168 Modern Food Microbiology
Cheeses
Most but not all cheeses result from a lactic fermentation of milk. In general, the process of
manufacture consists of two important steps:
1. Milk is prepared and inoculated with an appropriate lactic starter. The starter produces lactic acid,
which, with added rennin, gives rise to curd formation. The starter for cheese production may
differ depending on the amount of heat applied to the curds. S. salivarius subsp. thermophilus is
employed for acid production in cooked curds (up to 60
◦ C) because it is more heat tolerant than
either of the other more commonly used lactic starters; or a combination of S. salivarius subsp.
thermophilus and L. lactis subsp. lactis is employed for curds that receive an intermediate cook.
2. The curd is shrunk and pressed, followed by salting, and, in the case of ripened cheeses, allowed
to ripen under conditions appropriate to the cheese in question.
Although most ripened cheeses are the product of metabolic activities of the lactic acid bacteria, several
well-known cheeses owe their particular character to other related organisms. In the case of Swiss
cheese, a mixed culture of L. delbrueckii subsp. bulgaricus and S. salivarius subsp. thermophilus is
usually employed along with a culture of Propionibacterium shermanii or P. freundenreichii added
to function during the ripening process in flavor development and eye formation. (See Figure 7–1(C)
and (D)) for a summary of propionibacteria pathways and Figure 7–4 for pathway in detail.) These
organisms have been reviewed extensively by Hettinga and Reinbold.
33 For blue cheeses such as
Roquefort, the curd is inoculated with spores of Penicillium roqueforti, which effect ripening and
impart the blue-veined appearance characteristic of this type of cheese. In a similar fashion, either the
milk or the surface of Camembert cheese is inoculated with spores of Penicillium camemberti.
Two coryneform bacteria of the genus Brachybacterium have been recovered from the surfaces
of French Gruy` ere and Beaufort cheeses
65 but the role these organisms play in the ripening process
is unclear. In a study of L. monocytogenes in European red smear cheese (soft, semisoft, and hard),
5.8% of 329 test samples contained Listeria spp. with 6.4% being L. monocytogenes and 10.6% L.
innocua.
60 Eight samples contained >100 L. monocytogenes/cm
2 ; and two samples contained 10
4
cfu/cm
2 .
There are over 400 varieties of cheeses representing fewer than 20 distinct types, and these are
grouped or classified according to texture or moisture content, whether ripened or unripened, and if
ripened, whether by bacteria or molds. The three textural classes of cheeses are hard, semihard, and
soft. Examples of hard cheeses are all cheddar, Provolone, Romano, Parmesan, Gruy` ere, Emmental,
and Edam. All hard cheeses are ripened by bacteria over periods ranging from 2 to 16 months. Semihard
cheeses include Muenster, Roquefort, Limburger, and Gouda and are ripened by bacteria over periods
of 1–8 months. Blue and Roquefort are two examples of semihard cheeses that are mold ripened for
2–12 months. Limburger is an example of a soft bacteria-ripened cheese, and Brie and Camembert are
examples of soft mold-ripened cheeses. Among unripened cheeses are cottage, cream, Mozzarella,
and Neufchatel.
The low moisture content of hard and semihard ripened cheeses makes them insusceptible to
spoilage by most organisms, although molds can and do grow on these products as would be expected.
Some ripened cheeses have sufficiently low oxidation–reduction potentials to support the growth
of anaerobes. It is not surprising to find that anaerobic bacteria sometimes cause the spoilage of
these products when a w (water activity) permits growth to occur. Clostridium spp., especially C.
pasteurianum, C. butyricum, C. sporogenes, and C. tyrobutyricum, have been reported to cause late
gassiness of cheeses. One of these (C. tyrobutyricum) is well established as the cause of a butyric acid
Cheeses
Most but not all cheeses result from a lactic fermentation of milk. In general, the process of
manufacture consists of two important steps:
1. Milk is prepared and inoculated with an appropriate lactic starter. The starter produces lactic acid,
which, with added rennin, gives rise to curd formation. The starter for cheese production may
differ depending on the amount of heat applied to the curds. S. salivarius subsp. thermophilus is
employed for acid production in cooked curds (up to 60
◦ C) because it is more heat tolerant than
either of the other more commonly used lactic starters; or a combination of S. salivarius subsp.
thermophilus and L. lactis subsp. lactis is employed for curds that receive an intermediate cook.
2. The curd is shrunk and pressed, followed by salting, and, in the case of ripened cheeses, allowed
to ripen under conditions appropriate to the cheese in question.
Although most ripened cheeses are the product of metabolic activities of the lactic acid bacteria, several
well-known cheeses owe their particular character to other related organisms. In the case of Swiss
cheese, a mixed culture of L. delbrueckii subsp. bulgaricus and S. salivarius subsp. thermophilus is
usually employed along with a culture of Propionibacterium shermanii or P. freundenreichii added
to function during the ripening process in flavor development and eye formation. (See Figure 7–1(C)
and (D)) for a summary of propionibacteria pathways and Figure 7–4 for pathway in detail.) These
organisms have been reviewed extensively by Hettinga and Reinbold.
33 For blue cheeses such as
Roquefort, the curd is inoculated with spores of Penicillium roqueforti, which effect ripening and
impart the blue-veined appearance characteristic of this type of cheese. In a similar fashion, either the
milk or the surface of Camembert cheese is inoculated with spores of Penicillium camemberti.
Two coryneform bacteria of the genus Brachybacterium have been recovered from the surfaces
of French Gruy` ere and Beaufort cheeses
65 but the role these organisms play in the ripening process
is unclear. In a study of L. monocytogenes in European red smear cheese (soft, semisoft, and hard),
5.8% of 329 test samples contained Listeria spp. with 6.4% being L. monocytogenes and 10.6% L.
innocua.
60 Eight samples contained >100 L. monocytogenes/cm
2 ; and two samples contained 10
4
cfu/cm
2 .
There are over 400 varieties of cheeses representing fewer than 20 distinct types, and these are
grouped or classified according to texture or moisture content, whether ripened or unripened, and if
ripened, whether by bacteria or molds. The three textural classes of cheeses are hard, semihard, and
soft. Examples of hard cheeses are all cheddar, Provolone, Romano, Parmesan, Gruy` ere, Emmental,
and Edam. All hard cheeses are ripened by bacteria over periods ranging from 2 to 16 months. Semihard
cheeses include Muenster, Roquefort, Limburger, and Gouda and are ripened by bacteria over periods
of 1–8 months. Blue and Roquefort are two examples of semihard cheeses that are mold ripened for
2–12 months. Limburger is an example of a soft bacteria-ripened cheese, and Brie and Camembert are
examples of soft mold-ripened cheeses. Among unripened cheeses are cottage, cream, Mozzarella,
and Neufchatel.
The low moisture content of hard and semihard ripened cheeses makes them insusceptible to
spoilage by most organisms, although molds can and do grow on these products as would be expected.
Some ripened cheeses have sufficiently low oxidation–reduction potentials to support the growth
of anaerobes. It is not surprising to find that anaerobic bacteria sometimes cause the spoilage of
these products when a w (water activity) permits growth to occur. Clostridium spp., especially C.
pasteurianum, C. butyricum, C. sporogenes, and C. tyrobutyricum, have been reported to cause late
gassiness of cheeses. One of these (C. tyrobutyricum) is well established as the cause of a butyric acid