Starter Cultures and Bacteriophages ◾ 163
quickly. The cells can be exposed to adverse physical and chemical environments (or stresses) that
can reduce survival, growth, and metabolism. 5 Freezing and thawing and freeze-drying and rehydration are known to cause cell damage, leading to cell death and cell injury (Chapter 10). This
can lead to situations wherein a concentrated culture fails to meet the needs of initial viable cell
concentrations (10 6–7 cells/mL or cells/g of raw materials) and the relatively short lag in the fermentation process. Culture producers have been successful in reducing cell damage by using cryoprotectants in the suspending menstrua and freezing the cells rapidly at a very low temperature. Some
of the causes of cell viability loss are thawing and refreezing, thawing for a long time before using,
mixing thawed cultures with or rehydrating dried cultures in concentrated solutions of other constituents (such as curing salt and spice mixtures used in sausage fermentation), and long storage
at –20°C or at higher temperatures. Many of these occur in the product-processing environment
as a result of lack of knowledge of the people handling the cultures. For the best performance of
concentrated cultures, the directions from culture producers need to be strictly followed.
Inhibitors in Raw Materials
Milk can contain either antibiotics, given to the animals to treat some infections (mastitis), or
sanitizers (from the equipment). Meat can contain ingredients used to make fermented sausages,
such as phosphate or nitrite, and sanitizers from equipment. These factors can prevent or reduce
the growth of starters.
Bacteriophages of Lactic Acid Bacteria
The role of bacteriophages (or phages) in starter-culture failure in food fermentation has been recognized for a long time. A short discussion on their life cycle that results in starter failure as well
as some current practices used to overcome phage problems in food fermentation are presented
here. 6–8 Bacteriophages are also used in controlling foodborne pathogens to improve food safety
(see Chapter 38) and to detect foodborne pathogens as a diagnostic tool (see Chapter 42).
Morphology and Characteristics
Bacteriophages are filterable viruses of bacteria widely distributed in the environment, especially
in food fermentation environments. A phage contains several proteins (that make the head, tail,
tail fiber, and contractile sheath) and DNA, which can be linear or circular, double stranded of
approximately 20 to 55 kb in size. The double-stranded DNA molecule is packed in the head,
which can be round or hexagonal (prolate or isometric, respectively).
Several schemes have been proposed at different times to classify the bacteriophages of lactic acid bacteria. Currently, a scheme is used in which the family, group, and morphotype are
classified together. The three families Myoviridae, Siphoviridae, and Podoviridae are included in
three groups, A (with contractile tails), B (with noncontractile long tails), and C (with noncontractile short tails), respectively (Figure 14.2). They can have three morphotypes, namely 1 (with
small isometric heads), 2 (with small prolate heads), and 3 (with large prolate heads), respectively.
Lactococcus lactis ssp. lactis and ssp. cremoris appear to have many wide varieties of phages. They
have been divided into 12 species, of which three are in the Siphoviridae family, and are important because of their virulent nature. The three in the Siphoviridae family are 936-species (all are
virulent with small isometric heads, for example, sk1), c2 species (all are virulent with prolate
heads, for example, c2), and P335 species (have both virulent and temperate phages and small
quickly. The cells can be exposed to adverse physical and chemical environments (or stresses) that
can reduce survival, growth, and metabolism. 5 Freezing and thawing and freeze-drying and rehydration are known to cause cell damage, leading to cell death and cell injury (Chapter 10). This
can lead to situations wherein a concentrated culture fails to meet the needs of initial viable cell
concentrations (10 6–7 cells/mL or cells/g of raw materials) and the relatively short lag in the fermentation process. Culture producers have been successful in reducing cell damage by using cryoprotectants in the suspending menstrua and freezing the cells rapidly at a very low temperature. Some
of the causes of cell viability loss are thawing and refreezing, thawing for a long time before using,
mixing thawed cultures with or rehydrating dried cultures in concentrated solutions of other constituents (such as curing salt and spice mixtures used in sausage fermentation), and long storage
at –20°C or at higher temperatures. Many of these occur in the product-processing environment
as a result of lack of knowledge of the people handling the cultures. For the best performance of
concentrated cultures, the directions from culture producers need to be strictly followed.
Inhibitors in Raw Materials
Milk can contain either antibiotics, given to the animals to treat some infections (mastitis), or
sanitizers (from the equipment). Meat can contain ingredients used to make fermented sausages,
such as phosphate or nitrite, and sanitizers from equipment. These factors can prevent or reduce
the growth of starters.
Bacteriophages of Lactic Acid Bacteria
The role of bacteriophages (or phages) in starter-culture failure in food fermentation has been recognized for a long time. A short discussion on their life cycle that results in starter failure as well
as some current practices used to overcome phage problems in food fermentation are presented
here. 6–8 Bacteriophages are also used in controlling foodborne pathogens to improve food safety
(see Chapter 38) and to detect foodborne pathogens as a diagnostic tool (see Chapter 42).
Morphology and Characteristics
Bacteriophages are filterable viruses of bacteria widely distributed in the environment, especially
in food fermentation environments. A phage contains several proteins (that make the head, tail,
tail fiber, and contractile sheath) and DNA, which can be linear or circular, double stranded of
approximately 20 to 55 kb in size. The double-stranded DNA molecule is packed in the head,
which can be round or hexagonal (prolate or isometric, respectively).
Several schemes have been proposed at different times to classify the bacteriophages of lactic acid bacteria. Currently, a scheme is used in which the family, group, and morphotype are
classified together. The three families Myoviridae, Siphoviridae, and Podoviridae are included in
three groups, A (with contractile tails), B (with noncontractile long tails), and C (with noncontractile short tails), respectively (Figure 14.2). They can have three morphotypes, namely 1 (with
small isometric heads), 2 (with small prolate heads), and 3 (with large prolate heads), respectively.
Lactococcus lactis ssp. lactis and ssp. cremoris appear to have many wide varieties of phages. They
have been divided into 12 species, of which three are in the Siphoviridae family, and are important because of their virulent nature. The three in the Siphoviridae family are 936-species (all are
virulent with small isometric heads, for example, sk1), c2 species (all are virulent with prolate
heads, for example, c2), and P335 species (have both virulent and temperate phages and small