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COFFEE ROASTING

PROGRESS ON COFFEE ROASTING: A PROCESS CONTROL TOOL FOR A CONSISTENT ROAST DEGREE – ROAST AFTER ROAST Chahan Yeretzian, Flurin Wieland & Alexia N. Gloess Zurich University of Applied Science, Institute of Chemistry and Biological Chemistry

Marco Keller, Andreas Wetzel & Stefan Schenker Bühler AG

A real-time automated process control tool for coffee roasting was developed to consistently and accurately achieve a targeted roast degree. It is based on timeresolved on-line monitoring of volatile organic compounds (VOC) in the off-gas of a drum roaster, using Proton-Transfer-Reaction Time-of-Flight Mass-Spectrometry (PTR-TOF-MS). These experiments provide a detailed, real-time picture of the evolution of the roasting process with the aim of controlling the process and consistently achieving a targeted roast degree.

The flavour of a freshly prepared cup of coffee is the final expression and perceptible result of a long chain of transformations which link the seed to the cup. These include agricultural factors such the variety of the plant, the chemistry of the soil, the weather and the altitude at which the coffee is grown. Combined with the way the cherries are picked, further processed and stored, a green bean is obtained that contains all the ingredients necessary for the later development of the typical coffee aroma. Yet, the green beans give no clue as to what they might become once roasted. They have neither the characteristic smell nor the taste of a good cup of coffee. To reveal the typical coffee flavour, coffee has to be roasted. From a scientist’s point of view, roasting is the collection of a large number of heat induced time and temperature dependent physical and chemical transformations. It turns a hard, spongy to bite, green / grassy smelling bean into an intensely fragrant, brittle, and extractable dark brown bean. These changes, and in particular the development of the substances responsible for the smell, taste, and brown colouration, have still not sufficiently been elucidated. Therefore, the commonly used

FIGURE 1 A simplified scheme showing the main classes of volatile compounds of roasted coffee formed from non-volatile precursors in the green beans during roasting

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methods of roasting have largely been found empirically. In Figure 1, an overview of

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COFFEE ROASTING TABLE 1 Coffee was roasted applying three different hot-air inlet temperatures (L, low; M, medium; H, high), each to three different roast degrees (L, light; M, medium; D, dark), yielding a total of nine different time-temperature roasting profiles, each labelled by a two-letter code (first column). For each profile, 3 to 5 roasting trials were performed (# of trials). The total number of trials that were included in the subsequent data analysis (principle component analysis) was 42. Results are reported as mean values at a 95 per cent confidence level. The roast degree was measured in values of Colorette. FOG is the gas flow in the off-gas line. The three last columns give the values T1, T2 and T3 for each roast profile, as measured by the respective temperature sensors (see text)

Code LL LM LD ML MM MD HL HM HD

# of trials 4 5 6 5 3 5 5 5 4

hot-air inlet temperature low low low medium medium medium high high high

roast degree light medium dark light medium dark light medium dark

roasting time / s 1130 ± 25 1199 ± 9 1258 ± 11 831 ± 1 869 ± 14 907 ± 3 546 ± 7 572 ± 8 593 ± 8

roast degree / Colorette 109.9 ± 1.2 90.4 ± 0.7 77.5 ± 1.1 110.3 ± 0.9 91.1 ± 0.6 77.8 ± 1.0 109.5 ± 1.4 91.1 ± 1.2 77.9 ± 0.5

mean FOG / g·s-1 70.4 ± 0.6 68.7 ± 0.5 67.7 ± 0.2 69.6 ± 0.4 70.2 ± 0.5 69.4 ± 0.3 67.6 ± 0.6 67.1 ± 0.5 66.7 ± 0.3

