122 ◾ Fundamental Food Microbiology
Streptococcus
Only one species, Streptococcus thermophilus, has been used in dairy fermentation. A change in
designation to Str. salivarius ssp. thermophilus was suggested but not made. They are used in dairy
fermentation. The Gram-positive cells are spherical to ovoid, 0.7–0.9 μm in diameter, and exist in
pairs to long chains (Figure 11.1). The cells grow well at 37°C–40°C but can also grow at 52°C.
They are facultative anaerobes and, in glucose broth, can reduce the pH to 4.0 and produce l(+)lactic acid. They ferment fructose, mannose, and lactose, but generally not galactose and sucrose.
Cells can survive at 60°C for 30 minutes. Their natural habitat is unknown although they are
found in milk.
Leuconostoc
The Gram-positive cells are spherical to lenticular, arranged in pairs or in chains, nonmotile,
nonsporulating, catalase negative, and facultative anaerobes (Figure 11.1). The species grow well
between 20°C and 30°C with a range of 1°C–37°C. Glucose is fermented to d(–)-lactic acid,
CO 2 , ethanol, or acetic acid, with the pH reduced to 4.5–5.0. The species grow in milk but may
not curdle. Also, arginine is not hydrolyzed. Many produce dextran while growing on sucrose.
Citrate is utilized to produce diacetyl and CO 2 . Some species can survive at 60°C for 30 minutes.
Leuconostoc species are found in plants, vegetables, silage, milk and some milk products, and raw
and processed meats.
At present, five species are known: Leuconostoc mesenteroides, Leu. paramesenteroides, Leu. lactis,
Leu. carnosum, and Leu. gelidum. Leu. mesenteroides has three subspecies: subsp. mesenteroides, ssp.
dextranicum, and ssp. cremoris. Leu. mesenteroides ssp. cremoris and Leu. lactis are used in some dairy
and vegetable fermentations. Many of these species, particularly Leu. carnosum and Leu. gelidum,
have been associated with spoilage of refrigerated vacuum-packaged meat products.
Leuconostocs are morphologically heterogenous and may contain genetically diverse groups of
bacteria. Recently, two new genera have been created from it: Weisella and Oenococcus. Oen. oeni is
found in wine and related habitats and is used for malolactic fermentation in wine.
Pediococcus
These cells are spherical and form tetrads, but they can be present in pairs. Single cells or chains
are absent (Figure 11.1). They are Gram-positive, nonmotile, nonsporulating, facultative anaerobes. They grow well between 25°C and 40°C; some species grow at 50°C. They ferment glucose
to l(+)- or dl-lactic acid, some species reducing the pH to 3.6. Depending on the species, they
can ferment sucrose, arabinose, ribose, and xylose. Lactose is not generally fermented, especially
in milk, and milk is not curdled. Some strains may have weak lactose-hydrolyzing capability,
especially in broth containing lactose as a carbohydrate source.
Depending on the species, they are found in plants, vegetables, silage, beer, milk, and fermented vegetables, meats, and fish. The genus has seven to eight species, of which Pediococcus
pentosaceus and Pediococcus acidilactici are used in vegetables, meat, cereal, and other types of fermented foods. They have also been implicated in ripening and flavor production of some cheeses
as secondary cultures. These two species are difficult to differentiate, but compared with Ped. acidilactici, Ped. pentosaceus ferments maltose, does not grow at 50°C, and is killed at 70°C in 5 minutes. Ped. halophilus, used in fermentation of high-salt products, is now classified as Tet. halophilus.
Streptococcus
Only one species, Streptococcus thermophilus, has been used in dairy fermentation. A change in
designation to Str. salivarius ssp. thermophilus was suggested but not made. They are used in dairy
fermentation. The Gram-positive cells are spherical to ovoid, 0.7–0.9 μm in diameter, and exist in
pairs to long chains (Figure 11.1). The cells grow well at 37°C–40°C but can also grow at 52°C.
They are facultative anaerobes and, in glucose broth, can reduce the pH to 4.0 and produce l(+)lactic acid. They ferment fructose, mannose, and lactose, but generally not galactose and sucrose.
Cells can survive at 60°C for 30 minutes. Their natural habitat is unknown although they are
found in milk.
Leuconostoc
The Gram-positive cells are spherical to lenticular, arranged in pairs or in chains, nonmotile,
nonsporulating, catalase negative, and facultative anaerobes (Figure 11.1). The species grow well
between 20°C and 30°C with a range of 1°C–37°C. Glucose is fermented to d(–)-lactic acid,
CO 2 , ethanol, or acetic acid, with the pH reduced to 4.5–5.0. The species grow in milk but may
not curdle. Also, arginine is not hydrolyzed. Many produce dextran while growing on sucrose.
Citrate is utilized to produce diacetyl and CO 2 . Some species can survive at 60°C for 30 minutes.
Leuconostoc species are found in plants, vegetables, silage, milk and some milk products, and raw
and processed meats.
At present, five species are known: Leuconostoc mesenteroides, Leu. paramesenteroides, Leu. lactis,
Leu. carnosum, and Leu. gelidum. Leu. mesenteroides has three subspecies: subsp. mesenteroides, ssp.
dextranicum, and ssp. cremoris. Leu. mesenteroides ssp. cremoris and Leu. lactis are used in some dairy
and vegetable fermentations. Many of these species, particularly Leu. carnosum and Leu. gelidum,
have been associated with spoilage of refrigerated vacuum-packaged meat products.
Leuconostocs are morphologically heterogenous and may contain genetically diverse groups of
bacteria. Recently, two new genera have been created from it: Weisella and Oenococcus. Oen. oeni is
found in wine and related habitats and is used for malolactic fermentation in wine.
Pediococcus
These cells are spherical and form tetrads, but they can be present in pairs. Single cells or chains
are absent (Figure 11.1). They are Gram-positive, nonmotile, nonsporulating, facultative anaerobes. They grow well between 25°C and 40°C; some species grow at 50°C. They ferment glucose
to l(+)- or dl-lactic acid, some species reducing the pH to 3.6. Depending on the species, they
can ferment sucrose, arabinose, ribose, and xylose. Lactose is not generally fermented, especially
in milk, and milk is not curdled. Some strains may have weak lactose-hydrolyzing capability,
especially in broth containing lactose as a carbohydrate source.
Depending on the species, they are found in plants, vegetables, silage, beer, milk, and fermented vegetables, meats, and fish. The genus has seven to eight species, of which Pediococcus
pentosaceus and Pediococcus acidilactici are used in vegetables, meat, cereal, and other types of fermented foods. They have also been implicated in ripening and flavor production of some cheeses
as secondary cultures. These two species are difficult to differentiate, but compared with Ped. acidilactici, Ped. pentosaceus ferments maltose, does not grow at 50°C, and is killed at 70°C in 5 minutes. Ped. halophilus, used in fermentation of high-salt products, is now classified as Tet. halophilus.