Raw milk is illegal in many countries as it can be contaminated
with potentially harmful microbes. Contamination
can also spoil the milk, making it taste bitter
and turn thick and sticky. Now scientists have
discovered new species of bacteria that can grow
at low temperatures, spoiling raw milk even when
it is refrigerated.
According to research, the microbial
population of raw milk is much more complex than
previously thought. "When we looked at the
bacteria living in raw milk, we found that many
of them had not been identified before,"
said Dr Malka Halpern from the University of Haifa,
Israel. "We have now identified and described
one of these bacteria, Chryseobacterium oranimense,
which can grow at cold temperatures and secretes
enzymes that have the potential to spoil milk.“
New technologies are being developed
to reduce the initial bacterial counts of pasteurized
milk to very low levels. Most enzymes will be
denatured at the high temperatures used during
pasteurisation, which means they will stop working.
However, the heat-stable enzymes made by cold-tolerant
bacteria will still affect the flavour quality
of fluid milk and its products. Because of this,
research into cold-tolerant bacteria and the spoilage
enzymes they produce is vital.
"Milk can be contaminated
with many different bacteria from the teat of
the cow, the udder, milking equipment and the
milking environment," said Dr Halpern. "Milk
is refrigerated after collection to limit the
growth of microbes. During refrigeration, cold-tolerant,
or psychrotolerant, bacteria that can grow at
7°C dominate the milk flora and play a leading
role in milk spoilage. Although we have not yet
determined the impact on milk quality of C.
oranimense and two other novel species (C.
haifense and C. bovis) that were
also identified from raw milk samples, the discovery
will contribute to our understanding the physiology
of these organisms and of the complex environmental
processes in which they are involved. There is
still a lot to learn about the psychrotolerant
bacterial flora of raw milk.“
There is an ongoing debate about the benefits
and risks of drinking unpasteurised milk. Some
people believe the health benefits resulting from
the extra nutrient content of raw milk outweigh
the risk of ingesting potentially dangerous microbes,
such as Mycobacterium bovis, which can
cause tuberculosis, and Salmonella species.
Because of these risks, many countries have made
the sale of unpasteurised milk illegal.
Pasteurisation involves heating
milk to around 72°C for 15-20 seconds in order
to reduce the number of microbes in the liquid
so they are unlikely to cause disease. Some bacteria
produce extracellular enzymes that are remarkably
heat tolerant and can resist pasteurisation. Lipase
enzymes cause flavour defects and proteases can
lead to bitterness and reduced yields of soft
cheese.
Raw milk is consumed in rural
areas of Europe and is also available in large
cities. Distribution of unpasteurised milk is
legal in England, Wales and Northern Ireland but
illegal in Scotland. There are around 275 establishments
in England that are approved by the Food Standards
Agency to sell raw milk. However, the green-top
bottles must display a warning that indicates
the content has not been heat-treated and may
contain harmful organisms. Furthermore, farmers
are not allowed to sell unpasteurised dairy products
if their herd is suspected to be infected with
bovine tuberculosis.
"In Israel, dairy companies
estimate that cold-tolerant bacteria can cause
a 10% loss of milk fats and proteins. When researchers
looked at these bacterial communities, they found
that 20% of the bacteria isolated were found to
be novel species and 5% of these were members
of the genus Chryseobacterium,"
said Dr Halpern. "Because of their effect
on milk quality, it is important that we develop
sensitive and efficient tools to monitor the presence
of these cold-tolerant bacteria.“
Source:
www.sciencedaily.com