Benchtop (60 MHz) proton NMR spectroscopy for quantification of 16-O-methylcafestol in lipophilic extracts of ground roast coffee

We present a method for analysing the lipophilic fraction extracted from ground coffee beans using 60 MHz proton ( ¹H) NMR spectroscopy. In addition to the triglycerides from coffee oil, spectral features are seen from a range of secondary metabolites, such as various diterpenes. We demonstrate quantitation of a peak attributed to one such compound, 16-O-methylcafestol (16-OMC), which is of interest as a coffee species marker. It is present in low concentrations (<<50 mg/kg) in Coffea arabica L. (‘Arabica’) beans, but in orders of magnitude greater concentrations in other coffees, in particular the other commercially grown species C. canephora Pierre ex A. Froehner (commonly known as ‘robusta’). A series of coffee extracts spiked with 16-OMC analytical standard are used to establish a calibration, and to estimate 16-OMC concentrations in a range of different coffees (Arabicas and blends with robustas). To validate the method, values obtained are compared with an analogous quantitation method that uses high field (600 MHz) NMR spectroscopy.