Measuring beer color with a micro-spectrometer

By Gary Spingarn, Hamamatsu Corporation (March 2017)

Color measurement is a quality control process in which a highly sensitive spectrometer generates a quantitative spectrum across ultraviolet and visible wavelengths. This is a well-known process in the paint, food, and textile industries used to ensure accurate results and reduce observer perception error.

It can also be used to verify beer type.

Because beer color is a quantifiable characteristic that helps identify ingredients and processes, the American Society of Brewing Chemists (ASBC) used it to create a reliable, accurate method to verify its consistency across different beer batches. Their Standard Reference Method (SRM) quantifies beer color by measuring absorption of attenuated light at 430 nm wavelength passing through 1 cm3 of beer.

Absorption is then multiplied with the SRM constant 12.7 to find the beer's SRM number on a 2 to 40+ scale, with higher numbers indicating darker beers:

It's necessary to obtain original SRM color data using spectrometers in transmission mode, which relies heavily on correct sample placement and geometry to achieve desirable accuracy. But to verify SRM numbers, reflection mode is a viable alternative that enables verification without the meticulous lab setup transmission mode requires. A white LED light is pointed at the beer sample, and the light reflected from the sample hits the spectrometer:

Anshuman Das at the MIT Media Lab and Tata Center for Technology and Design created a wireless process to detect beer type by using the C12666MA micro-spectrometer in reflection mode and sending the data to his mobile app called Biiru:

The beer is set on chrome-plated polished bases. A light source is set above the sample. The spectrometer generates an analog signal, which is converted into digital by the Arduino and Bluetooth board built by Mr. Das. The Biiru app wirelessly receives this data and creates a traditional spectrum.

In reflection geometry, the micro-spectrometer captures the sample's spectrum and looks at the ratios of blues, greens and reds. It measures intensities at 438, 500, 530, 535, 560, 580, and 600 nm wavelengths. The ratios of all these wavelengths with each other, I(438)/I(500), I(438)/I(530), etc., yield an array of 21 combinations on the app:

The app compares the unknown beer to a spectral library of known SRM data. The closest beer match is calculated when the spectrum feeds into the wavelength ratio algorithm:

This operation produces a single array of differences for each beer comparison. The app software takes the mean of each array, with the smallest difference indicating the closest match. For example, if the difference between the mean of the unknown sample is 1.4 for Bud Light and 0.01 for Sam Adams Boston Lager, then the app will identify the sample as Sam Adams:

Smaller and more portable technology not only creates easier ways to perform established applications, it also encourages the development of new ones like this demo or quickly and accurately testing fruit ripeness.

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