survive into the finished beer, i.e. utilized, is termed the "utilization". Under
homebrewing conditions, utilization generally tops out at 30%.
Several factors in the wort boil influence the degree to which isomerization occurs.
Unfortunately how all these factors affect the utilization is complicated and not well
understood. Empirical equations have been developed which give us at least some
ability to estimate IBUs for homebrewing.
The utilization is influenced by the vigor of the boil, the total gravity of the boil, the
time of the boil and several other minor factors. The vigor of the boil can be
considered a constant for each individual brewer, but between brewers there
probably is some variation. The gravity of the boil is significant because the higher
the malt sugar content of a wort, the less room there is for isomerized alpha acids.
The strongest bittering factors are the total amount of alpha acids you added to the
wort, and the amount of time in the boil for isomerization. Understandably then,
most equations for IBUs work with these three variables (gravity, amount, and
time) against a nominal utilization. As mentioned earlier, the utilization for alpha
acids in homebrewing is generally accepted as topping out at about 30%. The
utilization table on the next page lists the utilization versus time and gravity of the
boil. This allows you to estimate how much each hop addition is contributing to the
total bitterness of the beer. By incorporating a factor for gravity adjustment, the
IBU equation allows for direct comparisons of total hop bitterness across beer
styles. For instance, 10 AAUs in a Pale Ale would taste pretty bitter while 10 AAUs
would hardly be noticed in a high gravity Stout. Gravity is not the total difference
between styles however, the yeast also yields a particular flavor and sweetness
profile which the hop bitterness balances against. As the maltiness of the beer
increases, so does the relative balance between hop bitterness and malt sweetness.
A very sweet American Brown Ale needs about 40 IBUs to yield the same balance of
flavor as a Bavarian Oktoberfest of the same gravity does with 30 IBUs.
This brings up a good question, how bitter is bitter? Well, in terms of IBUs, 20 to 40
is considered to be the typical international range. North American light beers, like
Coorsú, have a bitterness of only 10-15 IBUs. More bitter imported light beers, like
Heinekenú, have a bitterness closer to 20-25. American microbrews like Samuel
Adam Boston Lagerú have a bitterness of about 30 IBUs. Strong bitter ales like
Anchor Liberty Aleú and Sierra Nevada Celebration Aleú have bitterness of 45 or
While more experimentation and analysis needs to be done to accurately predict
hop bittering potential, the IBU equations described on the next page have become
the common standard by which most homebrewers calculate the final bitterness in
the beer. Everyone who uses these equations is in the same ballpark and that is
close enough for comparison.
5.5 Hop Bittering Calculations
For those of you who dislike math, I will make this as straightforward as possible.
We will use the following example:
6 lbs. of Amber DME
1.5 oz of 6.4% AA Perle hops (60 minutes)
1 oz of 4.6% AA Liberty hops (15 minutes)
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For a 5 gallon recipe, we will boil 1.5 oz of Perle hops for 60 minutes for Bittering
and 1 oz of Liberty for 15 minutes for Finishing. The recipe calls for 6 lbs. of dry
malt extract and it will be boiled in 3 gallons of water because of the pot size. The
remaining water will be added in the fermenter.
The first step is to calculate the Alpha Acid Units (AAUs).
AAU = Weight (oz) x % Alpha Acids (whole number)
AAU (60) = 1.5 oz x 6.4 = 9.6 AAUs of Perle and AAU (15) = 1 oz x 4.6 = 4.6 AAUs
Whenever a brewer is using AAUs in a recipe to describe the quantity of hops, it is
important to specify how long each addition is boiled. The boiling time has the
largest influence on how bitter a hop addition makes the beer. If no times are
specified, then the rule of thumb is that bittering hops are boiled for an hour and
finishing hops are boiled for the last 10-15 minutes. Many brewers add hops at 15
or 20 minute intervals and usually in multiples of a half ounce (for ease of
To calculate how much bitterness the final beer will have from these hop additions,
we apply factors for the recipe volume (V), gravity of the boil and the boil time. The
time and gravity of the boil are expressed as the utilization (U). The equation for
IBU = AAU x U x 75 / Vrecipe
75 is a constant for the conversion of English units to Metric. The proper units for
IBUs are milligrams per liter, so to convert from ounces per gallon a conversion
factor of 75 (74.89) is needed. For the metric world, using grams and liters, the
factor is 10. (For those of you paying attention to the units, the missing factor of
100 was taken up by the % in the AAU calculation.)
