On-Line Monitoring of Coffee Roasting by Proton-Transfer-Reaction

Chapter 10

On-Line Monitoring of Coffee Roasting by Proton-Transfer-Reaction Mass-Spectrometry 1

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C . Yeretzian , A . Jordan , H. Brevard , and W . Lindinger 1

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N e s t l é Research Center, Vers-Chez-les-Blanc, 1000 Lausanne 26, Switzerland Institut für Ionenphysik, Leopold-Franzens-Universität, Technikerstr. 25, 6020 Innsbruck, Austria

Downloaded by FUDAN UNIV on March 2, 2017 | http://pubs.acs.org Publication Date: September 7, 2000 | doi: 10.1021/bk-2000-0763.ch010

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Volatiles emitted during roasting of coffee beans were monitored on-line by Proton-Transfer-Reaction Mass-Spectrometry. In a first series of experiments, roasting was performed with a large batch of beans. The dynamics of the roasting process - drying of beans in the endothermic phase, transition to the exothermic phase and emission of pyrolysis reaction products - were clearly reflected in the data. In a second experimental series, phenomena occurring at the single bean level were studied. Strong bursts of volatiles were observed that coincided with the popping sounds of beans. These experiments showed, in real-time, the complex processes that take place during roasting.

Coffee is mainly consumed for the pleasurable sensory experience it gives consumers. Its quality is assessed largely on the basis of the aroma (nose) and flavor (mouth) by expert coffee-tasters, and the highest quality beans command a considerable premium. To bring out this delicious, rich and strong flavor requires a tremendous empirical knowledge from cultivators and processors alike. The art of the cultivators consists in producing a green coffee bean, that contains all the ingredients necessary for the development of this flavor. Yet, the green bean itself does not have the characteristic look, smell or taste of a good cup of coffee and gives no clue as to final quality. To bring out this flavor, coffee has to be roasted. Roasters, all over the globe, use their eyes, nose, ears coupled with largely empirical experience to transform an unpalatable seed into a delicious coffee. Though the art of coffee roasting is old, its chemistry is still little understood. It is a time-temperature-dependent process, whereby chemical changes are induced by

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© 2000 American Chemical Society

Roberts and Taylor; Flavor Release ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


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pyrolysis within the coffee beans, together with physical changes in their internal structure. During these transformations, a whole range of different volatile organic compounds (VOCs) is generated, some of which are responsible for the flavor of the coffee beverage. Out of the 900 different VOCs identified in coffee (1), about 30 are believed to be key flavor compounds (2,3,4). The objective was to monitor on-line time-intensity profiles of VOCs emitted during roasting, and thus to relate process parameters to chemical transformations leading to the formation of the typical coffee aroma. For this, we have investigated the headspace (HS) of green (unroasted) coffee, and monitored the release of VOCs during roasting by Proton-Transfer-Reaction Mass-Spectrometry (PTR-MS). Here, we present some preliminary results from this work, and demonstrate the potential of PTR-MS for on-line process monitoring.

Experimental Roasting is inherently a dynamic process, during which important chemical transformations take place. Over the last few years, several on-line HS techniques have been implemented (5,6,7), which are capable of capturing on-line such fast processes. One particularly interesting approach is PTR-MS, which combines soft, sensitive and efficient chemical ionization with mass analysis.

Proton-Transfer-Reaction Mass-Spectrometry (PTR-MS) Technical aspects of PTR-MS have already been discussed in the literature (8,9,10) and elsewhere in this book (C. Yeretzian et al.). Here only a brief overview will be given. PTR-MS is in essence a chemical-ionization mass-spectrometer (Figure 1). Neutral HS-gas is swept with air, continuously injected into a chemical ionization (CI) cell, ionized by proton transfer from H 0 , and mass analyzed. What distinguishes PTR-MS from other traditional CI-approaches is that the generation of the primary H 0 and the chemical ionization of VOCs are individually controlled and spatially and temporally separated processes. This allows (i) maximizing signal intensity by increasing the generation of primary reactant ions, H 0 , in the ion source, (ii) reducing fragmentation and clustering by optimizing the conditions for proton transfer in the drift tube, and (iii) quantifying VOCs. The three key features of PTR-MS can be summarized as follows. First, it is fast. Time dependent variations of HS profiles can be monitored with a timeresolution of better than one second. Second, the volatile compounds do not experience any work-up or thermal stress, and measured mass spectral profiles closely reflect genuine HS distributions. Finally, measured mass spectral intensities can be directly related to absolute HS concentrations, without calibration or use of standards. +

3

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+

3



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Figure 1: Schematic representation of PTR-MS. It consists of three chambers. In the first chamber, nearly pure HjO is generated by electrical discharge in H2O vapor. A small field drives H^O ions through an orifice into the drift tube (chemical ionization chamber), while neutral VOCs are introduced coaxial to the orifice, into the drift tube. VOCs with proton affinities exceeding 166.5 kcal/mol ionize by proton transfer from H^0 and are accelerated out of the drift tube into the mass filter. +

+

+

Emissions from Green Coffee Beans. The emission of volatile compounds from green coffee beans was measured for both Arabica (Bordes-Sto-Domingo) and Robusta (Indonesia). Green beans (30g) were put in a 500 ml glass vial with two openings on the top cover. Through one opening, HS-gas was sampled at a rate of 17 mL/min and replaced by air entering from the other opening. The HS-gas was directly introduced into the drift tube of the PTR-MS and the mass spectrum averaged over 30 minutes. In a separate experiment the HS profile of the empty glass vial was measured and subtracted from the green bean spectrum (background subtraction).

