82 Chapter 3
cooking stage of manufacture, when initial
levels of log 5 cfu/ml were used to artifi cially
contaminate the cheese milk. At similar
initial concentrations, Y. enterocolitica was
recoverable in 30 - day - old semi - hard cheeses
but undetectable later in the ripening cheeses.
A 3 - log increase in Y. enterocolitica popula-
tions was observed when pasteurized milk
for Colby cheese manufacture was inoculated
to contain 10^2 to 10^3 cfu/ml with surviving
cells detected after eight weeks of ripening
(3 ° C). Growth and survival on the surface of
ripened Brie (4 ° C to 20 ° C) and during the
manufacture of Turkish feta cheese have
been reported (Little and Knochel, 1994 ;
Erkman, 1996 ).
Their fi ndings refl ect the ability of Y.
enterocolitica to survive and proliferate in
milk and milk products at refrigeration tem-
peratures. However, the organism does not
survive pasteurization or even lesser heat
treatments of milk and milk products. For
example, heat treatment of milk at 60 ° C
for 1 to 3 minutes effectively inactivates
Y. enterocolitica (Lee et al., 1981 ), and
hence does not survive the cooking step in
cottage cheese manufacture (Golden and
Hou, 1996 ). However, given its ability to
grow at low temperature and the fact that
dairy processing plants may harbor the
organism, it is important that post pasteuriza-
tion contamination is minimized to reduce
any risks.
Campylobacter jejuni. Campylobacter
jejuni enteritis outbreaks are frequently asso-
ciated with consumption of unpasteurized
cow ’ s milk. However, fermented dairy prod-
ucts, including cheeses, are rarely associated
with campylobacteriosis. One such sporadic
instance was associated with the consump-
tion of locally produced unpasteurized fresh
cheese in Kansas in 2007 (CDC 2009 ). Of the
101 people who ate the cheese, 66% became
ill, although all cheese samples tested nega-
tive for Campylobacter. However, the infre-
quent isolation of the organism from milk or
milk products, even in epidemiologically2001 ). The increased pH (6.5 to 7.0) and rip-
ening storage temperature (10 ° C to 12 ° C)
associated with Camembert - type cheeses
was found to have a cumulative positive
effect on staphylococcal growth (Meyrand
et al., 1998 ). The thermal stress (high cook-
ing temperatures), long ripening times, and
low water activity associated with the Italian
hard cheese varieties such as Grana Padano
and Parmigiano - Reggiano present suffi cient
obstacles for the proliferation of Staph.
aureus during production (Ercolini et al.,
2005 ). Bachmann and Sphar (1995) assessed
the survival of Staph. aureus in semi - hard
and hard Swiss cheeses and detected a > 5 -
log 10 cfu/g decrease over 24 hours and 60
days for the respective cheeses.
Yersinia entercolitica. Worldwide studies
indicate that Y. enterocolitica is fairly
common in raw milk, with reported levels
varying between 4% and 81%. However,
many isolates from these studies have been
classifi ed as avirulent environmental strains.
The predominant virulent strains, 0 : 3, 0 : 6,
30, 0 : 8, 0 : 10k, and 0 : 13, have been epide-
miologically associated with outbreaks in
pasteurized milk or milk products. Many of
these outbreaks have resulted from post pas-
teurization cross contamination (Ryser, 2001 ;
Greenwood and Hooper, 1990 ).
Although no outbreaks associated with
cheese have been reported, De Boer et al.
(1986) detected four (4.5%) Y. enterocolitica
positive samples from Brie and Camembert
samples tested and one positive sample from
50 blue - veined cheeses. Yersinia spp., includ-
ing Y. enterocolitica , have also been detected
in raw milk cheeses (Hamama et al., 1992 ).
Other studies have failed to recover Y. entero-
colitica from Canadian produced cheddar
and Italian cheese (Schiemann, 1978 ) and
commercially produced pasteurized Brazilian
soft cheese (Aroujo et al., 2002 ). Bachmann
and Sphar (1995) studied the survival of Y.
enterocolitica in raw milk hard and semi -
hard Swiss cheese. In the hard cheese, the
organism was not detectable after the curd