Gravity of the Boil
The recipe volume is 5 gallons. The gravity is figured by examining the amount and
concentration of malt being used. As noted in the previous chapter, dry malt
extract typically yields about 40 pts/lb./gal. Since this recipe calls for 6 lbs. of
extract to be used in 5 gallons, the calculated OG = 6 x 40 / 5 = 48 or 1.048
But, since we are only boiling 3 of the 5 gallons due to of the size of the pot, we
need to take into account the higher gravity of the boil. The boil gravity becomes 6
x 40 / 3 = 80 or 1.080
It is the gravity of the boil (1.080) that is used in figuring the Utilization. As you will
see in the next section, hop utilization decreases with increasing wort gravity. The
higher concentration of sugars makes it more difficult for the isomerized alpha acids
to dissolve. I use the initial boil gravity in my utilization calculation; others have
suggested that the average boil gravity should be used. (The average being a
function of how much volume will be boiled away during the boiling time.) This gets
rather complicated with multiple additions, so I just use the initial boil gravity to be
conservative. The difference is small—overestimating the total bitterness by 1-3
The utilization is the most important factor. This number describes the efficiency of
the isomerization of the alpha acids as a function of time. This is where a lot of
experimentation is being conducted to get a better idea of how much of the hops
are actually being isomerized during the boil. The utilization numbers that Tinseth
published are shown in Table 7. To find the utilizations for boil gravities in-between
the values given, simply interpolate the value based on the numbers for the
bounding gravities at the given time.
For example, to calculate the utilization for a boil gravity of 1.057 at 30 minutes,
look at the utilization values for 1.050 and 1.060. These are .177 and .162,
respectively. There is a difference of 15 between the two, and 7/10ths of the
difference is about 11, so the adjusted utilization for 1.057 would be .177 - .011 =
The Utilizations for 60 minutes and 15 minutes at a Boil Gravity of 1.080 are 0.176
and .087, respectively. Inserting these values into the IBU equations gives:
IBU(60) = 9.6 x .176 x 75 / 5 = 25 (rounded to nearest whole number) and
IBU(15) = 4.6 x .087 x 75 / 5 = 6
Giving a grand total of 31 IBUs.
Table 7 - Utilization as a function of Boil Gravity and Time
Utilization numbers are really an approximation. Each brew is unique; the variables
for individual conditions, i.e. vigor of the boil, wort chemistry, or for losses during
fermentation, are just too hard to get a handle on from the meager amount of
published data available. Then why do we bother, you ask? Because if we are all
working from the same model and using roughly the same numbers, then we will all
be in the same ballpark and can compare our beers without too much error. Plus,
when the actual IBUs are measured in the lab, these models are shown to be pretty
Click here for a nomograph that calculates the IBUs for each addition.
Click here for a metric nomograph.
Hop Utilization Equation Details
For those of you who are comfortable with the math, the following equations were
generated by Tinseth from curve fitting a lot of test data and were used to generate
Table 7. The degree of utilization is composed of a Gravity Factor and a Time
Factor. The gravity factor accounts for reduced utilization due to higher wort
gravities. The boil time factor accounts for the change in utilization due to boil time:
Utilization = f(G) x f(T)
f(G) = 1.65 x 0.000125^(Gb - 1)
f(T) = [1 - e^(-0.04 x T)] / 4.15
The numbers 1.65 and 0.00125 in f(G) were empirically derived to fit the boil
gravity (Gb) analysis data. In the f(T) equation, the number -0.04 controls the
shape of the utilization vs. time curve. The factor 4.15 controls the maximum
utilization value. This number may be adjusted to customize the curves to your own
system. If you feel that you are having a very vigorous boil or generally get more
utilization out of a given boil time for whatever reason, you can reduce the number
a small amount to 4 or 3.9. Likewise if you think that you are getting less, then you
can increase it by 1 or 2 tenths. Doing so will increase or decrease the utilization
value for each time and gravity in Table 7.