Mean Values at the final time T1 / °C T2 / °C T3 / °C 316.5 ± 0.9 184.6 ± 0.4 179.1 ± 0.5 321.0 ± 1.2 192.2 ± 0.7 185.2 ± 0.7 327.0 ± 1.4 198.9 ± 0.4 189.0 ± 0.3 368.3 ± 0.4 185.7 ± 0.4 183.1 ± 0.5 374.9 ± 1.4 192.7 ± 0.5 188.3 ± 0.2 380.2 ± 0.6 198.3 ± 0.5 191.9 ± 0.2 462.4 ± 1.2 185.9 ± 0.5 185.4 ± 0.4 468.8 ± 1.9 191.4 ± 0.6 188.8 ± 0.4 472.6 ± 0.9 195.4 ± 1.2 191.1 ± 0.7

important chemical transformations leading

process. Indeed, we believe that consistency,

coffee variety and a target flavour profile

from the non-volatile green coffee precursors to

roast after roast, is one of the most important

accomplished, the sign of a good roast-master is

the main classes of volatile compounds formed

criterion of a good roast. Once a specific roasting

the capacity to understand and control the

during roasting are schematically summarised.

process has been established for a given green

relevant raw material and roasting parameters

From a practitioner’s perspective, roasting can be regarded as a skill, which sometimes approaches an art form. It is in essence the search for the perfect time-temperature path for a given green coffee raw material to express its full flavour potential. From an economic perspective, coffee is the second most valuable commodity exported by developing countries – second to crude oil1. With an annual production in 2010 of 133 million bags (60 kg/bag) and an average price of USD 4.45/kg, this amounts to a total value

RoastMaster™20 – uniform and efficient coffee roasting to the core. Now small and medium-sized companies can use cutting-edge roasting technology by Bühler. The RoastMaster™20 comes with a state-of-the-art control system including a touch screen for comfortable parameter optimization and process control. The proven drum roasting technology with variable drum speed and the separate cooling and heating fans assure perfect roasting of every single bean. High quality materials provide excellent operational reliability and make the RoastMaster™20 the ideal solution for all who ask for nothing less than a perfect product for a competitive price.

of worldwide traded coffee in 2010 of USD 35 billion. Coffee is a commodity of high economic importance for coffee producing as

Bühler AG, Coffee, CH-9240 Uzwil, Switzerland, T +41 71 955 11 11, F +41 71 955 35 82 [email protected], www.buhlergroup.com

well as for coffee consuming countries. Most remarkably, coffee has been, next to cotton, the second most performing commodity

RoastMaster™20

of 2010 – which has logged an impressive

PLC control system : Easy-to-use solution with integrated touch panel, parameter memory, temperature trend and safety features.

45 per cent return in 2010. There is no doubt that green coffee is a crop of global, economic importance. But even the best green coffee can be spoiled if not properly roasted. Hence,

Flexible roasting profile for best flavour: Hot-air system with high proportion of convective heat transfer assures uniform roasting of every single bean for highest product quality.

roasting is of crucial economic importance and defines whether the potential in the green beans is expressed in the cup and also materialises in economic terms. It is indispensable to consistently master the roasting process

Reliable drum roasting technology: Variable drum speed for optimal heat transfer and product quality.

and get the most out of an ever increasingly valuable crop, particularly in times of high commodity prices.

Functional and durable machine design : High-quality materials and innovative features for a long lifetime and superior reliability.

In view of the outstanding economic importance of roasting, we have, since 1996, been engaged in developing novel state-of-theart analytical technologies, with the aim of increasing the consistency of the roasting

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COFFEE ROASTING significance to the odour of coffee. In order to monitor detailed, flavour relevant information on the evolution of the coffee roasting process in real-time, one needs methods capable of monitoring VOCs at high time-resolution, with high sensitivity and chemical selectivity in the off-gas of a roaster. Several techniques have been developed to monitor on-line the off-gas during coffee roasting 3-28. In 1996, the first of a series of exploratory studies using different laser ionisation schemes coupled to Time-of-Flight Mass-Spectrometry was published 23,24,29,30. Applying multivariate statistical methods on time-resolved intensity traces of nine volatile coffee compounds, the roasting degree could be roughly traced for a range of roasting temperatures (200-250°C). This study demonstrated the feasibility of on-line control of the coffee roasting process, yet lacked precise and repeated measurements of the VOC profiles by a tight control of the roasting process23. Later, in FIGURE 2 Schematic representation of the experimental strategy to establish an on-line process control for coffee roasting that achieves a consistent roast degree, roast after roast. For each roasting cycle, a series of on- and off-line measurements are performed. The on-line measured values include three temperature profiles; (i) the temperature of the heating gas entering the roasting, (ii) the off-gas temperature and (iii) the temperature inside the roaster. Two flow meters also measure the flow of the heating gas entering the roaster and the off-gas flow. Furthermore, the off-gas of the roaster was measured on-line by PTR-ToF-MS. At the end of each roasting cycle, the GC-MS and sensory profiles of the extracts and the roast degree were measured off-line. Combining on- with off-line date yielded a comprehensive characterisation of each roasting process, and allowed establishing a process control tool for the coffee roasting, as explained in the text

