Online monitoring of coffee roasting by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS): towards a real-time process control for a consistent roast profile


A real-time automated process control tool for coffee roasting is presented to consistently and accurately achieve a targeted roast degree. It is based on the online monitoring of volatile organic compounds (VOC) in the off-gas of a drum roaster by proton transfer reaction time-of-flight mass spectrometry at a high time (1 Hz) and mass resolution (5,500 m/Δm at full width at half-maximum) and high sensitivity (better than parts per billion by volume). Forty-two roasting experiments were performed with the drum roaster being operated either on a low, medium or high hot-air inlet temperature (= energy input) and the coffee (Arabica from Antigua, Guatemala) being roasted to low, medium or dark roast degrees. A principal component analysis (PCA) discriminated, for each one of the three hot-air inlet temperatures, the roast degree with a resolution of better than ±1 Colorette. The 3D space of the three first principal components was defined based on 23 mass spectral profiles of VOCs and their roast degree at the end point of roasting. This provided a very detailed picture of the evolution of the roasting process and allowed establishment of a predictive model that projects the online-monitored VOC profile of the roaster off-gas in real time onto the PCA space defined by the calibration process and, ultimately, to control the coffee roasting process so as to achieve a target roast degree and a consistent roasting.

Online monitoring of coffee roasting by real-time analysis of the roaster off-gas using PTR-ToF-MS. In a first phase, 42 calibration experiments were conducted at three different roasting temperatures and to three final roast degrees, to generate the 3D space defined by the three first principle components PC1, PC2 and PC3. Inverted triangles mark the dark roast degree, square medium and circle light, respectively. The hot-air inlet temperature is marked as follows: high (black), medium (grey), low (white). The different hot-air inlet temperatures and roast degrees are clearly separated. In a second phase, an online monitored PTR-ToF-MS spectrum of a roasting process was projected onto the 3D space, allowing following in real-time the roasting process and halting the roasting with a precision better that ± 1 Colorette roast degree.

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  1. 1.

    Pendergrast M (2009) Coffee second only to oil? Is coffee really the second largest commodity? Tea Coffee Trade J 181:38–41

    Google Scholar 

  2. 2.

    Schenker S, Perren R, Escher F, Heinemann C, Huber M, Pompizzo R (2002) Impact of roasting conditions on the formation of aroma compounds in coffee beans. J Food Sci 67(1):60–66

    Article  CAS  Google Scholar 

  3. 3.

    Baggenstoss J, Poisson L, Kaegi R, Perren R, Escher F (2008) Coffee roasting and aroma formation: application of different time–temperature conditions. J Agric Food Chem 56(14):5836–5846

    Article  CAS  Google Scholar 

  4. 4.

    Moon JK, Shibamoto T (2009) Role of roasting conditions in the profile of volatile flavor chemicals formed from coffee beans. J Agric Food Chem 57(13):5823–5831

    Article  CAS  Google Scholar 

  5. 5.

    Sivetz M, Desrosier NW (1979) Coffee bean processing. In: Coffee technology. AVI Publishing, Westport, pp 209–278

  6. 6.

    Gutierrez-Osuna R (2002) Pattern analysis for machine olfaction: a review. IEEE Sens J 2(3):189–202

    Article  Google Scholar 

  7. 7.

    Biasioli F, Yeretzian C, Dewulf J, van Langenhove H, Märk T (2011) Direct injection mass spectrometry (DIMS): adding the time dimension to (B)VOC analysis. Trac-Trends Anal Chem 30(7):1003–1017

    Google Scholar 

  8. 8.

    jublot L, Linforth RST, Taylor AJ (2005) Direct atmospheric pressure chemical ionisation ion trap mass spectrometry for aroma analysis: speed, sensitivity and resolution of isobaric compounds. Int J Mass Spec 243(3):269EP–277EP

    Article  Google Scholar 

  9. 9.

    Taylor AH, Linforth RST (2003) Direct mass spectrometry of complex volatile and non-volatile flavour mixtures. Int J Mass Spec 223–224:179–191

    Article  Google Scholar 

  10. 10.

    Taylor AJ (1996) Volatile flavor release from foods during eating. CRC Crit Rev Food Sci Nutr 36(8):765–784

    Article  CAS  Google Scholar 

  11. 11.

