Wake up and smell the coffee

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  • Published: Jun 9, 2008
  • Author: Jon Evans
  • Channels: Ion Chromatography
thumbnail image: Wake up and smell the coffee

This may come as something of a surprise, but scientists know next to nothing about an important family of compounds that help give your morning cup of coffee its distinctive look and taste. But that could all change thanks to work recently conducted by a team of Dutch food chemists, who by probing the properties of these compounds has come up with an anion exchange chromatography-based method for isolating them for detailed study.

The family of compounds do at least have a name: melanoidins. They are brown-coloured, nitrogen-containing polymers formed when sugar molecules and amino acids react together at high temperatures and low moisture levels, in a process known as the Maillard reaction. They make up around 25% of the dry weight of coffee, being responsible for its brown colour and much of its taste. They have also been linked to its antioxidant and antibacterial properties.

But that is about all that is known about them, with scientists not yet having worked out the chemical structure of a single melanoidin.

The problem is that the melanoidin family is huge and incredibly varied. The Maillard reaction produces numerous reaction products (not all of which are melanoidins) when a single sugar molecule reacts with a single amino acid. So when coffee beans rich in proteins and numerous different sugars are roasted, the resultant Maillard reactions produce a whole plethora of different melanoidins.

Separating all these melanoidins from the other products of the Maillard reaction, as well as from the many small molecules present in coffee such as caffeine, chlorogenic acids and minerals, has proved to be a challenge. This is especially the case for the smaller, low molecular weight (LMw) melanoidins, which have therefore received less attention than their larger, high molecular weight (HMw) cousins.

To address this oversight, researchers from Wageningen University and the Anglo-Dutch consumer products giant Unilever, led by Wageningen's Henk Schols, used numerous different forms of chromatography to probe the properties of LMw melanoidins.

First off, after removing the HMw molecules from the coffee with membrane dialysis, the researchers used solid phase extraction (SPE) to split the remaining LMw molecules into six different fractions based on their polarity (the degree to which a molecule's positive and negative charges are separated). Their assumption was that melanoidins would generally be less polar than the other small molecules in coffee, meaning that the last three fractions would contain more melanoidins than the first three.

Using a rough-and-ready measurement of the melanoidin concentration in each of the fractions, based on the fact that brown compounds absorb light with a wavelength of 405nm, the researchers discovered that their assumption was probably correct. Confirmation came when they analysed each of the fractions with high performance liquid chromatography. This showed that the peaks representing small molecules such as caffeine found in the first three fractions gradually disappeared in the last three fractions (especially the last two), to be replaced by a broad bump assumed to be melanoidins.

Finally, Schols and his team analysed each of the fractions with anion-exchange chromatography (AEC) to determine whether LMw melanoidins possess a negative charge, as earlier research had revealed that HMw melanoidins do. Although they found that LMw melanoidins do indeed possess a negative charge, it is not as large as in HMw melanoidins. In addition, it varies according to polarity, with more polar melanoidins possessing greater negative charges.

Combining these findings, Schols and his team were able to develop a simple method for isolating LMw melanoidins from coffee. For although the last three fractions contained 64% of the melanoidins in only 15% of the dry weight of the coffee, the first of these three fractions contained a substantial amount of non-melanoidin molecules, especially caffeine.

So the researchers turned to AEC to separate the neutral caffeine molecules from the slightly negatively-charged melanoidins. By then combining this AEC fraction with the last two SPE fractions, Schols and his team found they could produce a 'pure' melanoidin fraction, offering high concentrations of melanoidins for in-depth study.

Such in-depth studies should hopefully reveal more about these elusive family members.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Coffee and roasted coffee beans

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