What’s Your
O?
ne of the most important contributions of
hops to beer is bitterness. Bitterness pro-
vides a counterpart to the sweetness of the
malt to create a balanced beer. If you’ve
ever made an IPA that turned out more like
a bock, you know that making an accurate
estimate of the amount of bitterness impart-
ed by the hops is paramount to success in
brewing. This article will compare several
methods used to estimate hop bitterness.
The bitterness of hops is derived from
the bitter resins in the yellow lupulin glands.
These resins, or crystalline weak acids, orig-
inally were categorized into alpha-, beta-
and gamma-fractions (De Clerck, 1957).
The alpha- and beta-fractions are collec-
tively known as the soft resins because they
are soluble in hexane. The gamma resin
fraction is now referred to as the hard resin
fraction because it is insoluble in hexane.
The alpha-fraction is composed of a
group of related chemicals called the alpha
acids. Alpha acid, often referred to in litera-
ture simply as humulone, is comprised of
the chemicals humulone, cohumulone,
adhumulone, prehumulone and posthumu-
lone (Fix, 1989). Each variety differs only by
what is present on a side chain of the humu-
lone molecule. The alpha acids will dissolve
in hot wort, up to 250 mg/L at a pH of 5 and
a temperature of 212 degrees F (100 degrees
C). They are not very soluble in beer, with
its lower pH and temperature, and will pre-
cipitate out if their concentration is higher
than 5 mg/L at a pH of 4 and temperature of
32 degrees F (0 degrees C) (Hough et al.,
1982). During the kettle boil, the alpha acids
undergo a molecular rearrangement called
isomerization. The resultant chemicals are
called iso-alpha acids, and there is a corre-
sponding version for each humulone (iso-
humulone, isocohumulone, etc.). The iso-
alpha acids are much more soluble in wort
and beer, and they are the primary source
of bitterness in beer.
The beta-fraction of the hop resins is
composed of the beta acids and many other
chemicals, including the oxidation pro