    Linforth RST, Savary I, Pattenden B, Taylor AH (1994) Volatile compounds found in expired air during eating of fresh tomatoes and in the headspace above tomatoes. J Sci Food Agric 65:241–247

    Article  CAS  Google Scholar 

  12. 12.

    Linforth RST, Taylor A (1993) Measurement of volatile release in the mouth. Food Chem 48:115–120

    Article  CAS  Google Scholar 

  13. 13.

    Yeretzian C, Jordan A, Brevard H, Lindinger W (2000) Time-resolved headspace analysis by proton-transfer-reaction mass-spectrometry. In: Roberts DD, Taylor AJ (eds) ACS Symposium Series 763. ACS, Washington, pp 58–72

    Google Scholar 

  14. 14.

    Lindinger W, Hansel A, Jordan A (1998) Proton-transfer-reaction mass spectrometry (PTR-MS): on-line monitoring of volatile organic compounds at pptv levels. Chem Soc Rev 27:347–354

    Article  CAS  Google Scholar 

  15. 15.

    Lindinger W, Hansel A, Jordan A (1998) On-line monitoring of volatile organic compounds at pptv levels by means of proton-transfer-reaction mass spectrometry (PTR-MS): medical applications, food control and environmental research. Int J Mass Spectrom Ion Process 173:191–241

    Article  CAS  Google Scholar 

  16. 16.

    Lindinger W, Hansel A (1997) Analysis of trace gases at ppb levels by proton transfer reaction mass spectrometry (PTR-MS). Plasma Sources Sci Technol 6:111–117

    Article  CAS  Google Scholar 

  17. 17.

    Hansel A, Jordan A, Holzinger R, Prazeller P, Vogel W, Lindinger W (1995) Proton transfer reaction mass spectrometry: on-line trace gas analysis at the ppb level. Int J Mass Spectrom Ion Process 149/150:609–619

    Article  CAS  Google Scholar 

  18. 18.

    Lindinger C, Pollien P, Ali S, Yeretzian C, Märk T (2005) Unambiguous identification of volatile organic compounds by proton-transfer reaction mass spectrometry coupled with GC/MS. Anal Chem 77(13):4117–4124

    Article  CAS  Google Scholar 

  19. 19.

    Lindinger C, Labbe D, Pollien P, Rytz A, Juillerat MA, Yeretzian C, Blank I (2008) When machine tastes coffee: instrumental approach to predict the sensory profile of espresso coffee. Anal Chem 80(5):1574–1581

    Article  CAS  Google Scholar 

  20. 20.

    Blake RS, Monks PS, Ellis AM (2009) Proton-transfer reaction mass spectrometry. Chem Rev 109(3):861–896

    Article  CAS  Google Scholar 

  21. 21.

    Hanley L, Zimmermann R (2009) Light and molecular ions: the emergence of vacuum UV single-photon ionization in MS. Anal Chem 81(11):4174–4182

    Article  CAS  Google Scholar 

  22. 22.

    Geissler R, Saraji-Bozorgzad MR, Groêger T, Fendt A, Streibel T, Sklorz M, Krooss BM, Fuhrer K, Gonin M, Kaisersberger E, Denner T, Zimmermann R (2009) Single photon ionization orthogonal acceleration time-of-flight mass spectrometry and resonance enhanced multiphoton ionization time-of-flight mass spectrometry for evolved gas analysis in thermogravimetry: comparative analysis of crude oils. Anal Chem 81(15):6038–6048

    Article  CAS  Google Scholar 

  23. 23.

    Muhlberger F, Hafner K, Kaesdorf S, Ferge T, Zimmermann R (2004) Comprehensive on-line characterization of complex gas mixtures by quasi-simultaneous resonance-enhanced multiphoton ionization, vacuum-UV single-photon ionization, and electron impact ionization in a time-of-flight mass spectrometer: setup and instrument characterization. Anal Chem 76(22):6753–6764

    Article  CAS  Google Scholar 

  24. 24.

    Dorfner R, Ferge T, Yeretzian C, Kettrup A, Zimmermann R (2004) Laser mass spectrometry as on-line sensor for industrial process analysis: process control of coffee roasting. Anal Chem 76(5):1386–1402

    Article  CAS  Google Scholar 

  25. 25.

