NMR

Testing for robusta in Arabica coffee
in minutes using X-Pulse

Application Note 11

Introduction
Coffee is one of the most widely traded commodities in the world. The
trade is made up of two main varieties, commonly known as Arabica
and robusta, with Arabica accounting for 60-70% of the world market
and robusta most of the rest. Arabica is generally considered to be ofhigher quality, and sells on world commodity markets for about twice
the price of robusta. There is therefore the potential for economic fraud,with unscrupulous traders adding robusta to Arabica and still labelling
the product “100% Arabica”. Analytical methods are therefore needed
to detect the presence of robusta coffee in products labelled as Arabica.
Methods
The official method for testing for the presence of robusta ininterfering peaks from other compounds present in the samples.

Arabica is DIN 10779, which is a complex HPLC method that

Figure 1 illustrates where this peak appears in relation to spectra

takes over 7 hours and involves the use of acetonitrile (a knownof various coffee samples. The peak at 3.16ppm is labelled (i)carcinogen). Faster and easier methods are therefore desirable.

in the figure. The researchers at Quadram showed that theThe HPLC method looks for the presence of a diterpene
compound known as 16-O-methylcafestol (16-OMC), which
is present in both varieties of coffee but only at trace levels in

integrated area around this peak correlates very well with theamount of robusta in a sample of Arabica, as shown in Figure 2,
with R2 of 0.99 and RMSE of 0.6% w/w.

Arabica. The trace levels of 16-OMC in Arabica are not detectedby the HPLC method, so any detection is deemed to be a result of
adulteration of the sample with robusta.
It has long been known that high-field NMR can detect thepresence of 16-OMC in coffee, but the use of high-field NMR forsuch a test is also complicated and expensive. So, researchers atthe Quadram Institute Bioscience (formerly the Institute of FoodResearch) decided to see whether low-field (benchtop) NMR could
be used instead. They used a 60MHz X-Pulse to do this work,and quickly discovered that 16-OMC could be detected at 60MHz
sufficiently well to offer the possibility of a method to detect
adulteration of Arabica by robusta.

Figure 1: NMR spectra of 16-OMC and various coffee samples

A recent surveillance study of retail purchased “100% Arabica”

As mentioned earlier, there are trace amounts of 16-OMC incoffees using this method found that 6 out of 60 samples

Arabica, so allowance must be made for these naturally-

displayed signals commensurate with adulteration at levels ofoccurring levels when trying to assess potential adulteration

3–30% w/w.*with robusta.

The X-Pulse method
16-OMC has a number of clear peaks in the NMR spectrum, butone in particular, at 3.16ppm, is in a region where there are noAlthough the trace amounts vary from sample to sample, they
are generally equivalent to an addition of approximately 1% ofrobusta into Arabica, so this is the lower limit of detection ofadulteration. The complete statistical analysis generated by the
Quadram researchers indicates that Arabicas adulterated withrobusta at the 1% w/w level will be detected in about half of allcases. This rises to 90% at the 2% level, and at the 3% level it isunlikely that any adulterated sample will pass undetected.
Experimental method
To carry out this measurement using X-Pulse, 10g of groundcoffee beans are stirred with 30ml chloroform for 5 minutes,
then filtered and dried in a vortex evaporator for 30 minutes.
The dried extract is then re-dissolved in 800µl of deuterated
chloroform and filtered directly into standard 5mm NMR
tubes. Measurement time in X-Pulse is approximately 40
minutes, making a total preparation and measurement time ofapproximately 90 minutes per sample.
An alternative method is available, dissolving 1g of sample
directly into deuterated chloroform and then filtering into theNMR tube. Total measurement time for this method is about

Figure 2: Calibration of robusta in Arabica, using integrated areaaround 3.16ppm

Acknowledgements

45 minutes per sample, but because the concentration step

Oxford Instruments is grateful for permission from Quadramis missed the detection limit for robusta in Arabica is higher.

Institute Bioscience (QIB) and researchers Kate Kemsley et. al. toSamples containing at least 20% robusta are very likely to be

use their data in the preparation of this Application Note. The

detected, and a substantial proportion of samples

NMR method described is available exclusively through Oxford

containing only 10% robusta will be identified.

Instruments by agreement with QIB.

*Reference: Gunning, Y, Defernez. M. et al. Food Chemistry248 (2018) 52–60

visit nmr.oxinst.com/x-pulse for more information or email magres@oxinst.com

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