The
Yeast, the Starter, his Wort and her Oxygen:
An excerpt from "How To Brew Your First Beer, Rev. D.2" by
John J. Palmer
January 20, 2001
Yeast
There are several aspects to yeast; it is the other major factor in determining
the flavor of the beer. Different yeast strains will produce different beers
when pitched to identical worts. Yeast is available both wet and dry, for Ale
and Lager, et cetera. For the first-time brewer, a dry Ale yeast is highly
recommended. There are several brands available, including Coopers, Edme,
Nottingham, and Red Star. All of these listed will produce good results.
Ale yeast are referred to as top-fermenting because much of the fermentation
action takes place at the top of the fermenter, while Lager yeasts would seem to
prefer the bottom. While many of today's strains like to confound this
generalization, there is one important difference, and that is temperature. Ale
yeasts like warmer temperatures, going dormant below 55F (12C), while Lager
yeasts will happily work at 40F. Using Lager yeast at Ale temperatures 65-70F
(18-20C) produces Steam Beer, or what is now termed California Common Beer.
Anchor Steam Beer (tm) was the founder of this unique style.
Starter
Liquid yeast must be and all yeast should be, pitched to a Starter before
pitching to the beer in the fermenter. Using a starter gives yeast a head start
and prevents weak fermentations from under-pitching. Dry Yeast should be
re-hydrated before pitching. Re-hydrating dry yeast is simple.
1. Put 1 cup of warm (90F, 35C) boiled water into a sterile jar and stir in the
yeast. Cover with Saran Wrap and wait 10 minutes.
2. Stir in one teaspoon of sugar.
3. Cover and place in a warm area out of direct sunlight.
4. After 30 minutes or so the yeast should be actively churning and foaming.
This is now ready to pitch.
Liquid yeast is regarded as superior to Dry yeast because of the refinement of
yeast strains present and little risk of bacterial contamination during
manufacture. Liquid yeast allows for greater tailoring of the beer to a
particular style. However, the amount of yeast in a liquid packet is much less
than the amount in the dry. For best results, it needs a starter. The packet
must be squeezed and warmed to 80F at least two days before brewing. One day
before, it should be pitched to a wort starter made from 1/4 cup of DME and a
pint of water that has been boiled and cooled to 75F (25C). Adding a quarter
teaspoon of yeast nutrient is also advisable. Let this sit in the same warm
place until brewing time the next day. Some foaming or an increase in the white
yeast layer on the bottom should be evident. The Starter process may be repeated
to provide even more yeast to the wort to insure a strong fermentation.
The Wort and her Oxygen
The use of oxygen in brewing is a double-edged sword. The yeast need
oxygen to grow and multiply enough to provide a good fermentation. When the
yeast has first been pitched, whether to the starter or the beer, it first seeks
to reproduce. The yeast makes use of the dissolved oxygen in the wort for this.
Boiling the wort drives out the dissolved oxygen, which is why aeration of some
sort is needed prior to fermentation. The yeast first use up all of the oxygen
in the wort for reproduction, then get down to the business of turning sugar
into alcohol and CO2 as well as processing the other flavor compounds.
On the other hand, if oxygen is introduced while the wort is still hot, the
oxygen will oxidize the wort and the yeast cannot utilize it. This will later
cause oxidation of the beer which gives a wet cardboard taste. The key is
temperature. The generally accepted temperature cutoff for preventing hot wort
oxidation is 80F. In addition, if oxygen is introduced after the fermentation
has started, it will not be utilized by the yeast and will later cause the wet
cardboard or sherry-like flavors.
This is why it is important to cool the wort rapidly to below 80F, to prevent
oxidation, and then aerate it by shaking or whatever to provide the dissolved
oxygen that the yeast need. Cooling rapidly between 90 and 130F is important
because this region is ideal for bacterial growth to establish itself in the
wort.
Most homebrewers use cold water baths around the pot or copper tubing Wort
Chillers to accomplish this cooling in about 20 minutes or less. A rapid chill
also causes the Cold Break material to settle out, which decreases the amount of
protein Chill Haze in the finished beer.
Aeration of the wort can be accomplished several ways: shaking the container,
pouring the wort into the fermenter so it splashes, or even hooking up an
airstone to an aquarium air pump and letting that bubble for an hour. For the
latter method, (which is popular) everything must be sanitized! Otherwise,
Infection City. These instructions recommend shaking the starter and
pouring/shaking the wort.
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