Brewers Yeast
Pure Culture Maintenance
Once yeast has been selected, accepted, and fully proven for use in brewing, it is essential that a pure culture (working master culture) is maintained in the laboratory yeast bank for prolonged periods. Some small breweries do not maintain a master culture but rather purchase fresh slants for each in-house propagation cycle or hold stock cultures at independent third party institutions. Some of the more common methods brewers can use for maintaining the purity and characteristics of their yeast strains are sub-culturing, desiccation, lyophilization, and freezing in liquid nitrogen.
Sub-Culturing
Sub-Culturing on Agar Slants
A popular method of sub-culturing involves maintaining the cultures on a medium suitable for yeast growth. Yeast cultures are best kept on agar slopes in 28 ml screw-capped McCartney bottles. Aluminum caps with rubber liners are preferred since bottles fitted with plastic caps suffer from poor survival rates (13).
Sub-culturing on Agar Slants with an Oil Overlay
Alternatively, if the agar slants are overlayed with sterile mineral oil, the known shelf life increases to at least two years. After inoculation and incubation for 3 days at 25°C, the culture is overlaid with a layer of sterile B.P. mineral oil (29).
Dessication
This method uses purified silica gel as a desiccant, or squares of Whatman No. 4 filter paper (29). It is more suitable for strains used by collection curators or for research rather than for industrial strains. The shelf life can be up to 3 to 6 years.
Lyophilization
Lyophilization or freeze-drying is another popular technique among research laboratories and culture collections. Cultures are rapidly frozen followed by drying under vacuum.
Freezing in Liquid Nitrogen
With this method pure cultures are kept in vials and submerged in liquid nitrogen (-196°C), thereby maintaining viability and genetic integrity for tens of years.
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