Calculating the IBUs for each hop addition will help you to design your own beer
recipes. You will not be a slave to any recipe book but will be able to take any beer
style, any combination of malts, and plan the amount of hops to make it a beer you
know you will like.
Garetz, M., Using Hops: The Complete Guide to Hops for the Craft Brewer
(HopTech, Danville, California, 1994).
Pyle, N., Ed., The Hop FAQ
Tinseth, G., The Hop Page
Tinseth, G., personal communication, 1995.
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Chapter 6 - Yeast
What Is It?
There was a time when the role of yeast in brewing was unknown. In the days of
the Vikings, each family had their own brewing stick that they used for stirring the
wort. These brewing sticks were regarded as family heirlooms because it was the
use of that stick that guaranteed that the beer would turn out right. Obviously,
those sticks retained the family yeast culture. The German Beer Purity Law of 1516
- The Reinheitsgebot, listed the only allowable materials for brewing as malt, hops,
and water. With the discovery of yeast and its function in the late 1860's by Louis
Pasteur, the law had to be amended.
Brewer's Yeast (Saccharomyces cerevisiae) is considered to be a type of fungus. It
reproduces asexually by budding- splitting off little daughter cells. Yeast are
unusual in that they can live and grow both with or without oxygen. Most micro-
organisms can only do one or the other. Yeast can live without oxygen by a process
that we refer to as fermentation. The yeast cells take in simple sugars like glucose
and maltose and produce carbon dioxide and alcohol as waste products.
Along with converting sugar to ethyl alcohol and carbon dioxide, yeast produce
many other compounds, including esters, fusel alcohols, ketones, various phenolics
and fatty acids. Esters are the molecular compound responsible for the fruity notes
in beer, phenols cause the spicy notes, and in combination with chlorine, medicinal
notes. Diacetyl is a ketone compound that can be beneficial in limited amounts. It
gives a butter or butterscotch note to the flavor profile of a beer and is desired to a
degree in heavier Pale Ales, Scotch Ales and Stouts. Unfortunately, Diacetyl tends
to be unstable and can take on stale, raunchy tones due to oxidation as the beer
ages. This is particularly true for light lagers, where the presence of diacetyl is
considered to be a flaw. Fusel alcohols are heavier molecular weight alcohols and
are thought to be a major contributor to hangovers. These alcohols also have low
taste thresholds and are often readily apparent as "sharp" notes. Fatty acids,
although they take part in the chemical reactions that produce the desired
compounds, also tend to oxidize in old beers and produce off-flavors.
6.1 Yeast Terminology
The following are some terms that are used to describe yeast behavior.
Attenuation This term is usually given as a percentage to describe the percent of
malt sugar that is converted by the yeast strain to ethanol and CO2. Most yeast
strains attenuate in the range of 65 - 80%. More specifically, this range is the
"Apparent" attenuation. The apparent attenuation is determined by comparing the
Original and Final gravities of the beer. A 1.040 OG that ferments to a 1.010 FG
would have an apparent attenuation of 75%.
(From FG = OG - (OG x %) => % att. = (OG-FG)/OG)
The "Real" attenuation is less. Pure ethanol has a gravity of about 0.800. If you had
a 1.040 OG beer and got 100% real attenuation, the resulting specific gravity would
be about 0.991 (corresponding to about 5% alcohol by weight). The apparent
attenuation of this beer would be 122%. The apparent attenuation of a yeast strain
will vary depending on the types of sugars in the wort that the yeast is fermenting.
Thus the number quoted for a particular yeast is an average. For purposes of
discussion, apparent attenuation is ranked as low, medium, and high by the
65-70% = Low
71-75% = Medium
76-80% = High
Flocculation This term describes how fast or how well a yeast clumps together and
settles to the bottom of the fermenter after fermentation is complete. Different
yeast strains clump differently and will settle faster or slower. Some yeasts layers
practically "paint" themselves to the bottom of the fermenter while others are ready
to swirl up if you so much as sneeze. Highly flocculant yeasts can sometimes settle
out before the fermentation is finished, leaving higher than normal levels of diacetyl
or even leftover fermentable sugars. Pitching an adequate amount of healthy yeast
is the best solution to this potential problem.
