hist-brewing: Medieval yeast sharing

Jeff Renner nerenner at umich.edu
Thu Dec 9 07:11:30 PST 1999

> "Sarah Wakely" <sarahbrew at hotmail.com> asked:
>I read
>somewhere that medieval brewers 'shared' their yeasts when they were done
>with them - how did this work? Does fermentation occur naturally if the ale
>is left alone? Is using a modern yeast authentic?

Beer wort will ferment "spontaneously" if left to the vagaries of its
surroundings, but it's a dicey thing, and it will almost certainly end up
sour, as with a Belgian lambic.  It is a matter of conjecture if
many/most/all historic beer was sour.  However, brewers of old were fully
aware of what yeast did (and called it God-is-good), even if they didn't
know what it was, and knew that if they added it to a new batch of beer,
good results were far less chancy.

Ale yeast (tradtional strains, at least) will form a "top crop" of thick,
pasty yeast, which can be remarkably free of other microflora.  This can be
harvested with a spoon.  I have an ESB going right now that is about ready
for this.  The first few days I skimmed the thin brown proteinaceous crud
off the head.  It is now (~66 hours at 10 am Thursday) forming a kind of
pancake of yeast, which will be probably 1/4 thick or more (8 gallon batch)
by this evening.  To harvest, I pat the back of a sanitized serving spoon
onto the yeast and it all sticks, leaving clear beer behind.  I scrape this
into a sanitized jar and go back for more yeast.  It is a clean creamy
color, about as thick as peanut butter, and remarkably pure, at least
judging by the results of repitching - no bad beer.  I will probably get
about 3/4 cup of this very thick yeast.

I have repitched into five or six consecutive batches before I get tired of
the yeast strain, and often share it with other members of the Ann Arbor
Brewers Guild.  As a matter of fact, I got this yeast from another member.
I refrigerate it for up to three or four weeks.  Midieval brewers wouldn't
have kept it but would have shared it or reused it themselves immediately.

Are modern yeasts authentic?  Not in one way - they are pure strains, all
descended from a single cell, but these single cell strains are very old,
typically collected from old (perhaps extinct) breweries.  Old yeasts were
very likely mixed strains.  A very few traditional brewers in England still
use stable mixed strains which seem to keep out contaminants.  I suspect
(as do others) that the near universal adoption of single cell strains has
resulted in less complex flavors.

In the eighteenth century in England, big scale commercial brewing (of
porter) developed, and beer was stored in huge tuns for a year or more to
allow it to become "stale," or sour (also called "hard").  This was blended
with new, or "mild" beer for porter.  Lactobacillus spp. and Acetobacter
spp. evidently produced the sourness, and Brettanomyces spp. contributed to
other prized flavors of stale beer.  These bacteria and yeasts were likely
in the original reptiched yeast, although they may have been in the brewing


Jeff Renner in Ann Arbor, Michigan USA, c/o nerenner at umich.edu
"One never knows, do one?"  Fats Waller, American Musician, 1904-1943. 

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