Many brewers wonder what difference
bottle conditioning makes in brewing. One aspect of bottle
conditioning is the presence of live yeast its effects on the aging
beer.

Based on an on-line discussion, which I
had with fellow brewers, I designed an experiment where I added a
small amount of live yeast to my Doppelbock when I bottled the
carbonated beer after cold conditioning.

The recipe, which I brewed about 7
month before this sampling, was very similar to the recipe posted on
braukaiser.com. It was brewed with an enhanced double decoction. The
first week of primary fermentation was done at 8 C (46 F) and the 2^nd
week was done at 10 C (50 F). This was followed by a 1 month
maturation at 12-13 C (54-56 F) during which time the beer was racked
off the primary yeast into a Cornelius keg with shortened dip tube
where it was allowed to reach its final gravity of 4.8 Plato. To help
this maturation the beer was kraeusened with WLP830 (German Lager) 2
weeks into this maturation rest even though the primary fermentation
itself was done with WLP833 (Ayinger Lager). The attenuation going
into the following cold conditioning was 73% while the attenuation
limit of the wort was 76.6%. During the maturation rest the beer also
built up natural carbonation.

The beer was cold conditioned for about
1 month and 2 weeks and then bottled straight from cold conditioning
at 1 C (34 F) into chilled bottles. The bottles were not purged with
CO2 prior to filling. Oxygen scavenging caps were used because these
were the only ones I had at hand. 3 of the bottles received about 300
Million cells of an active WLP830 (German Lager) culture. This
yielded about 1 Million cells per ml in these bottles. No sugar or
other fermentables were added since the beer was already carbonated.

After bottling the bottles were stored
in the basement. The ambient temperature started at about 13 C (56 F)
at bottling time and rose to about 17 C (64 F) over the following 3
months. 2 days before sampling the bottles were cooled to 10 C (50
C).

3 month after bottling and about 7
month after brewing I sampled a bottle that was bottles without
additional yeast (non-yeasted sample) and a bottle that was bottled
with additional yeast (yeasted sample) side-by-side. At the time of
sampling I knew which was which but didn't know what to expect from
the yeasted sample.

Aroma:

The non-yeasted sample showed the
typical dark fruit (including black currant) and malt aroma of a
German Doppelbock with a hint of alcohol while the yeasted sample
showed a much more restraint aroma. The malt notes and dark fruit
notes were rather faint. The aroma was more that of the young beer.
There was also a hint of alcohol in its aroma.

Appearance:

Both beers were clear at serving
temperature. The yeasted sample formed a thin, yet dense, yeast layer
on the bottom of the bottle.

Foam stability:

My standard foam stability test is to
take a Koelsch glass, pour the beer down the middle to let it foam up
until the foam reaches the top of the glass. Then the time it takes
for the foam to collapse and show the beer surface is taken. For both
samples it took more than 13 min for that to happen.

Taste:

The taste experience was similar to the
aroma experience. The non-yeasted beer showed a much stronger and
more complex taste while the yeasted sample was more subdued. Both
samples did not exhibit any off-flavors. In both cases the bitterness
was low and did not linger into the finish. The non-yeasted sample
was a bit sweeter which was also reflected in its lower attenuation

Mouthfeel:

Both samples exhibited the same
mouthfeel.

Stats:

	non yeasted beer	yeasted beer
OE	18.0 Plato
attenuation limit	76.6%
yeast addition	none (actual residual yeast count was not assessed)	~ 1 Million/ml
yeast viability at tasing time (methylene blue staining)	n/a	~ 60%
AE	4.7 Plato	4.5 Plato
attenuation	74%	75%
attenuation delta	2.6%	1.6%
pH (beer)	4.50	4.51

The yeasted beer did ferment a little
further since at bottling time a small amount of residual fermentable
sugar was available as can be seen from the attenuation to
attenuation limit difference of 2.6% for the non-yeasted beer.

Conclusion:

The result of this tasting did surprise
me yet supports my thinking that the hallmark flavor of German Bocks
and Doppelbocks is in fact the product of oxidation and staling of
the beer. It is assumed that the yeasted beer sample did not exhibit
that flavor as strongly since the yeast was able to scavenge the
oxygen that had been introduced during the bottling process.

The added yeast did not affect the head
retention negatively in this case. One way it can do this is by
releasing excessive amounts of Proteinase A into the beer which can
break down foam proteins.

The pH was not negatively affected
either which is a sign that there was not an excessive amount of
yeast autolysis or not enough yeast to make a difference.

The idea that big dark beers benefit
from small amounts of post fermentation oxygenation has also been
brought up by fellow home brewer Fred Bonjour and warrants further
investigation into optimal oxygenation rates ad well as aging times.