    Dorfner R, Ferge T, Kettrup A, Zimmermann R, Yeretzian C (2003) Real-time monitoring of 4-vinylguaiacol, guaiacol and phenol during coffee roasting by resonant laser ionisation time-of-flight mass-spectrometry. J Agric Food Chem 51(19):5768–5773

    Article  CAS  Google Scholar 

  26. 26.

    Mühlberger F, Wieser J, Ulrich A, Zimmermann R (2002) Single photon ionization (SPI) via incoherent VUV-excimer light: robust and compact time-of-flight mass spectrometer for on-line, real-time process gas analysis. Anal Chem 74:3790–3801

    Article  Google Scholar 

  27. 27.

    Dorfner R, Ferge T, Uchimura T, Yeretzian C, Zimmermann R, Kettrup A (2002) Laser/chemical ionisation—mass spectrometry as an on-line analysis technique for monitoring the coffee roasting process. ASIC-19eme Colloque Scientifique International sur le Café, ASIC, Paris

  28. 28.

    Zimmermann R, Heger HJ, Kettrup A, Boesl U (1997) A mobile resonance-enhanced multiphoton ionization time-of-flight mass spectrometry device for on-line analysis of aromatic pollutants in waste incinerator flue gases: first results. Rapid Commun Mass Spectrom 11(10):1095–1102

    Article  Google Scholar 

  29. 29.

    Heger HJ, Zimmermann R, Dorfner R, Beckmann M, Griebel H, Kettrup A, Boesl U (1999) On-line emission analysis of polycyclic aromatic hydrocarbons down to pptv concentration levels in the flue gas of an incineration pilot plant with a mobile resonance-enhanced multiphoton ionization time-of-flight mass spectrometer. Anal Chem 71:46–57

    Article  CAS  Google Scholar 

  30. 30.

    Zimmermann R, Heger HJ, Yeretzian C, Nagel H, Boesl U (1996) Application of laser ionization mass spectrometry for on-line monitoring of volatiles in the headspace of food products: roasting and brewing of coffee. Rapid Commun Mass Spectrom 10:1975–1979

    Article  Google Scholar 

  31. 31.

    Yeretzian C, Jordan A, Badoud R, Lindinger W (2002) From the green bean to the cup of coffee: investigating coffee roasting by on-line monitoring of volatiles. Eur Food Res Technol 214:92–104

    Article  CAS  Google Scholar 

  32. 32.

    Yeretzian C, Jordan A, Brevard H, Lindinger W (2000) On-line monitoring of coffee roasting by proton-transfer-reaction mass-spectrometry. In: Roberts DD, Taylor AJ (eds) ACS Symposium Series 763. ACS, Washington, pp 112–123

    Google Scholar 

  33. 33.

    Dorfner R, Zimmermann R, Kettrup A, Yeretzian C, Jordan A, Lindinger W (1999) Vergleich zweier massenspektrometrischer Verfahren zur Direktanalyse in der Lebensmittelchemie. Lebensmittelchemie 53:32–34

    Google Scholar 

  34. 34.

    Friedman JH (1984) A variable span scatterplot smoother. Laboratory for Computational Statistics

  35. 35.

    Esbensen KH, Guyot D, Westad F, Houmøller LP (2002) Multivariate data analysis: in practice: an introduction to multivariate data analysis and experimental design, 5th edn. CAMO Software AS, Oslo

  36. 36.

    Kessler W (2007) Multivariate datenanalyse in der bio- und prozessanalytik. Wiley-VCH, Weinheim

    Google Scholar 

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We acknowledge Bühler AG for financial support.

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Correspondence to Chahan Yeretzian.

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Published in the special issue Analytical Sciences in Switzerland with guest editors P. Dittrich, D. Günther, G. Hopfgartner, and R. Zenobi.

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Wieland, F., Gloess, A.N., Keller, M. et al. Online monitoring of coffee roasting by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS): towards a real-time process control for a consistent roast profile. Anal Bioanal Chem 402, 2531–2543 (2012).

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  • Process analysis
  • Foods
  • Beverages
  • Gas sensors
  • Quality assurance/control
  • Sampling
  • Agriculture