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Yeast Propagation
Theoretically yeast can go through a cycle of fermenting, skimming and storage indefinitely. However yeasts are prone to change due to contamination with “wild” yeast or beer spoilage bacteria, or to mutation during one of the millions of cell divisions that take place during each fermentation.
If this is allowed to go unchecked it can cause serious problems in a brewery.
Broughton Ales have their own strain of yeast which originally came from the Vaux Brewery in Sunderland.
Over the intervening 20 years or so, this yeast would have developed and changed to the point where its fermentation characteristics would have altered to a considerable degree. In particular this means a change in the flavour profile of the beer produced using this yeast.
As well as producing alcohol and carbon dioxide yeasts also produce, during fermentation, a whole range of other products including higher alcohols, esters, ketones, aldehydes, acids etc. These all have an effect on the flavour of the final beer.
Each strain of yeast will produce varying amounts of these by-products of fermentation and thus the particular strain becomes very important.
A number of years ago Broughton employed the services of a microbiologist to isolate our particular strain and to cultivate it on agars slopes in an incubator. Subsequently this isolate was sent to the National Collection of yeast cultures where it was freeze dried and is now stored. (ref 338).
Every 20 generations (approx. 20 weeks) we propagate fresh yeast from this supply in our yeast propagator.
This ensures the stability of our strain and helps ensure that the beer is consistent and that the beer made this week taste just like last weeks beer and so on.
Broughton Ales brew a lager, Angel Organic Lager, and this beer is fermented in a slightly different way to the ales. The yeast used is a different species of yeast to that used to ferment ales (Saccharomyces cerevisae for ale and Saccharomyces uvarum for lager).
Being a different type of yeast it produces a different flavour profile during fermentation. It also ferments better from the point of view of flavour at the much lower temperature of 11-12°C which also, of course, means that the fermentation takes up to 14 days compared with 4 or 5 days for ale fermentation. Another factor about lager yeast is that during fermentation it settles to the bottom of the FV (hence the term bottom fermenting yeast as apposed to top fermenting yeast)
Because Broughton only brew Angel lager occasionally the yeast is bought in fresh from a yeast manufacturer each time it is required. This also gets round the problem of recovering yeast from the bottom of the FV.
