Picture the scene; A
Scandinavian farm house stands close to a small, Viking village, with
a fresh stream feeding a nearby well.
Barley, the staple
grain harvested during the Viking Age in Scandinavia, has been soaked
in the stream to start the germination process. It is kilned at a
fairly low temperature for the time in the local sauna producing a paler than
average malt, and mixed into water in a large vessel. The starches
convert to sugars before the spent grain is used as feed for the village’s
livestock.
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A log fire burns underneath the kettle, boiling the wort. Juniper berries and herbs are added for flavour
before being fed in to a large vat. The concoction cools overnight
and in the morning, the Clans Jarl stirs the wort with his staff and
gives it life. Because of the Jarls intervention, in the following
few days, the wort transforms to beer and sustains every man, woman and
child in the village.
The staff is a treasured family heirloom, providing beer for the clan
for generations, but why? As well as life, the stir gave the beer a unique taste, a
taste that the villagers had been accustomed to for many years. The
staff must have been revered as magical, or Godly, but in fact it was
not the staff that turned wort in to beer. It was microorganisms
living within the knots and grain of the wood. It would be
another thousand years before science could prove that it was no
magic or work of God that nourished this village, but that it was in
fact Yeast.
What is yeast?
Yeast is the living
component of our beers. It consumes the sugars from our wort,
resulting in the creation of by-products including alcohol and carbon
dioxide. Most commercial breweries filter the yeast out of the
finished product before bottling or kegging the beer, but those brewed at home
retain a small amount of yeast which allows the beer to carbonate
naturally and mature in the bottle. Yeast also contributes to the
taste of a beer. Some brews call for clean, neutral yeast to allow
the malt or hop characters to shine through. In others, like German
Weissbier’s, the yeast is the focal point, making banana and clove
flavours amongst others.
What does yeast need to
successfully ferment our wort?
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Beer at high krausen |
The right environment
is critical for yeast to reproduce and prosper. They need food which
is provided in the form of sugars from our malts. They also need a
clean, sanitary environment with little competition from other
bacteria and oxygen is required to give them a good start. The number
of viable yeast cells relative to the volume of the brew is
important too. The optimal amount required differs between styles and
higher alcohol beers generally require a higher pitching rate than
lower ones.
Another very important
aspect to fermentation is temperature. Too hot and the yeast will
die, too cold and they will hibernate. Generally speaking, for ale
yeast, a steady temperature between 17-21 degrees C is a good range,
personally for most beers; I ferment at 18 degrees C.
Which type can I use?
Yeast is either sold in liquid form, or dry, in small packets with granular looking contents. Dry yeast has been dehydrated in order to increase the shelf life. You can either rehydrate the dry yeast in a little warm water before pitching, or just sprinkle the contents on top of the wort after transferring to the fermenter. Rehydrating will increase the amount of viable, healthy yeast cells, but it does come with an additional risk of contamination. Some argue that the risk outweighs the benefits, and personally I have done both, with little difference in the end product.
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| A Yeast Starter |
Liquid yeast is
normally packaged in a vial which contains a lower cell count than a
dry yeast packet. If you are brewing a low ABV beer then you may
just want to empty the vial directly into the wort. For most beers
however, it is best practice to make a yeast starter. This, in short
is a process where you would make a mini brew with malt extract and
water. You would boil this for 15 minutes to sanitise, allow to cool
to room temperature and then pitch the yeast. In the following few days,
the yeast will ferment the mini brew and grow their numbers to a
level adequate for pitching in to your wort.
Although
the yeast is filtered out of most kegged and bottled beers, cask
conditioned ale still contains live yeast. The cask is often filled
just before the yeast finishes consuming the sugars, meaning that it
finishes the job in the cask. The carbon dioxide produced by this
secondary fermentation is now sealed in the keg and is absorbed back
into the beer to produce a naturally gentle carbonation. When
arriving at the pub, the beer is ‘green’ but matures and
conditions over the following few days or weeks before the finished
product is poured in to your glass.
I hope this post has
helped you understanding of one of the most vital ingredients of
beer. Of course there are many in depth works on the subject, this
just being a short introduction.
