78 Chapter 3
3.3 ), the majority have been associated with
raw or unpasteurized cheese milk. During the
period from 1980 to 1982, an outbreak caused
by S. muenster in a raw - milk cheddar was
traced to a single farm where one cow was
shedding the organism (Wood et al., 1984 ).
A second large outbreak occurred in Canada
during 1984 and was linked to the consump-
tion of pasteurized cheddar cheese. The con-
taminated cheese contained S. typhimurium
phage type 10, subgroup I and II bacteria.
Investigators revealed confounding factors
which may have contributed to the infection,
such as pasteurization process lapses and
contamination from personnel (Donnelly,
2004 ).
After a substantial number of outbreak -
associated cases of salmonellosis occurred in
Minnesota in 1989, S. javiana and S. oran-
ienburg were isolated from 2 of 68 blocks
(3%) of pasteurized mozzarella cheese.
Again, post pasteurization contamination
through handling or factory environment
were believed the most likely sources (El
Gazzer and Marth 1992 ). Improper pasteuri-
zation was the cause of a previous outbreak
associated with pasteurized mozzarella
(Altekruse et al., 1998 ).
More recent outbreaks have been associ-
ated with raw or unpasteurized milk used in
the production of soft cheeses, in which low -
level contamination had gone undetected.
Cross contamination of milk from poultry or
other animals or lack of systematic surveil-
lance for salmonella may have contributed to
the outbreaks. Hard (Cantal) and semi - hard
(Morbier) cheeses have been involved in two
French outbreaks in which the implicated
strains were S. enteritidis and S. typhimurium,
respectively. Salmonella typhimurium has
been the predominant serotype in outbreaks
from several countries, with 10 associated
deaths (De Buyser et al., 2001 ). Fifty percent
of immuno - compromised cases died in a
French outbreak associated with S. dublin.
Both S. typhimurium and S. dublin are fre-
quently recovered from raw milk, dairy6.0 during ripening (Ryser, 2001 ). While
growth is not observed in hard cheeses such
as cheddar or Colby, survival and persistence
has been observed for 70 to 434 days
(Cheddar, pH 5.0 to 5.15) (Pearson and
Marth, 1990 ). Listeria monocytogenes sur-
vived more than 90 days in Trappist cheese
(pH 4.7 to 5.42) and feta cheese at pH 4.6. In
experiments in which cottage cheese was
manufactured from pasteurized milk and
inoculated with 10^4 to 10^8 cfu/ml L. monocy-
togenes , the curd cooking temperature
reduced levels to less than 10 to 100 cfu/g.
No growth of the L. monocytogenes occurred
due to lactic acid content and associated low-
ering of the pH (Ryser et al., 1985 ). During
the manufacture of mozzarella cheese, the
curds undergo severe heat treatments (80 ° C
to 90 ° C for 20 minutes) during the stretching
(fi latura) process. Challenge studies have
indicated that stretching curd at 66 ° C for 5
minutes or 77 ° C for 1 minute can effectively
control L. monocytogenes during the produc-
tion of mozzarella cheese (Kim et al., 1998 ).
In a study of pathogen survival in Swiss
hard and semi - hard cheeses made from raw
milk, with respective curd - cooking tempera-
tures of 53 ° C for 45 minutes and 42 ° C for 15
minutes, Bachmann and Spahr (1995)
detected only L. monocytogenes (of all patho-
gens tested) after 90 days of aging with
growth observed on the semi - hard cheese
surface. The very hard Italian Grana cheeses
(which have a good microbiological safety
record), including Parmigiano - Reggiano and
Grana Padana, have extended aging periods
(more than 12 to 24 months). This elongated
maturation coupled with low water activity
(a w ), cheese curd cooking temperature - time
of 53 ° C to 56 ° C for 85 minutes, and mold -
holding temperature of 52 ° C for 10 hours at
pH 5.0 ensures that L. monocytogenes do not
survive the manufacturing process or are
rapidly killed during the early stages of rip-
ening (Yousef and Marth, 1990 ).
Salmonella spp. Of 20 Salmonella out-
breaks implicating cheese since 1980 (Table