Spectroscopy: Shining a Light on Coffee

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The art of coffee tasting gets a boost from science as researchers use infrared analyzers to predict quality, detect defects, and uncover fraud.

Spectrometry is a technology that empowers astronomers to probe the far reaches of the universe. But back here on earth, it’s a handy tool for analyzing coffee. The technique is based on the fact that light and other electromagnetic radiation emitted from or reflected by any material displays a specific pattern of wavelengths that is unique to the element or molecule present. A spectrometer can determine the exact chemical composition of a coffee, such as a blend of roasted beans, and even predict its cup quality. 

The advantage of this technique is that it is fast, cheap, and reliable. Only a very small sample is needed — half a gram, for example — and the test is not destructive, according to Veronica Belchior, a licensed Q-grader who is also a scholar of food science at Federal University of Minas Gerais, in the heart of Brazil’s coffeeland.

Another advantage: a spectroscopic analyzer is portable, so it can be taken to a farm, post-harvest processing center, roastery, or cupping event, Belchior said in a talk at the Specialty Coffee Expo this year in Portland, Oregon.

Spectroscopic techniques can be used to analyze coffee in any form: green or roasted whole beans; ground; a blend; instant; and even brewed coffee. It can be used to optimize the roasting process; to classify a coffee’s grade; to differentiate by species and variety; and to verify geographical origin.

The type of spectrometry used in analysis of coffee is based on one of three methods: mid-infrared (IR), near infrared, and Raman spectroscopy.

Roast optimization

Probably the most important and transformational step in processing coffee is roasting, and spectroscopy-based methods are especially beneficial here, Belchior said.

“We can see the physical, chemical, and structural transformation of the beans because of the heat, and the roasting process impacts the quality of the beverage. So, in this case, using spectroscopy techniques to evaluate or to confirm some parameters of the roasting can be useful for the coffee industry.”

For example, the analysis can be used to predict the length of each phase of the roast and when to stop the process. It is even possible to plan an entire roast profile. You can manage a roast according to color, the varietal, or the composition of a blend.

Spectroscopy can also help monitor and manage the change in acidity during the roasting process.  It can be used to create a model that will predict the decrease, increase, and peak of acidity at each stage.

Measuring the roast’s sucrose content, for example, predicts the process since the sugar level declines as the roast advances.  Color too is indicative. Spectroscopy based methods are somewhat less useful in predicting when the roast will start to crack and finish cracking, however.

Sensory attributes

Sensory analysis of specialty coffee is done by trained personnel under the strict protocols of a cupping. The cuppers score and classify a coffee based on its physical attributes and the quality of the beverage. The whole process is time consuming and often lacks reproducibility, Belchior said. Different water or different equipment can affect results.

Spectroscopic methods can be useful to confirm the coffee classification, however, when inconsistencies occur.

One method predicts the sensory characteristics of coffee samples by comparing a sample’s spectroscopic profile to results evaluated using the SCA protocol. Similarly, correlation with known profiles can be used to identify different species, varieties, production regions, drying conditions, post-harvest processes, storage time, roasts, and blends. 

In one study, researchers analyzed 24 samples of ground and roasted coffee in capsules. They used spectroscopy to create models to predict aroma and flavor based on analysis of carbohydrates, proteins, and carboxylic acid.  Acidity was predicted by measuring carboxylic and well as chlorogenic acid. Results were compared to assessment of sensory characteristics by a panel of coffee tasters.

Defects and adulterants

Spectroscopic methods can be applied to detect defective beans as well as adulterants such as corn, barley, coffee husks, wood, rice, and others. “In the case of adulterants, and in the case of the presence of other things that are not coffee, we need to tell people that they are buying something that is not coffee, that is coffee with something else,” Belchior said.

Defective beans are those that are sour, broken, black, moldy, immature, or damaged by insects. All of these defects reduce the quality of the beverage. But once the coffee beans are roasted and ground, it’s not possible to detect the presence of defective beans without spectroscopy-based methods.

Identifying the geographic origin of a sample is possible with spectroscopy. This can be done to detect fraud, as when a seller claims that a coffee originates from a certain region, but actually does not. Spectroscopy also enables scientists to discriminate between arabica, liberica, and robusta beans, as well as hybrids.

Some techniques can detect the moisture content of ground and roasted coffee beans or unroasted coffee. Others can predict the caffeine content in green coffee beans.

Limitations

Spectroscopy-based methods must be conducted by skilled professionals who understand the techniques and the analytic system. Some techniques can score coffees as precisely as Q graders do.

Spectroscopy does not quantify the chemical composition. Instead, it makes a prediction based on differences between samples based on the presence of certain groups of compounds. So, it is not as accurate as chromatography. “But we can detect, confirm, and differentiate coffee by a certain chemical composition. And the chemical composition is important for us to understand and to know, because it differentiates the quality of the beverage as well,” Belchior said.

On the way

Belchior is certain that analysis of coffees using spectroscopy will become part of regular practice in assessment of quality, authenticity, origin, and roast profile in the near future. “With the machinery, the samples, the results, the chemo-metrics, and the confirmation of different information that is available in the coffee industry, we are able to receive more reliable results.”