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proliferation of bacteria before it is consumed (22,23). In one sampling study,
bacterial counts were measured in 48 samples, the bacterial counts did not
increase significantly over the two-day period and in 5 of these the count
actually fell (22). Rigorous testing by the Milk Marketing Board's Central
Testing Laboratories make it unlikely that antibiotics contaminating the milk
might have contributed to the decline in bacterial count. Whilst some organisms
may have continued to grow, the number dying was greater, resulting in an
overall decline. In another experiment, in milk held at 4C the bacterium
Campylobacter jejuni (a potential cause of acute gastro-entritis) "died most
rapidly in unpasteurised milk and was inactivated at an intermediate rate in
sterile milk" (23). If untreated milk is of high microbiological quality, it is
obviously not the breeding ground for bacteria that many people would have us
believe.
Once milk has been pasteurised or otherwise heat-treated, the ability to
restrict bacterial growth is lost or severely curtailed (17,23). The enzymes and
other infective agents are destroyed to varying degrees. This means that
bacteria contaminating milk after pasteurisation (e.g. from filling machines,
from the air, or even entering under the bottle cap) can grow more rapidly than
they would in untreated milk. Pasteurisation of human milk for hospital milk
banks has been reported in some instances to contribute to outbreaks of
digestive upsets in infants (24,25). The doctors writing this report concluded
that "the evidence suggests that pasteurisation not only eliminates pathogenic
bacteria but also damages bacteriostatic mechanisms, so making the milk more
susceptible to later contamination" (24). They concluded that "pasteurisation of
donated breast milk is unnecessary and it is not recommended" and that
"untreated breast milk can be safely stored at 4 - 6 C for 72 hours".
Conditions of hygiene in many of the larger dairies are fortunately extremely
good but nevertheless post-pasteurisation contamination remains a problem
(26,27) and some of the contaminating micro-organisms can grow quite well
5
at refrigeration temperatures. In a recent report from the National Institute
for Research in Dairying, 146 out of 158 samples of pasteurised milk from 50
dairies were contaminated with these kinds of bacteria. Although not pathogenic
they do contribute to spoilage (26). In most cases, the bacteria were present at
a very low and insignificant level so that the keeping quality of the milk was
still very good. Some, however, had bacterial counts sufficiently high that the
effective shelf-life was very much shorter.
Yet another group of bacteria, the so-called thermodurics, can survive
pasteurisation. The numbers present in bottled milk can vary quite widely and
the presence of those which grow well at low temperatures affects the shelf-life
of the pasteurised product (28). In addition, laboratory studies have shown that
when cow's milk is pasteurised, a germinant for certain bacterial spores can be
produced (29).
THE EFFECTS OF HEAT TREATMENT ON DISEASE BACTERIA
The major advantage of pasteurisation, if not the only one, is its ability to
destroy pathogenic bacteria. There are, however, two fundamental questions to be
asked: Does all pasteurised milk offer absolute protection from infection? Does
the consumption of all untreated milk post a significant health risk?
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