Microbiology of Fermented Food Production ◾ 179
Microbial Problems
In plain yogurt, flavor problems can be associated with the concentrations of acetaldehyde. A low
concentration gives a chalky and sour flavor, and too much acetaldehyde can give a green flavor.
Similarly, too much diacetyl gives a buttery aroma. Too much acid production during storage
causes a sour taste. Proteolysis and accumulation of bitter peptides during storage are associated
with bitter flavor. Production of exopolysaccharides by the starter can give a viscous and ropy texture (which can be desirable in some situations). Growth of yeasts during storage can also produce
a fruity flavor, especially in yogurt containing fruits and nuts. In colored, flavored, and blended
yogurt, many of these problems are masked. During long storage, molds can grow on the surface
(also see Chapter 20).
Cheeses
Cheeses are made by coagulating the casein in milk with lactic acid produced by lactic acid bacteria, without or with the enzyme rennin, followed by collecting the casein for further processing,
which may include ripening. 6 The process was probably accidentally discovered in the Middle East
in approximately 7000 b.c. from the coagulation of milk stored in a calf stomach by lactic acid
produced by lactic acid bacteria (and probably rennin in the stomach). At present, many varieties
of cheeses are made worldwide, which probably use more than 20% of the total milk produced.
In the United States, the total production of different varieties of cheese in 1982 was 4.4 billion
pounds (2 billion kg) and, in 1987, this increased to 5.3 billion pounds (2.4 billion kg). Because
of the worldwide increase in cheese consumption, cheese production will continue to increase not
only in the United States, but also in other countries, especially in dairy-rich countries, such as
Europe and New Zealand.
Cheese varieties have been grouped in different ways. Examples of several varieties based on
starter cultures used and some important secondary flora are listed here.
Unripened Cheese
Soft
− Cottage cheese with starters Lac. lactis ssp. lactis and cremoris and Leuconostoc mesenteroides ssp. cremoris.
− Mozzarella cheese with starters Str. thermophilus and Lab. delbrueckii ssp. bulgaricus.
Ripened Cheese
Soft
− Brie cheese with starter Lac. lactis ssp.; Penicillium sp. and yeasts are secondary flora.
Semihard
− Gouda cheese with starters Lac. lactis ssp. and Leuconostoc spp.; dairy Propionibacterium
may be secondary flora.
− Blue cheese with starter Lac. lactis ssp., Leuconostoc spp.; Penicillium roquefortii, yeasts,
and micrococci are secondary flora.
Hard
− Cheddar cheese with starters Lac. lactis ssp.; some lactobacilli and pediococci (and probably enterococci) are secondary (or associative) flora.
Microbial Problems
In plain yogurt, flavor problems can be associated with the concentrations of acetaldehyde. A low
concentration gives a chalky and sour flavor, and too much acetaldehyde can give a green flavor.
Similarly, too much diacetyl gives a buttery aroma. Too much acid production during storage
causes a sour taste. Proteolysis and accumulation of bitter peptides during storage are associated
with bitter flavor. Production of exopolysaccharides by the starter can give a viscous and ropy texture (which can be desirable in some situations). Growth of yeasts during storage can also produce
a fruity flavor, especially in yogurt containing fruits and nuts. In colored, flavored, and blended
yogurt, many of these problems are masked. During long storage, molds can grow on the surface
(also see Chapter 20).
Cheeses
Cheeses are made by coagulating the casein in milk with lactic acid produced by lactic acid bacteria, without or with the enzyme rennin, followed by collecting the casein for further processing,
which may include ripening. 6 The process was probably accidentally discovered in the Middle East
in approximately 7000 b.c. from the coagulation of milk stored in a calf stomach by lactic acid
produced by lactic acid bacteria (and probably rennin in the stomach). At present, many varieties
of cheeses are made worldwide, which probably use more than 20% of the total milk produced.
In the United States, the total production of different varieties of cheese in 1982 was 4.4 billion
pounds (2 billion kg) and, in 1987, this increased to 5.3 billion pounds (2.4 billion kg). Because
of the worldwide increase in cheese consumption, cheese production will continue to increase not
only in the United States, but also in other countries, especially in dairy-rich countries, such as
Europe and New Zealand.
Cheese varieties have been grouped in different ways. Examples of several varieties based on
starter cultures used and some important secondary flora are listed here.
Unripened Cheese
Soft
− Cottage cheese with starters Lac. lactis ssp. lactis and cremoris and Leuconostoc mesenteroides ssp. cremoris.
− Mozzarella cheese with starters Str. thermophilus and Lab. delbrueckii ssp. bulgaricus.
Ripened Cheese
Soft
− Brie cheese with starter Lac. lactis ssp.; Penicillium sp. and yeasts are secondary flora.
Semihard
− Gouda cheese with starters Lac. lactis ssp. and Leuconostoc spp.; dairy Propionibacterium
may be secondary flora.
− Blue cheese with starter Lac. lactis ssp., Leuconostoc spp.; Penicillium roquefortii, yeasts,
and micrococci are secondary flora.
Hard
− Cheddar cheese with starters Lac. lactis ssp.; some lactobacilli and pediococci (and probably enterococci) are secondary (or associative) flora.