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Mid-infrared spectroscopy

Watch this video showing how FTIR can be used to glean detailed information on chemical components present in ground roast coffee using coffee beans.
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Coffee is an important, high-value commodity, but it’s vulnerable to fraud across its long supply chain. The Arabica species yields the highest-value beans. These are prized for their smooth, rounded flavour. Beans of the other main species, Robusta, are much cheaper. And as their name suggests, they produce a rougher brewed drink. Premium coffee products are often made from 100% Arabica beans, and are sold at a higher price. This makes fraudulent substitution with Robusta economically attractive, and it can be difficult to detect in a ground roast coffee, or an instant coffee product. In chemical terms, coffee beans are complicated mixtures of 100s of different organic compounds present in concentrations that range from trace amounts up to 10s of percent by weight.
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The main bioactive compound, caffeine, is present in both bean types at around 2 to 4 percent by weight. Coffee beans also contain quite high concentrations of lipids, from 10 to 15 percent by weight, present as triglycerides and free fatty acids. This oily component is important for sensory properties, for carrying flavour compounds, and for mouth feel. Think of the rich, creamy layer on top of an espresso. Here we are analysing ground roast coffee beans with the same ATR method previously used for examining oils. ATR is very versatile. The only requirement is that there is good contact between the sample and the crystal. For solids such as ground coffee, we use a clamp to press the sample down onto the crystal surface.
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The ATR crystal is not damaged by this, as it is made of diamond, one of the hardest materials known. The chemical complexity of coffee is reflected in the mid-infrared spectrum, which contains many overlapping peaks that can be thought of as an intricate chemical fingerprint of the particular sample. Some of the more prominent peaks can be assigned to individual compounds. You might recognise one of these from the spectra of oils, the sharp peak at 1,744 wave numbers, which comes from vibrations of carbonyl bonds in the lipid component of the coffee. This peak is potentially useful for detecting fraud.
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Arabica beans contains significantly more lipids than Robusta, and this can be clearly seen here in a comparison of spectra from the two types. So if an unusually small carbonyl peak is found in a spectrum from ground Arabica coffee, it could be an indication that some substitution with Robusta beans has taken place. The coffees being analysed here are capsule products, and we are looking to distinguish between regular and decaffeinated samples. Caffeine also has a characteristic infrared spectrum, with a pair of very strong features at around 1,650 and 1,700 wave numbers, which are seen clearly in spite of the heavily overlapped spectrum. These features are absent in decaffeinated samples. Other main components of ground roast coffee are carbohydrates and mild reaction products.
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These are a wide range of compounds produced from combinations of carbohydrates and amino acids that are formed during the roasting process. All of these different compounds contribute unique signatures that add together to give a mid-infrared spectrum which is characteristic, not only of the chemical composition of the original beans, but also of the specific way in which these have been processed. So FTIR spectroscopy can offer a flexible means of quality control at various points in the coffee production process, particularly when it is combined with mathematical pattern recognition to deal with the complex, overlap nature of the spectrum.
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Another technique which provides information on molecular composition, but through a very different mechanism from the infrared techniques we’ve considered so far, is nuclear magnetic resonance spectroscopy, or NMR.
This video shows how FTIR can be used to analyse a solid sample – coffee beans – to glean detailed information on a range of chemical components present in ground roast coffee.
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