Lag Time This term refers to the amount of time that passes from when the yeast
is pitched to when the airlock really starts bubbling on the fermenter. A long
lagtime (more than 24 hours) indicates that the wort was poorly aerated, not
enough yeast was pitched and/or that the yeast was initially in poor shape.
6.2 Yeast Types
There are two main types of yeast, ale and lager. Ale yeasts 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 about 55°F (12°C), while lager yeasts will happily work at 40°F.
Using certain lager yeasts at ale temperatures 60-70°F (18-20°C) produces a style
of beer that is now termed California Common Beer. Anchor Steam Beer revived
this unique 19th century style.
6.3 Yeast Forms
Yeast come in two main product forms, dry and liquid. (There is also another form,
available as pure cultures on petri dishes or slants, but it is generally used as one
would use liquid yeast.) Dry yeast are select, hardy strains that have been
dehydrated for storability. There are a lot of yeast cells in a typical 7 gram packet.
For best results, it needs to be re-hydrated before it is pitched. For the first-time
brewer, a dry ale yeast is highly recommended.
Dry yeast is convenient for the beginning brewer because the packets provide a lot
of viable yeast cells, they can be stored for extended periods of time and they can
be prepared quickly on brewing day. It is common to use one or two packets (7 -
14 grams) of dried yeast for a typical five gallon batch. This amount of yeast, when
properly re-hydrated, provides enough active yeast cells to ensure a strong
fermentation. Dry yeast can be stored for extended periods (preferably in the
refrigerator) but the packets do degrade with time. This is one of the pitfalls with
brewing from the no-name yeast packets taped to the top of a can of malt extract.
They are probably more than a year old and may not be very viable. It is better to
buy another packet or three of a reputable brewer's yeast that has been kept in the
refrigerator at the brewshop. Some leading and reliable brands of dry yeast are
DCL Yeast, Yeast Labs (marketed by G.W. Kent, produced by Lallemand
Canada), Cooper's, DanStar (produced by Lallemand
), Munton & Fison and Edme.
Dry yeasts are good but the rigor of the dehydration process limits the number of
different ale strains that are available and in the case of dry lager yeast, eliminates
them almost entirely. A few dry lager yeasts do exist, but popular opinion is that
they behave more like ale yeasts than lager. DCL Yeast markets two strains of dry
lager yeast, Saflager S-189 and S-23, though only S-23 is currently available in a
homebrewing size. The recommended fermentation temperature is 48-59°F. I
would advise you to use two packets per 5 gallon batch to be assured of a good
The only thing missing with dry yeast is real individuality, which is where liquid
yeasts come in. Many more different strains of yeast are available in liquid form
than in dry.
Liquid yeast used to come in 50 ml foil pouches, and did not contain as many yeast
cells as in the dry packets. The yeast in these packages needed to be grown in a
starter wort to bring the cell counts up to a more useful level. In the past few
years, larger 175 ml pouches (Wyeast Labs) and ready-to-pitch tubes (White Labs)
have become the most popular forms of liquid yeast packaging and contain enough
viable cells to ferment a five gallon batch.
6.4 Yeast Strains
There are many different strains of brewer's yeast available nowadays and each
strain produces a different flavor profile. Some Belgian strains produce fruity esters
that smell like bananas and cherries, some German strains produce phenols that
smell strongly of cloves. Those two examples are rather special, most yeasts are
not that dominating. But it illustrates how much the choice of yeast can determine
the taste of the beer. In fact, one of the main differences between different beer
styles is the strain of yeast that is used.
Most major breweries generally have their own strain of yeast. These yeast strains
have evolved with the style of beer being made, particularly if that brewery was a
founder of a style, such as Anchor Steam. In fact, yeast readily adapts and evolves
to specific brewery conditions, so two breweries producing the same style of beer
with the same yeast strain will actually have different yeast cultivars that produce
unique beers. Several yeast companies have collected different yeasts from around
the world and offer them to home brewers. Some homebrew supply shops have
done the same, offering their own brands of many different yeasts.
