When pitchable quantities of "liquid" yeast are provided to a commercial microbrewery by a commercial yeast producer, in what stage of the life of the yeast is the yeast supplied? It is my understanding--please correct me where I am wrong--that commercial yeast producers typically propagate the yeast aerobically with constant infusion of medium at rates that maintain very low glucose concentrations in the culture--conditions that avoid induction of the Crabtree effect of high glucose.

At some point in the propagation, the infusion of the medium is discontinued and the yeast are allowed to consume the remainder of the fermentables and to flocculate. Is this the stage in which the pitchable quantities of yeast are sold to commercial microbreweries? If not, tell us what the yeast producers provide to commercial breweries.

-Fred L. Johnson

RESPONSE:

The method you described is probably used by some commercial yeast producer brewing for liquid brewing yeast and we use a similar approach for our dry yeast production. However at Siebel we produce the yeast upon request and use very well aerated step up propagations for that. For these batch propagations we use regular all malt brewers' wort which is supplemented with extra nutrients, like minerals and vitamins, to ensure optimum growth and high viability and vitality.

When all the sugars are consumed in the last batch propagation a sample is taken for QC and the propagation vessel is cooled to allow the yeast to settle. The media/beer is then decanted to concentrate the yeast. Once all QC tests are finished and the yeast meets all our specifications it is shipped cooled to the customer. This whole process takes up to 2 weeks, which means that the customer will receive the yeast 5 to 6 days after the propagation is finished. That is how long it takes to complete QC and for shipping.

Tobias