1998, the coffee roasting process was investigated by an emerging, alternative on-line technology, Proton-Transfer-Reaction MassSpectrometry (PTR-MS) 31-33. Here, chemical ionisation of neutral VOCs is achieved by proton transfer from H3O+. The protonated VOC ions are subsequently analysed by a quadruple mass filter.

such as to consistently achieve the same

represent coffee flavour compounds. While

In 2009, an improved version of PTR-MS was

targeted sensory profile.

close to 1,000 VOCs were reported in coffee,

introduced which replaces the Quadropole by a

most are odourless and less than 50 are of

Time-of-Flight MS 34. Triggered by this tech -

As we enter the 21st century, it seems unlikely that the basic technology of applying heat to the beans will fundamentally change. Yet, one important innovation that will most probably affect future generations of roasters is intelligent on-line control tools. In this article, we summarise work from by our group on on-line analysis of the roasting process, and discuss our most recent contribution to the subject. The ultimate objective of our research is to develop on-line technologies for controlling the roasting process so as to achieve highest consistency for the flavour profile of roasted coffee, using real-time process control tools.

On-line analysis of off-gas from the coffee roaster Roast-gas is composed of a complex mixture of gases. These are primarily non-odorous inorganic gases such as CO2, CO, N2 and H2O2, and less than one per cent of the volume of the gases are VOCs, only a small fraction of which

FIGURE 3Temperature profiles during the roasting experiments. T2 represents a convoluted temperature trace between the actual bean temperature and the surrounding hot-air temperature. The brown solid line shows the temperature profile for a high (H) hot air inlet temperature. The dotted red line shows the temperature profile for a medium (M) and the orange one for a low (L) hot air inlet temperature. For each hot air temperature profile, coffee was roasted to a light (L), a medium (M) and a dark (D) roast degree. So e.g. HL, HM and HD represent the end points of the roasting cycles at high hot air inlet temperature, to a light (ML), a medium (MM) and a dark (MD) roast, respectively. All roasting experiments start at 210°C

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COFFEE ROASTING nological innovation, we revisited the on-line

determined in Colorette values, as provided

component analysis (PCA), the PTR-TOF-MS

monitoring of VOCs in the roaster off-gas, now

from a Probat Colorette 3b. Except for the roast

profiles at the end of each of the 42 roasting

using a PTR-TOF-MS35. The PTR-TOF 8000 from

degree, the off-line data (GCMS and sensory) will

experiments with the corresponding off-line

Ionicon Analytik GmbH achieves a detection

not be discussed here and will be the subject of

measured roast degrees. This generated a

limit of better than ppbv for a time resolution of

a forthcoming publication.

3D-space, defined by the three first principle

one second, and a mass resolution of up to 5500

On-line PTR-TOF-MS data: Out of hundreds

m/Δm (FWHM)4,36. Figure 2 (opposite) depicts

of m/z ion signals monitored by PTR-TOF-MS

the analytic approaches implemented in this

during each roasting cycle, traces were

Once the PTR-TOF-MS were calibrated

most recent study.

grouped into 23 different characteristic

against roast degree, on-line recorded

components PC1, PC2 and PC3, as shown in Figure 5 on page 26.

