Microbiological Aspects of Dairy Ingredients 67scale equipment at the extended holding time
of 25 seconds (Grant et al., 2002b ) and
culture positive in a survey of retail pasteur-
ized milk in the UK (Grant et al., 2002a ).
Other researchers, however, contest that
because of the lethality of the pasteurization
process, with regard to Map, and the low
numbers of the organism likely to be present
in bulk milk, that the risk is negligible. In the
authors ’ opinion, the debate, on both fronts,
will be protracted.
Salmonella spp. Salmonella spp. are not
heat resistant, certainly at the high water
activity of pasteurized milk. If present in
the product it is due to inadequate pasteuriza-
tion heat treatments or post - pasteurization
contamination (Boor and Murphy, 2002 ). It
is not considered psychrotrophic, although
some strains grow at refrigeration tempera-
tures, albeit very slowly. Pasteurized milk
has been documented as causing salmonel-
losis outbreaks in the United States and UK
due to S. typhimuruim. This is an infective
process, in that the viable organism must be
ingested, rather than a preformed toxin. As a
result it has a longer incubation time than
toxin - mediated food poisoning with pyrexial
illness, diarrhea, and vomiting of several
days ’ duration and in some instances a sys-
temic infection.
Prion proteins. Precursor infective prion
proteins (PrP c ) have been detected in milk but
not in the bovine spongiform encephalopathy
(BSE) associated isoform (PrP sc , Franscini
et al., 2006 ). In the UK, the Southwood
Working Party concluded that the risk of
transmission of new variant Creutzfeldt -
Jakob disease (the human form of BSE) via
bovine milk was remote on the basis of expe-
rience with scrapie, a similar spongiform
encephalopathy affecting sheep. However,
the group recommended that infected ani-
mals and their milk not enter the human food
chain.
One of the problems in making this assess-
ment is that in some cases a mouse bioassay
was used for detection of the infective priontion (Johnson, 1984 ). Dairy products are
responsible for very few cases of food poi-
soning from this organism, because high
numbers are required to elicit the symptoms
and these would produce overt spoilage that
would deter consumption (Goepfert et al.,
1972 ).
Mycobacterium avium subsp. paratuber-
culosis (Map) : The Map organism is the
known cause of Johne ’ s disease (paratuber-
culosis) of cattle, particularly dairy cattle,
which presents as untreatable diarrhea,
weight loss, and reduction of milk yield.
The disease has a severe economic impact
and many countries have control programs
in place with varying degrees of success.
Vaccination of animals is problematic
because of a possible cross - reaction with the
tuberculin test for bovine tuberculosis.
The organism has been implicated as a
cause or contributory factor in Crohn ’ s
disease of humans (Behr and Kapur, 2008 ).
This presents as an infl ammation of the gas-
trointestinal tract which results in weight loss
and constipation due to occlusions of the
colon that may require surgical intervention.
The condition is incurable and severely
impairs the quality of life of those who suffer
from it and their immediate families. Debate
rages on two fronts, fi rst on the contribution,
if any, that Map makes to Crohn ’ s disease,
and second on the ability of Map to survive
commercial pasteurization, even when the
holding time is increased from 15 to 25
seconds. The evidence for a causal link
between Map and Crohn ’ s disease was suf-
fi ciently strong for the UK government to
adopt the precautionary principle (Rubery,
2001 ) and advocate strategies to minimize
exposure of the public to the organism.
However, the evidence suggests that Crohn ’ s
disease is likely to be a multifactorial condi-
tion with genetic predisposition involving the
Nod2 gene (Bentley et al., 2008 ).
Regarding heat resistance, the organism
was found to be culture positive when natu-
rally infected milk was pasteurized in pilot