Starter Cultures and Bacteriophages ◾ 161
A fairly recent advance has been the availability of custom-designed starter cultures to meet
the specific needs of a food processor. An understanding of the genetic basis of some important
desirable traits, as well as traits for phage attack inhibition in starter cultures, has helped produce
designer cultures.
Concentrated Cultures
In controlled fermentation of food, a starter is added to a raw material at a level of approximately
10 6–7 live cells/mL or live cells/g for the fermentation to proceed at the desired rate. In the conventional process, a bulk culture with approximately 10 8–9 cells/mL needs to be inoculated at approximately a 1% level in the raw material. A cheese processor who uses 100,000 gal or more milk per
day needs 1000 gal or more bulk starter daily. This large volume is produced from mother cultures
through several intermediate transfers, involving more handling at the processing facilities and
possible introduction of phages as the phages are more abundant in the processing environment
(Figure 14.1). In contrast, in the production of concentrated culture, most handling is done by culture producers under controlled environmental conditions, thereby minimizing phage problems. The
strains are grown in suitable media to obtain high cell density (some to more than 10 10 cells/mL). The
cells are harvested by centrifugation and resuspended in a liquid at a concentration of approximately 10 12 cells/mL. A 360 mL frozen concentrate culture in the DVS system can be added to
a 5000-gal milk vat to get the desired initial concentrations of viable cells (10 6–7 cells/mL). The
---------------------------------Transport to processors
Freeze (or dry)
Suspend in protective
media (large volume,
Processing vat
- -------------------------------------------------------Concentrated cultures
Conventional cultures
Culture
producer
Freeze-dried culture
(small volume, single
strains)
Culture
producer
Stocks (frozen/freezedried)
--------Food
processors
Stocks
(single strains)
Inoculate in liquid
media (small volume,
single)
Mother culture
(single strains)
Inoculate in liquid
mixed or single)
Harvest cells (centrifugation)
Intermediate cultures
(mixed or single)
media
Bulk cultures
(mixed or single)
--------------------Food
processors
Processing vat
Figure 14.1 Production steps and use of conventional and concentrated cultures and changes
in culture handling by culture producers and food processors.
A fairly recent advance has been the availability of custom-designed starter cultures to meet
the specific needs of a food processor. An understanding of the genetic basis of some important
desirable traits, as well as traits for phage attack inhibition in starter cultures, has helped produce
designer cultures.
Concentrated Cultures
In controlled fermentation of food, a starter is added to a raw material at a level of approximately
10 6–7 live cells/mL or live cells/g for the fermentation to proceed at the desired rate. In the conventional process, a bulk culture with approximately 10 8–9 cells/mL needs to be inoculated at approximately a 1% level in the raw material. A cheese processor who uses 100,000 gal or more milk per
day needs 1000 gal or more bulk starter daily. This large volume is produced from mother cultures
through several intermediate transfers, involving more handling at the processing facilities and
possible introduction of phages as the phages are more abundant in the processing environment
(Figure 14.1). In contrast, in the production of concentrated culture, most handling is done by culture producers under controlled environmental conditions, thereby minimizing phage problems. The
strains are grown in suitable media to obtain high cell density (some to more than 10 10 cells/mL). The
cells are harvested by centrifugation and resuspended in a liquid at a concentration of approximately 10 12 cells/mL. A 360 mL frozen concentrate culture in the DVS system can be added to
a 5000-gal milk vat to get the desired initial concentrations of viable cells (10 6–7 cells/mL). The
---------------------------------Transport to processors
Freeze (or dry)
Suspend in protective
media (large volume,
Processing vat
- -------------------------------------------------------Concentrated cultures
Conventional cultures
Culture
producer
Freeze-dried culture
(small volume, single
strains)
Culture
producer
Stocks (frozen/freezedried)
--------Food
processors
Stocks
(single strains)
Inoculate in liquid
media (small volume,
single)
Mother culture
(single strains)
Inoculate in liquid
mixed or single)
Harvest cells (centrifugation)
Intermediate cultures
(mixed or single)
media
Bulk cultures
(mixed or single)
--------------------Food
processors
Processing vat
Figure 14.1 Production steps and use of conventional and concentrated cultures and changes
in culture handling by culture producers and food processors.