important part of keeping people healthy.
What does pasteurisation involve?
The times and temperatures needed for pasteurisation vary. Different bacteria have different tolerances to pasteurisation. In the graph below, anywhere along a line is just as lethal for that particular bacteria. The higher the temperature, the less time it takes to kill the bacteria.
Small-scale cheese makers can use batch pasteurization. They hold a 4–500L vat of milk at 63˚C for 30 minutes. However, this system doesn’t work for large milk factories. Instead they use high temperature-short time (HTST) pasteurization where milk is pumped through a plate heat exchanger and held at 72˚C for 15 seconds. In this set up, 100–1000L of milk can be processed each minute!
Pasteurisation is just one part of the food chain. Farmers and vets must also work together for healthy cows producing good quality milk (for example Australia has eradicated tuberculosis and brucellosis from cattle). In addition, cooling, quick transport and good cleaning of equipment and trucks are needed to help keep milk fresh, tasty and safe.
Pioneering galore
Pasteurisation isn’t the only thing Louis Pasteur is famous for. He also pioneered:
- Rabies and anthrax vaccines for animals
- The sterilization of surgical instruments
- Pure bacterial starters for cheese making
Starters are what begin the cheese making process (you can read more about them here). The picture below shows bacteria growing on two different agar plates. Each little dot or circle is a bacterial colony, and each colour is a different type of bacteria.
On the left is what you might imagine the starter for the 3300 year old Egyptian cheese to look like: a bit of a Licorice allsorts – of whatever was in or on the udder at the time. In comparison, the picture on the right is grown from a commercial culture (the lilac is Lactococus lactis and the navy is Streptococcus thermophilus). After pasteurisation, adding commercial cultures allows for predictable and larger scale cheese production.
Schooling @ home idea
Pasteurisation was invented 150 years ago – this is only a smidgen of time in the 7000 years of dairy history. Brainstorm inventions for the next 150 years!
