Measuring Bitterness

Home brewers and small commercial brewers use a variety of formulae to determine how much of a particular type of hop should be added to the kettle in order to set the bitterness of the beer to a desired level. They seldom obtain feedback as to whether they reached this goal or not. A trained taster can estimate bitterness to within probably 5 IBU but as most have no way of knowing the bitterness of the beers they are tasting it is difficult or impossible for them to develop this skill.

There are several ways to test the bitterness of beer but the International Method of  ASBC's Method of Analysis (MOA) Beer - 23A (Identical to the EBC's method with the exception of a few details) is probably the most common. As this involves fairly expensive equipment it is beyond the reach of nearly all home brewers and most craft brewers as well but there are laboratories who will do the test for you. Some homebrewers have access to the necessary equipment through work or school activities and watchful home brewers can sometimes find used but serviceable spectrophotometers through university or commercial sales.

It should be noted that the MOA begins with the following:

"Reports of the Subcommittee on Determination of Isohumulones in Beer for 1967 and 1968 indicate that Bitterness Units (BU) as determined in Method A below, express the bitter flavor of beer satisfactorily, regardless of whether the beer was made with fresh or old hops."

To appreciate the implications of 'satisfactorily' we note that the other ASBC methods (Beer-23C and Beer-23D) of bitterness determination assay the iso alpha acids present in the beer. When hops are old or when extracts are used to bitter the beer the results of the assay are less than that given by the International Method. This implies that bitterness is not a simple matter of mg/L iso alpha acid.

The procedure as prescribed by ASBC will be described with the corresponding EBC instructions in brackets if they are different.

  1. Transfer 10 mL of cold carbonated beer to a 50 mL centrifuge tube. Dip the tip of the pipet into octanol and shake off excess. The octanol prevents foam from obscuring the pipet reading.[Decarbonate beer and free it of foam before pipetting 10 mL into a 50 mL centrigfuge tube].
  2. Add 1 mL 3N hydrochloric acid and 20 mL spectrographic grade 2,2,4 Trimethyl-pentane (iso-octane). [Add 0.5 mL 6N hydrochloric acid, 3 glass balls and 20 mL spectrographic grade iso-octane]
  3. Close the centrifuge tube and shake vigorously for 15 minutes.
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The shaking can be done by hand (and the writer even knows the operator of a commercial laboratory that does it that way - he has tremendously developed wrists) but obviously a macine like the one illustrated here makes the analyst's life much easier and insures uniformity of results.

  1. If the phases do not separate centrifuge the tube until they do. [Centrifuge the tube for 3 minutes at 3000 rpm.]
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Depending on what the author assumes to be the protein content of the beer there will be two or three phases in the centrifuge tube after shaking. This illustration shows three: the beer at the bottom, a slush and clear liquid at the top. Sometimes one gets just beer and clear liquid but sometimes one gets just beer and slush. It is in the case where there is no clear liquid at all or only a fraction of a milliliter of liquid that centrifugation is required and it must be very vigorous centrifugation indeed. Another brewer told the writer that the centrifuge must supply so many g's that the bottom of the tube begins to deform but he didn't know the little trick the writer discovered. By 'crushing' the slush one can express the liquid just as would happen if one squeezed ice slush in his hands. If slush is present crush it with a pipet tip before centrifuging. If you do this much milder centrifugation will serve. The writer uses an old clinical centrifuge with success.

  1. While shaking or centrifuging are underway prepare an iso-octane blank by touching a toothpick or micropipet tip which has been dipped in octanol to the inside of a 1 cm quart curvet. Then pipet in 1 - 3 mL  iso-octane depending on how high you must fill cuvets for accurate reading in your spectrophotometer and close (cap) it.
  2. As soon as possible after the completion of shaking or centrifugation pipet clear iso-octane from the centrifuge tube into another 1 cm quartz cuvet and close. Be careful not to transfer any suspended flocs of the slush. The closure prevents evaporation of iso-octane which is beneficial both because the concentration of isohumulone in the cuvet is not increased as octane evaporates and because the interior of the spectrophotometer is not exposed to hydrocarbon fumes.
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  1. Set the photometer to 275 nanometers wavelength, zero the instument with the blank and read the absorption. Multiply the absorption value by 50.
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The instrument from which this screen shot was taken does the multiplication automatically and thus reads directly in IBU (International Bitterness Units).

aj@wetnewf.org© A.J. deLange 2013