Process monitoring of coffee roasting by PTR-TOF-MS Arabica green coffee beans from Guatemala, from the growing region of Antigua, were used throughout all roasting experiments. Roasting was performed on a Petroncini TT 15/20 drum coffee roaster with a capacity of 15-20 kilograms per batch. Batches of 15.6 kilograms green coffee were roasted at three different hot-air inlet temperatures (high (H), medium (M) and low (L)) and each to three different roasting

FIGURE 4 The m/z time–temperature profiles of two selected lead traces are shown. For each high, medium and low hot air inlet temperatures, the profiles to a light, medium and dark roast degree are essentially overlapping in both cases

degrees (dark (D), medium (M) and light (L)). To monitor each roasting cycle, the roaster was

families using statistical approaches 35. For

PTR-TOF-MS profiles can be projected in real-

fitted with several sensors, as outlined in Figure

each family, a lead trace was selected (often the

time onto the 3D space of the first three

2 opposite. Table 1 on page 23 summarises the

most intense m/z signal in each family)

principle components and the roasting process

characteristic parameters for each roasting

and included in the following PCA analysis.

monitored on-line for the roast degree. This is

process, whereas Figure 3 (opposite) shows the

Figure 4 shows two characteristic m/z lead

shown as a yellow trace for a roasting process at

experimentally measured time-evolutions of

PTR-TOF-MS time-intensity ion traces. They

a medium hot air inlet temperature. The trace

T2 for the various roasting profiles.

demonstrate two examples of very different

initially evolved through a drying phase. Later,

time-intensity behaviours.

an abrupt turn marks the transition to the

As outlined in Figure 2 (opposite), two different types of data were collected during the

In order to develop a predictive model and

exothermic phase, which is characterised by a

roasting cycles. Off-line data: roasted coffee at

ultimately a process control for a consistent

sudden change in the VOC profile. The following

the end of each roasting experiment was

roast degree, the on-line measured time-

phase of the roasting, represented by a straight

analysed off-line by (i) gas-chromatography

intensity profiles of the PTR-TOF-MS signals

trajectory in the 3D-PCA space, reflects the

mass-spectrometry (GCMS), (ii) profiled by a

were calibrated for the roast degree. This was

exothermic phase of the roasting. Then, a

human sensory panel and (iii) the roast degree

accomplished by linking, via a principle

second abrupt change in trajectory reflects the

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Taylor, A. J. Volatile Flavor Release from Foods during Eating. CRC Crit. Rev. Food Sci. Nutr. 1996, 36 (8), 765-784

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Sivetz, M.; Desrosier, N. W. Coffee Bean Processing. In Coffee Technology, AVI Publishing Co.: Westport Conn., 1979; pp 209-278

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Linforth, R. S. T.; Savary, I.; Pattenden, B.; Taylor, A. H. Volatile compounds found in expired air during eating of fresh tomatoes and in the headspace above tomatoes. J. Sci. Food Agric. 1994, 65, 241-247

15. Hansel, A.; Jordan, A.; Holzinger, R.; Prazeller, P.; Vogel, W.; Lindinger, W. Proton transfer reaction mass spectrometry: on-line trace gas analysis at the ppb level. International Journal of Mass Spectrometry and Ion Processes 1995, 149/150, 609-619

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Linforth, R. S. T.; Taylor, A. J. Direct atmospheric pressure chemical ionisation ion trap mass spectrometry for aroma analysis: Speed, sensitivity and resolution of isobaric compoundsA1 – Jublot,Lionel. Int. J. Mass Spec. 2005, 243 (3), 269EP-277

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Taylor, A. H.; Linforth, R. S. T. Direct mass spectrometry of complex volatile and non-volatile flavour mixtures. Int. J. Mass Spec. 2003, 223-224 (15 Jan 2003), 179-191

