72 Chapter 3
of such cheese - based products. The risk of
pathogen growth or cross contamination
through mishandling or temperature abuse in
catering, retail, or industrial settings cannot
be discounted. As indicated herein, the use of
raw - milk cheeses as an ingredient in products
can increase the risk of exposure to zoonotic
agents.
Disease - causing microorganisms are often
found in milk used for cheese making (Table
3.1 ). Raw milk quality is important in pro-
ducing all cheeses, particularly those made
from raw milk. Low bacterial counts and
low somatic cell counts are the key indi-
cators of milk quality; higher counts indicate
potential pathogen presence (Donnelly,
2004 ). Pasteurization of the milk (holding
pasteurization, 63 ° C for > 30 minutes; high -
temperature - short - time, 71.7 ° C for 15
seconds) effectively destroys most of the
microbial contaminants including pathogenic
bacteria (Zottola and Smith, 1991 ). In the
absence of pasteurization, U.S. regulations
stipulate that cheese must be held at a tem-
perature of not less than 1.7 ° C (35 ° F) for a
minimum of 60 days. Furthermore, some
cheeses are prepared from thermized milk, in
which a sub - pasteurization heat treatment is
applied to the milk, primarily to destroy psy-
chrotrophic bacteria. Such milk has been des-
ignated unpasteurized (De Buyser et al.,
2001 ) rather than raw, which the authors
defi ne as milk that has not been heat treated
above 40 ° C.
The temperatures used during the manu-
facture of cheese to ripen the milk or cook
the curd are not suffi ciently high to destroy
pathogenic microorganisms; therefore, if raw
milk is used in cheese manufacture, some
pathogenic microorganisms may survive the
manufacturing process and subsequently
present a public health risk. The UK Institute
of Food Science and Technology (IFST,
2000 ) has highlighted the potential public
health hazards posed by pathogenic bacteria,
particularly relating to soft and semi - soft
varieties. However, some research has noteddioxide by citric acid fermenting bacteria,
giving rise to the phenomenon of curd
fl oating.
Cheese
Cheese making is a complex microbiological
and biochemical process in which details are
specifi c to the cheese variety. Apart from
acid - coagulated cheeses, cheese making
essentially involves the coagulation of liquid
milk by added rennet and by acid developed
by starter lactic acid bacteria. Milk, usually
from bovine, caprine, ovine, or buffalo
sources, is prepared through standardization,
pasteurization, bactofugation, microfi ltra-
tion, or other processes prior to acidifi cation
and coagulation. The coagulum is cut, stirred,
cooked at an elevated temperature, pressed,
salted to encourage syneresis (dehydration),
and shaped through pressing. While the
nature and quality of the fi nished cheese is
determined largely by the manufacturing
process, the ripening phase, during which
cheeses tend to be stored at low temperature
for two weeks to two years, conveys the char-
acteristic texture and fl avor of the particular
cheese variety.
Cheese may be used directly as an ingredi-
ent in the home, food service, and industrial
sectors for the preparation of a variety of
culinary dishes, formulated foods, or assem-
bled foods (e.g., omelettes, sandwiches,
lasagna, pizza, sauces). Natural cheese is also
extensively used for the production of pro-
cessed cheese products (PCPs) and enzyme -
modifi ed cheeses (EMCs) and cheese
powders. The use of cheese as an ingredient
in other foods generally necessitates some
form of treatment, for example, comminu-
tion, heating/cooking, cooling, freezing,
thawing, or secondary processing such as
emulsifi cation, pasteurization, and spray -
drying. Secondary processing treatments,
especially of cheeses previously manufac-
tured from pasteurized or heat - treated milk,
should contribute to improved public safety