The potential of an established culture medium, Wallerstein Laboratories' nutrient agar, for the detection of wild yeasts has been evaluated and recommended for microbiological quality control in brewery laboratories. Wild yeast strains, including Saccharomyces species, can be differentiated from strains of Saccharomyces cerevisiae by the colour, form and rate of growth of their colonies on this medium within 2–3 days. The sensitivity of the method is such that one wild yeast can be detected in a Saccharomyces cerevisiae population of the order of 10° cells.

Some of the factors that contribute to the loss of viability of brewery yeast strains during lyophilization (freeze-drying) have been investigated. A lyophilization technique for the maintenance of brewery yeast strains with higher viabilities than those previously reported has been developed. Three lyophilized strains of ale yeast still had a survival rate of 60% after periods of storage of up to three years, while a lager yeast strain maintained a viability of approximately 50% during storage for eighteen months.

Methods employed in the isolation and storage of yeast clones are discussed. 10% wort‐gelatin is preferred as medium for development of giant colonies; at 18° C. good colony growth requires ca. 3 weeks. Owing to wort variations, initial comparisons should be made using the same wort. The incorporation in the medium of 0·1% of diphenyl as fungistatic has no adverse effect on the yeast. Growth records are made photographically and by verbal description.

A limited survey has been carried out on samples of pressed yeast from 10 different breweries, clones isolated from these yeasts being differentiated by their giant colony characteristics. The viability of pressed yeast packed in McCartney bottles is little affected by transportation and by storage under normal conditions for short periods. However, pressed yeast is not suitable for a survey of the type contemplated, and although head yeast shows greater viability it, too, is unsuitable as selective effects may occur in head formation; yeast from suspension during fermentation appears most profitable for this type of work.