6.4.1 Dry Yeast Strains
As I mentioned earlier, the dry ale yeast strains tend to be fairly similar,
attenuative and clean tasting, performing well for most ale styles. To illustrate with
a very broad brush, there are Australian, British and Canadian strains, each
producing what can be considered that country's style of pale ale. The Australian
type is more woody, the British more fruity, and the Canadian a bit more malty.
Fortunately with international interest in homebrewing growing as it is, dry yeast
strains and variety are improving. Some of my favorites are Nottingham (DanStar),
Whitbread (Yeast Labs), and Cooper's Ale.
Here is an incomplete list of dry yeast strains and their characteristics:
Cooper's Ale (Cooper's)
All-purpose dry ale yeast. It produces a complex woody, fruity beer at warm
temperatures. More heat tolerant than other strains, 65-75¡F; recommended for
summer brewing. Medium attenuation and flocculation.
Edme Ale (Edme Ltd.)
One of the original dry yeast strains, this produces a soft, bready finish. Medium
flocculation and medium-high attenuation. Fermentation range of 62-70°F.
London Ale (Lallemand
Moderate fruitiness suitable for all pale ale styles. Medium-high attenuation and
flocculation. Fermentation range of 64-70°F.
Nottingham Ale (Lallemand
A more neutral ale yeast with lower levels of esters and a crisp, malty finish. Can
be used for lager-type beers at low temperatures. High attenuation and medium-
high flocculation. Fermentation range of 57-70°F.
Munton and Fison Ale (Munton and Fison)
An all purpose ale yeast selected for a long shelf life. A vigorous starter, with
neutral flavors. Medium attenuation and high flocculation. Fermentation range of
Windsor Ale (Lallemand
Produces a full bodied, fruity English ale, but suitable for wheat beers also,
including hefe-weizen. Attenuation and flocculation are medium-low. Fermentation
range of 64-70°F.
Whitbread Ale (Yeast Lab)
An excellent pale ale yeast with a smooth crisp flavor and fruity aroma. Medium
attenuation and high flocculation. Fermentation range of 65-70¡F.
Safale S-04 (DCL Yeast)
A well-known commercial English ale yeast selected for its vigorous character and
high flocculation. This yeast is recommended for a large range of ale styles and is
especially well adapted to cask-conditioned ales.
Recommended temperature range of 64-75°F.
Saflager S-23 (DCL Yeast)
This lager strain is used by several European commercial breweries. This yeast
develops soft estery notes at the recommended temperature range of 48-59°F and
more ale-like characteristics at warmer temperatures. From what I have read, I am
speculating that this is a Kolsch or Alt-type yeast. This strain of yeast will produce a
lager character at 54°F, and homebrewers have reported good results with this
yeast. Given the recommended fermentation temperature range, these yeasts may
not respond well to lagering (extended secondary fermentation at low
temperatures) as described in Chapter 10, and probably should be maintained at
54°F for the duration of the time in the fermenter, approximately 2-3 weeks. I have
not used this yeast myself and cannot say for certain.
6.4.2 Liquid Yeast Strains
There are a lot of liquid yeasts to choose from and in order to keep this simple I will
just describe them by general strain. All of the brands of liquid yeast I can think of
(Wyeast, White Labs, Yeast Culture Kit Co., Yeast Labs, and Brew-Tek), are of very
good quality, and to describe each company offering of a particular strain would be
redundant. This is not to say that all of the cultivars of a type are the same; within
a strain there will be several cultivars that have different characteristics. You will
find that each company's offering will be subtly different due to the conditions
under which it was sampled, stored, and grown. You may find that you definitely
prefer one company's cultivar over another's. Detailed descriptions of each
company's cultivar will be available at your brewshop or on the company's website.
This is an incomplete list because new strains are being added to the market all the
All Purpose Ale Yeasts
American, Californian, or Chico Ale
A very "clean" tasting yeast, less esters than other types of ale yeast. Good for just
about any type of ale. This strain usually derives from that used for Sierra Nevada
Pale Ale. Medium attenuation, medium flocculation. Suggested fermentation
temperature is 68°F.
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