When brewing beers of different styles, there is an expectation of what that style should taste, look and smell like. To produce a beer to meet these consumer expectations the brewer’s association has developed style guidelines to conform to when making these beers. Two of the attributes specified in the guidelines are colour and bitterness both of which can be measured with a spectrophotometer. Colour is measured with in standard reference method (SRM) while bitterness is measured as international bittering units (IBU). IBU are a measurement of iso-alpha acids in beer.
Methods for using a spectrophotometer for measuring colour and bitterness are outlined by the American Society of Brewing Chemists (ASBC). Spectrophotometry projects a specific wavelength of light through a sample in a cuvette. Light metres measure the light transmittance through the sample. This transmittance can then be used to quantify an attribute of the sample.
To measure colour, the beer sample first needs to be degassed. If a carbonated sample was used, the CO2 bubbles would change the readings. Darker beers may need to be diluted to allow light transmittance through the sample. This dilution factor is then used in the final calculation. Light at a wavelength of 430 nm is passed through the sample in a 1cm cuvette. The result can then be calculated to provide the SRM of the beer.
The measurement of bitterness requires more sample preparation. Again the sample must be degassed and is then mixed with hydrochloric acid and isooctane. The mixture is the agitated to allow the iso-alpha acids to mix with the isooctane phase. The mixture is then centrifuged, and a sample of the iso-alpha acid containing isooctane phase is drawn for spectrophotometry. The absorbance at 275 nm is measured and then used to calculate IBU.
The spectrophotometry of beer is a rapid and easy method to determine colour and bitterness of beer samples.