Microbiological Aspects of Dairy Ingredients 77cheeses made from raw milk and in 2% of
cheeses made from heat - treated milk
(Loncarevic et al., 1995 ). Similar levels of
occurrence (1.1% to 8.2%) have been
detected in soft, semi - soft (un)ripened, blue -
veined cheeses in other European countries
(Lunden et al., 2004 ; Manfreda et al., 2005 ).
In the United States, more than 35 class 1
recalls of L. monocytogenes - contaminated
Mexican - style soft cheeses and 28 similar
recalls of imported European soft cheeses
were instigated between 1980 and 1998
(Ryser, 2001 ). Lower levels of contamination
(1.5% to 4%) have been recorded in hard
cheeses (Lunden et al., 2004 ). A number of
studies have reported the absence of L. mono-
cytogenes in mozzarella cheeses prepared
from both cow and buffalo milk (Banks,
2006 ). Recorded levels of L. monocytogenes
in retail raw milk cheeses in the UK (2004),
Ireland (2004), Sweden (1994), and Belgium
(2000) were 0.9%, 0.2%, 42%, and 47%,
respectively. The prevalence of L. monocyto-
genes in retail pasteurized milk cheeses was
0.2% (UK, 2005), 0.11% (Ireland, 2006), 1%
(Spain, 1997 to 1999), and 20% (Sweden,
1994) (Little et al., 2008 ).
Numerous studies have shown that the sur-
vival and growth of L. monocytogenes depends
on conditions during the manufacture, ripen-
ing, and storage of cheese, even when the
latter is performed at refrigeration tempera-
tures (Pitt et al., 2000b ). Given the sensitivity
of L. monocytogenes to pasteurization regimes,
pasteurized milk cheeses prove a minimal
health risk, provided post pasteurization/
processing contamination is avoided. Poor
hygiene, contaminated equipment/personnel,
or brine contamination may introduce L.
monocytogenes into cheeses (Banks, 2006 ).
Brine contamination presents a risk for soft
smear ripened and brined cheeses.
Growth of L. monocytogenes in cheese is
primarily confi ned to soft and semi - soft vari-
eties such as blue, brick, and Camembert,
where populations can increase to at least 10^6
cfu/g as the cheese attains a pH greater thanmilk; with respective D 10 values of 42 s and
2.7 s reported at 62.8 ° C and 71.7 ° C (Mackey
and Bratchell, 1989 ).
Undoubtedly, the largest number of
cheese - associated listeriosis outbreaks have
resulted from the use of raw milk for cheese
manufacture. Soft cheese made from raw
milk has been determined as the cause of
three outbreaks of listeriosis in France in
1995, 1997, and 1999. In 1995, 37 cases were
associated with consumption of a Brie - type
cheese, with 11 fatalities. No deaths resulted
from a similar type of epidemic caused by
two soft cheeses, manufactured by the same
establishment, in 1997. The incriminated L.
monocytogenes strains were serotype 2b in
both cases. A different L. monocytogenes
strain type, serotype 1/2a, was implicated in
a raw milk soft cheese consumed on - farm in
Sweden in 2001. Thirty - three people suffered
from febrile gastroenteritis after consump-
tion of the cheese but no deaths resulted.
Among the at - risk groups for human lis-
teriosis are unborn, newborn, neonates, the
elderly, and immuno - compromised individu-
als. The vulnerability of the unborn was illus-
trated in an outbreak of listeriosis among
Hispanic people in North Carolina. Eleven of
the 13 patients were pregnant and each had
consumed an illicitly produced Mexican -
style cheese contaminated with L. monocyto-
genes. The infection resulted in fi ve stillbirths,
three premature deliveries, and three infected
newborns (MacDonald et al., 2005 ). The
advice for at - risk groups is to avoid soft
cheese such as feta, Brie, Camembert, blue -
veined, and Mexican - style cheese or prod-
ucts which may contain these cheeses as
ingredients (McLauchlin, 1997 ).
Different prevalence levels of L. monocy-
togenes across cheese varieties and countries
have been reported. Studies on the occur-
rence of L. monocytogenes in or on soft
cheeses have shown contamination rates of
0.5% (Farber et al., 1987 ) to 15% (Beckers
et al., 1987 ). In Sweden, L. monocytogenes
was found in 42% of soft and semi - soft