10. Linforth, R. S. T.; Taylor, A. Measurement of volatile release in the mouth. Food Chem. 1993, 48, 115-120 11. Yeretzian, C.; Jordan, A.; Brevard, H.; Lindinger, W. TimeResolved Headspace Analysis by Proton-TransferReaction Mass-Spectrometry . In ACS Symposium Series 763 ; Roberts, D. D., Taylor, A. J., Eds.; ACS : Washington, DC, 2000; pp 58-72 12. Lindinger, W.; Hansel, A.; Jordan, A. Proton-transferreaction mass spectrometry (PTR-MS): on-line monitoring of volatile organic compounds at pptv levels. Chemical Society Reviews 1998, 27, 347-354 13. Lindinger, W.; Hansel, A.; Jordan, A. On-line monitoring of volatile organic compounds at pptv levels by means of Proton-Transfer-Reaction Mass Spectrometry (PTRMS): Medical applications, food control and environmental research. International Journal of Mass Spectrometry and Ion Processes 1998, 173, 191-241

16. Lindinger, C.; Pollien, P.; Ali, S.; Yeretzian, C.; Märk, T. Unambiguous Identification of Volatile Organic Compounds by Proton-Transfer Reaction Mass Spectrometry Coupled with GC/MS. Anal. Chem. 2005, 77 (13), 4117-4124 17. Lindinger, C.; Pollien, P.; Ali, S.; Yeretzian, C. Analysing On-line the In-Mouth Aroma of Foods by ProtonTransfer-Reaction Mass-Spectrometry. Anal. Chem. 2002, In preparation. 18. Lindinger, C.; Labbe, D.; Pollien, P.; Rytz, A.; Juillerat, M. A.; Yeretzian, C.; Blank, I. When Machine Tastes Coffee: Instrumental Approach To Predict the Sensory Profile of Espresso Coffee. Anal. Chem. 2008, 80 (5), 1574-1581 19. Blake, R. S.; Monks, P. S.; Ellis, A. M. Proton-Transfer Reaction Mass Spectrometry. Chemical Reviews 2009, 109 (3), 861-896 References continued on page 26

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COFFEE ROASTING first pop and a second sudden change in the

calibrated for the roast degree, based on the

roast degree, with a precision better than ± 1

VOC profile. The following and final phase then

calibration points established in the first phase

Colorette roast degree.

brings the beans from an initially light roast

of the experiments. The trajectory moves

This research demonstrates that a time-

towards progressively darker roasts. In this

precisely through the calibration points,

resolved analysis of the VOC profiles in the

phase of the roasting process, the trajectory of

allowing real-time following of the roasting

off-gas of a coffee roaster by PTR-TOF-MS

the PTR-TOF-MS in the 3D PCA space is

process and halting the roasting at a desired

provides a detailed picture of the evolution of the roasting process and allows establishing a real-time process control tool that ensures highest consistency of the roast degree. BIOGRAPHY

Dr. Chahan Yeretzian is Head of the Analytical & Physical Chemistry Group at the Zurich University of Applied Science (ZHAW) in Wädenswil. A major focus of his research is the chemistry and technology coffee. He established the first university degree on coffee (www.icbc.zhaw.ch/coffee), and is board member of the Swiss Chapter of the Speciality Coffee Association of Europe (SCAE). Dr. Yeretzian received his PhD in Chemistry from the University of Bern, Switzerland. A two year post-doctoral position at the University of California, Los Angeles (UCLA), and a subsequent Alexander von Humboldt Junior Award for a two year Fellowship at the Technical University of Munich, complemented his academic education. From 1995 to 2008, he held various R&D management positions at Nestlé. In 2008, he joined the faculty of the Institute of Chemistry and Biological Chemistry at ZHAW.

FIGURE 5The 42 calibration experiments, conducted at three different roasting temperatures and to three final roast degrees generated a 3D space, represented 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)

Dr. Yeretzian is co-author of more than 100 scientific publications and book chapters, co-editor of a book entitled ‘Expression of Multidisciplinary Flavour Science’ and regularly invited by both specialised and mainstream media to contribute articles and interviews around the subject of coffee. [email protected]

