Fundamental Food Microbiology, Third Edition - Fuad Fathir

176 FUNDAMENTAL FOOD MICROBIOLOGY
the phages are more abundant in the processing environment (Figure 13.1). 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 /ml). The cells are harvested by centrifugation and resuspended in
a liquid at a concentration of ca. 10 12 cells/ml. A 360-ml frozen concentrate culture
in the DVS system can be added to 5000-gal milk vat to get the desired initial
concentrations of viable cells (10 6–7 cells/ml). The suspending medium contains
cryoprotective agents to reduce cell death and injury during freezing and thawing.
The cells in a mixture in metal containers are frozen, either by using dry ice–acetone
(–78�C) or liquid nitrogen (–196�C), and stored at that temperature. They are transported
to processors, preferably by air, in Styrofoam boxes with sufficient amounts
of dry ice. A processor stores the containers at –20�C or below and uses it within
the time specified by the culture producer. Just before use, a container is thawed in
warm (45�C) potable water and added to the raw material. The directions for use
of these cultures are supplied by culture producers. 3
To produce freeze-dried concentrates, the liquid cell concentrates are shell frozen
(in thin layers or small droplets) in either dry ice acetone or in liquid nitrogen and
then dried under vacuum. The dried materials are packed in plastic bags under
vacuum and stored at –20�C or at refrigerated temperature. They are transported to
processors for quick delivery, either at ambient temperature or in boxes containing
ice packs. A processor stores the dried cultures at –20�C or in a refrigerator and
uses them within the specified time. Just before use, the dried culture is mixed with
warm water (preferably boiled and cooled or sterile) and added to the raw materials. 3
Culture
Producer
Conventional Cultures Concentrated Cultures
Freeze-dried culture
(small volume, single
strains)
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Food
Processors
Stocks
(single strains)
Ø
Mother culture
(single strains)
Culture
Producer
Stocks(frozen/freezedried)
Inoculate in liquid
media (small volume,
single)
Ø
Inoculate in liquid
media (large volume,
mixed or single)
Ø
Ø
Ø Harvest cells (centri-
Ø
fugation)
Ø
Intermediate cultures
Suspend in protective
(mixed or single)
media
Ø
Ø
Bulk cultures
Freeze (or dry)
(mixed or single)
Ø
Ø Transport to processors
Processing vat ---------------------------------------------------------Ø---------------------
Food
Processing vat
Processors
Figure 13.1 Production steps and use of conventional and concentrated cultures and changes
in culture handling by culture producers and food processors.
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