WHAT IS BITTERNESS
BITTERNESS IS A TASTE THAT IS DETECTED MOSTLY BY THE BACK PART OF THE TONGUE. IT IS A SENSATION THAT GROWS MORE PLEASURABLE WITH AGE.
The bitterness sensation appears to be initiated by Magnesium Sulphate, phenolic compounds and Iso Alpha Acid.
The most important compound in beer bitterness is Iso Alpha Acid, which is derived from Hops. However the influence of Ions such as Magnesium should not be forgotten. It is well known that metal Ions can have an effect on perceived bitterness as distinct from measured bitterness. The sensation of bitterness can be modified by other senses. Aroma, in particular, can enhance perceived bitterness.
INTERNATIONAL BITTERNESS UNITS (IBU)
INTERNATIONAL BITTERNESS UNITS OR IBU IS A SYSTEM MODERN BREWERS USE TO MEASURE THE PERCEIVED BITTERNESS OF A BEER.
This is measured by a chemical assay test that requires some fairly scary chemicals and an ultra violet spectrophotometer. The worldwide standard of Bitterness Units (BU) corresponds to parts-per-million (mg/L) of isomerized alpha acid in the finished beer, which corresponds to the perceived bitterness of the beer.
The BU scale is the measurement of the bitterness of a beer. Starting at zero, the scale is unrestricted, but generally beers don’t stray higher than 100 BUs. Anything above this is likely to be too bitter for most of us to taste much difference, and is certainly an acquired taste.
There is a European version (EBU) of the measurement that gives very slightly different numbers. but for the most part these systems can be thought of as the same. Most home brewers and small-scale brewers use a calculated BU number, which may vary considerably from measured numbers. depending on a large number of factors, especially at the higher end for super-hoppy beers like double IPAs and their ilk (greater than 80 BU or so), there is some doubt as to the reliability of the standard assays.
HOW IS IBU MEASURED
Iso Alpha Acid is more soluble in organic solvents than in water (beer is 95% water). This property is used to extract and measure the bittering substances from beer. A sample of beer is made more acidic by the addition of dilute hydrochloric acid. The acid is added because it improves the extraction conditions. The beer is then shaken together with an organic solvent called Iso-Octane. This is done in a special funnel fitted with a tap and a stopper. After a lot of shaking all the bittering substances will move into the Iso-Octane: timing is important for the shaking: 5mins. The funnel is known as a separating funnel.
A separating funnel is a pear-shaped, glassware funnel. It’s designed to permit the separation of liquids that don’t mix. The 2 liquids—Iso-Octane and acidified beer—are shaken together and the dense liquid separates to the bottom. It can then be poured off through the tap. The less dense liquid is left behind.
A spectrophotometer is used to measure the absorbance of the ISO-Octane extract at 275 nm and the result is expressed in Bitterness Units.
The spectrophotometer is an instrument which measures the amount of light of a specified wavelength which passes through a medium. According to Beer’s law, the amount of light absorbed by a medium is proportional to the concentration of the absorbing material or solute present. Thus the concentration of a colored solute in a solution may be determined in the lab by measuring the absorbency of light at a given wavelength. Wavelength (often abbreviated as lambda) is measured in nm. A spectrophotometer allows selection of a wavelength pass through the solution.