Using low-field NMR and solid-state NMR for the analysis of whole coffee samples

Solution nuclear magnetic resonance (NMR) spectroscopy has been used for many years in several studies of coffee. One of the most attractive features of this approach is the high resolution of the obtained spectra, provided the material is properly solubilized in the chosen solvent. However, the need to obtain liquid extracts and/or to solubilize the material is an issue that must be taken into consideration. This difficulty can be avoided if the coffee sample is analysed as a whole, without any extraction of solubilization process. This is possible in the case of NMR experiments conducted using solid-state NMR (SSNMR) methods [1,2] or in low-field NMR (LF-NMR) experiments [3,4]. The main advantage of these methods for the analyses of coffee is the possibility of conducting the experiments with no need of sample preparation or modification; the material (e.g., raw / roasted coffee beans or granules, spent coffee grounds, etc.) can be directly placed into the tube or rotor for the NMR experiment, so as to obtain spectra or relaxation curves representative of the whole sample. An example of this approach involves the use of ¹H LF-NMR experiments to distinguish the contributions due to arabica and robusta varieties present in coffee blends. As the oil content in arabica coffee is higher than in the robusta variety, the initial amplitude of the relaxation curves obtained in conventional Carr-Purcell-Meiboom-Gill (CPMG) experiments is significantly higher for arabica coffee [4]. A careful analysis of the exponential decay curve associated with the oil component (which is the component with the highest T₂ value in each sample) allows then the establishment of a correlation between the amplitude of the NMR signal and the arabica content in coffee blends [4]. These results illustrate the potential of LF-NMR relaxometry for studies aimed at screening and quality control of coffee-based products; given the low cost of the LF-NMR instruments and the non-destructive character of the technique, this method can be of high interest for practical applications in the coffee industry.

References

1. N. Kanai et al., Biosci. Biotechnol. Biochem., 83, 803-809 (2019).
2. J. C. C. Freitas et al., Food Chem., 409, 135317 (2023).
3. M.-L. Mateus et al., J. Food Eng., 81, 572-579 (2007).
4. R. O. Muniz et al., Food Res. Int., 174, 113610 (2023).

Acknowledgments: FAPES, CAPES, CNPq.