REFERENCES continued 20. Hanley, L.; Zimmermann, R. Light and Molecular Ions: The Emergence of Vacuum UV Single-Photon Ionization in MS. Anal. Chem. 2009, 81 (11), 4174-4182 21. Geissler, R.; Saraji-Bozorgzad, M. R.; Gro¦êger, T.; Fendt, A.; Streibel, T.; Sklorz, M.; Krooss, B. M.; Fuhrer, K.; Gonin, M.; Kaisersberger, E.; Denner, T.; Zimmermann, R. Single Photon Ionization Orthogonal Acceleration Time-ofFlight 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. 2009, 81 (15), 6038-6048 22. Muhlberger, F.; Hafner, K.; Kaesdorf, S.; Ferge, T.; Zimmermann, R. Comprehensive on-line charact erization of complex gas mixtures by quasisimultaneous 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. 2004, 76 (22), 6753-6764 23. Dorfner, R.; Ferge, T.; Yeretzian, C.; Kettrup, A.; Zimmermann, R. Laser mass spectrometry as on-line sensor for industrial process analysis: Process control of coffee roasting. Anal. Chem. 2004, 76 (5), 1386-1402 24. Dorfner, R.; Ferge, T.; Kettrup, A.; Zimmermann, R.; Yeretzian, C. 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. 2003, 51 (19), 5768-5773 25. Mühlberger, F.; Wieser, J.; Ulrich, A.; Zimmermann, R. Single Photon Ionization (SPI) via Incoherent VUVExcimer Light: Robust and Compact Time-of-Flight

Mass Spectrometer for On-Line, Real-Time Process Gas Analysis. Anal. Chem. 2002 26. Dorfner, R.; Ferge, T.; Uchimura, T.; Yeretzian, C.; Zimmermann, R.; Kettrup, A. Laser/Chemical Ionisation – Mass Spectrometry as an On-Line Analysis Technique for Monitoring the Coffee Roasting Process. In ASIC19eme Colloque Scientifique International sur le Café ; ASIC: Paris, 2002 27. Zimmermann, R.; Heger, H. J.; Kettrup, A.; Boesl, U. 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. 1997, 11 (10), 1095-1102 28. Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. 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 ResonanceEnhanced Multiphoton Ionization Time-of-Flight Mass Spectrometer. Anal. Chem. 1999, 71, 46-57 29. Zimmermann, R.; Heger, H. J.; Yeretzian, C.; Nagel, H.; Boesl, U. 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. 1996, 10, 1975-1979 30. Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. 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 ResonanceEnhanced Multiphoton Ionization Time-of-Flight Mass

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Spectrometer. Anal. Chem. 1999, 71, 46-57 31. Yeretzian, C.; Jordan, A.; Badoud, R.; Lindinger, W. From the green bean to the cup of coffee: investigating coffee roasting by on-line monitoring of volatiles. Eur. Food Res. Technol. 2002, 214, 92-104 32. Yeretzian, C.; Jordan, A.; Brevard, H.; Lindinger, W. Online Monitoring of Coffee Roasting by Proton-TransferReaction Mass-Spectrometry. In ACS Symposium Series 763; Roberts, D. D., Taylor, A. J., Eds.; ACS : Washington, DC, 2000; pp 112-123 33. Dorfner, R.; Zimmermann, R.; Kettrup, A.; Yeretzian, C.; Jordan, A.; Lindinger, W. Vergleich zweier massenspektrometrischer Verfahren zur Direktanalyse in der Lebensmittelchemie. Lebensmittelchemie 1999, 53, 32-34 34. Jordan, A.; Haidacher, S.; Hanel, G.; Hartungen, E.; Herbig, J.; MSrk, L.; Schottkowsky, R.; Seehauser, H.; Sulzer, P.; MSrk, T. D. An online ultra-high sensitivity Proton-transfer-reaction mass-spectrometer combined with switchable reagent ion capability (PTR+SRIMS). Int. J. Mass Spec. 2009, 286 (1), 32-38 35. Wieland, F.; Gloess, A.; Keller, M.; Wetzel, A.; Schenker, S.; Yeretzian, C. 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. 2011, 401, 1-13 36. Biasioli, F.; Yeretzian, C.; Märk, T. D.; Dewulf, J.; van Langenhove, H. Direct-injection mass spectrometry adds the time dimension to (B)VOC analysis. TrAC Trends in Analytical Chemistry 2011, 30 (7), 1003-1017