Standard Reference Method

Lore & Background

The SRM was adopted in 1950 by the American Society of Brewing Chemists, which recognized the need for an instrument-based measurement of color unburdened by the difficulties of the Lovibond system. The Lovibond system relies on visual comparison of the sample to tinted glass discs and is still in use in many industries including brewing. Beer colors measured in SRM and degrees Lovibond were approximately equal at the time of adoption, but modern analytical methods show that SRM and Lovibond diverge for darker colors.

The SRM number was originally defined as 10 times the absorbance of the beer measured in a 1/2-inch cell with monochromatic light at 430 nanometers. Modern spectrophotometers use 1 cm cuvettes, and application of the Bouguer–Beer–Lambert law shows the multiplier should be 12.7. The 430-nanometer wavelength corresponds to a deep blue (violet) light and was chosen, along with the multiplier, to make values comparable to those determined using the Lovibond system.

Both SRM and EBC systems demand that the beer be free of turbidity prior to measurement. In the SRM, a second measurement is taken at 700 nm; if the absorption at this wavelength is less than 0.039 times the absorption at 430 nm, the beer is considered turbidity free. If not, it is to be filtered or centrifuged. The augmented SRM method uses auxiliary 'deviation coefficients' to pick up the remaining spectral information, necessary for fruit beers and when subtle color differences in malt beers are to be characterized.

Reader's Guide

The Standard Reference Method is significant because it provides a reproducible, instrument-based measurement of beer color that replaced the subjective Lovibond visual comparison system. Its adoption by the American Society of Brewing Chemists in 1950 standardized color measurement across the brewing industry. The SRM value, though a single point in the absorption spectrum, conveys 92% of spectral information, making it remarkably effective for characterizing beer color. The method's legacy includes its relationship to the EBC system, where EBC is approximately twice SRM (EBC = SRM × 1.97). The SRM system also includes a turbidity check at 700 nm to ensure the beer has 'average spectral characteristics.' For darker beers or those exceeding the log linear limit of instruments, dilution is applied. The augmented SRM method extends the system's utility by adding deviation coefficients to capture the remaining spectral information, particularly for fruit beers and subtle malt beer color differences. The article notes that an earlier version of EBC color was based on absorption at 530 nanometers, but a formula for converting between old EBC and SRM should not be used as it is flawed and based on measurements no longer taken.

Did You Know?

• The relationship between SRM and Lovibond (ºL) is SRM = 1.3546 × ºL − 0.76.
• A second measurement at 700 nm is used in SRM to check for turbidity.