Milk is a wonderful substance for promoting growth. After
all, that’s what it’s designed to do. That can become a problem, though, when
what’s feeding off the milk’s nutrients is more sinister than a calf or a baby.
Milk can also be a veritable banquet for a host of microbial growth, and when
it’s stored at room temperature, bacteria and other pathogens soon begin to
proliferate. That’s why the process of pasteurisation is so important, and in
this article we’ll take a look at just how and why that is.
But first, just how dangerous is unpasteurised milk? Well,
the U.S. based Centers for Disease Control (CDC) says that imperfectly cared
for milk is the cause of almost three times more hospitalisations than any
other foodborne disease source, making it one of the world’s most perilous food
products. The list of potential pathogens in milk (which pasteurisation kills
the majority of) includes Salmonella, Listeria, Yersinia, Campylobacter,
Staphylococcus aureus, and more.
Pasteurisation, by killing all these pathogens and other
nasties that live in the milk, massively increases its shelf life.
High-temperature, short-time (HTST) pasteurised milk often has a refrigerated
shelf life of two to three weeks, whereas ultra-pasteurised (UHT) milk can last
much longer, sometimes two to three months. Combined with special packaging
technology, UHT milk can last as long as 9 months.
The process of pasteurisation has not always been the norm.
Before industrialisation, people kept dairy cows close by, meaning the milk was
fresh enough not to be dangerous. As supply chains lengthened, however, aging
milk began to become recognised as a source of disease. This led to the
development of traditional forms of pasteurisation in the 18th and 19th
centuries, whereby milk or cream was “scalded,” and quickly heated to kill
germs.
In the early 20th century, the standard method changed to a
longer, lower-temperature heating (e.g. 60°C for 20 minutes). This gradually
spread and became accepted practice, with some states in the U.S. implementing
mandatory pasteurisation laws as early as 1947, and the Federal Government
enacting country-wide legislation in 1973.
Nowadays, modern methods have reverted to the extremely high
heat, but heavily reducing the time of heating and add cooling to reduce curdling.
The two main categories of pasteurisation, HTST and UHT, both fall under this
description.
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Img source |
The above image shows a rough approximation of the
pasteurisation process. In the first one that we mentioned, HTST, milk is
forced between hot metal plates or pipes heated using hot water, which raises
the temperature of the milk to 72°C for 15 seconds. Milk that is simply labelled
pasteurised is usually treated with the HTST method.
For even longer-lasting milk, the UHT method is often
preferred. This nearly doubles the heating temperature to 140°C for 4 seconds
by spraying the milk through a nozzle into a chamber filled with high
temperature steam under pressure. After it reaches 140°C it is instantly cooled
in a vacuum chamber then packed in sterilised containers.
There are also a couple of less widely used alternative
pasteurisation methods such as ESL, which adds a microbial filtration step, and
home pasteurisation, heating the milk to 63°C for 30 minutes, which is still
FDA approved. The newest pasteurisation technology is microwave volumetric
heating (MVH), which uses microwaves to deliver energy evenly at the minimum
necessary hit, preserving heat-sensitive ingredients.
Pasteurisation methods are usually controlled and
standardised by national regulatory bodies, such as the FDA in the U.S., or the
FSA in the U.K. Generally, these agencies decree that milk must be at least HTST-treated
to qualify for the label of pasteurised, although specific temperature and time
designations can vary by authority.
Pasteurisation is a very effective food safety technique
when it comes to milk, and when done properly it kills 99.999% of viable
micro-organisms in milk. This destroys most yeast, moulds, common bacteria, and
other pathogens.
The process must take place under full quality control and
verification conditions. The milk must be heated evenly, with no part of the
milk getting shorter times or lower temperatures. In terms of monitoring,
microbiological measurement techniques can be used, but they can be costly and
time-consuming, meaning products may have already spoiled. Usually milk
pasteurisation efficacy is monitored by checking for the presence of alkaline
phosphatase, which is denatured by pasteurisation.
Overall, pasteurisation marks a key step in making one of
the most dangerous food products for foodborne pathogens into one of the
safest. Milk is so widely consumed in the Western world, and that’s largely
possible thanks to the benefits of pasteurisation.
Sam Franklin
With
a master’s in Literature, Sam inhales books and anything readable, spending his
working hours reformulating the info he gathers into digestible articles. When
not reading or writing, he likes to put his camera to work around the world,
snapping street photography from Stockholm to Tokyo. Too much of this time
spent in Japan teaching English has nurtured a weakness for sashimi, Japanese
whisky, and robot cafés.
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