Emission of Volatiles during Roasting. Two separate roasting experiments were performed. One with a larger load of green coffee beans, where beans were convectively heated by a strong flow of hot air,


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and a second setup, where just 6 coffee beans were roasted within a heated vessel and the HS was probed with a moderate flow of air.


Batch Roasting: In a first setup (see Figure 2), 40 grams of Arabica (Columbia) green beans were placed on a mesh inside a roasting vessel, and convectively heated with a strong flow of hot air (more that 500 L/min). The temperature of the air was maintained at 180°C, 185°C or 190°C. First the roaster was equilibrated to the roasting temperature, with hot air flowing through the setup for about 30 minutes. The green beans were then placed inside the roaster and the exhaust air was sampled ar 40 mL/min. This flow was diluted by addition of air at 171 mL/min and of this mixture, 11 mL/min were introduced into the drift tube. Single Bean Roasting: In the second setup, a glass vial acting as a roasting vessel, with two gas-feedthroughs, was placed inside an oven chamber. The oven was heated to 185°C or 195°C, and the temperature maintained throughout the roasting process. Via the gas-feedthroughs, a flow of 1700 mL/min air, preheated to the oven temperature, swept the HS of the roasting vessel. 1% of this air was led into the PTR-MS for on-line monitoring. Once the roasting vessel was equilibrated to the oven temperature, 6 green beans (approx. 1 gram) were placed inside the glass vial. The roasting then proceeded at constant air temperature (isothermal roasting) while the HS air was monitored by PTR-MS.

Figure 2: Setup for on-line analysis of coffee roast gas. In this configuration, 40 grams of coffee beans are roasted convectively with a strong flow of hot air.


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Results and Discussion


Emissions from Green Coffee Beans. The VOCs of green coffee beans have attracted rather limited attention so far, particularly in comparison to the large amount of published data on roasted coffee. This is due to the fact that most typical coffee aroma compounds are formed from non-volatile precursors during roasting, and only rarely could they be traced back to volatiles in the unroasted beans. Yet, in a few cases, flavor and off-flavor compounds have been characterized that appear in the green beans, survive the roasting process, and play a role for the flavor of the coffee beverage. The first comprehensive analysis of VOCs of green coffee beans dates back to 1968 (11). Some 50 compounds were reported, but their relative sensory importance was not investigated. Vitzthum et al. were the first to explicitly combine in 1976 instrumental and sensory methods on green coffee (12). More recently (1995), Holscher and Steinhart compiled a comprehensive literature review and added new results on the HS of green coffee (13). More than 200 compounds were listed in their paper. Finally, Grosch and Czerny reported detailed studies on the potent odorants of green beans, using aroma extract dilution analysis (14,15). A n important driver for research on green coffee flavor comes from the sporadic appearance of off-flavors that affect the cup quality. A baggy off-flavor was linked to the presence of some hydrocarbons in the green beans (16). 2,4,6-Trichloroanisole was found to be responsible for a harsh, phenolic, chemical, musty off-flavor (17,18). A musty/earthy off-flavor was linked to an increase in the concentration of geosmin and 2-methylisoborneol (19), while a bell-pepper, peasy off-note was related to higher concentrations of some alkyl-methoxy-pyrazines (12,14,20). Figure 3 shows a PTR-MS HS profile of Robusta (Indonesia) green beans. Based on a series of collateral PTR-MS experiments (see Ref. 8, and contribution by C. Yeretzian et al. in this book) and published work on green coffee volatiles (13,21,22), the most intense peaks have been tentatively assigned as listed in Table I. Based on this assignment, the most abundant compounds in the HS of green coffee are alcohols (methanol, ethanol and propanol), aldehydes and alkanes. Some organic acids seem also to contribute, although to a lesser extent. Besides Robusta, we also measured HS profiles from green Arabica (Bordes-StoDomingo) coffee. While both profiles show great similarities, some differences are noticed. The Arabica yields methanol and ethanol peaks an order of magnitude stronger. Whether this is related to differences in ripeness, post-harvest treatments or reflects real differences between species, remains to be answered by more systematic studies. Substantial emission of methanol was reported from leaves, and it is believed that vegetation is an important source of atmospheric methanol (23).


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Figure 3: PTR-MS HS profile of Robusta green coffee beans.

Emissions during Roasting. Arabica coffee (Columbia, 40g) was roasted at three different temperatures in the setup shown in Figure 2, and the roast gas was monitored on-line. Sixty masses were simultaneously monitored with a time resolution of about 40 seconds. In Figure 4, time intensity profiles are shown on a logarithmic intensity scale for roasting at 190 °C air temperature. For reasons of clarity, only 9 masses are included. Roasting of coffee beans can roughly be divided into two phases; (i) an endothermic phase during which the water content of the beans is reduced from initially 8-12% to just a few percent, (ii) and an exothermic phase where complex pyrolysis reactions take place in the nearly dry beans, to generate among others the typical coffee flavor compounds. A t the end of the roasting the moisture content of the beans is below 1%. Looking at Figure 4, we observe at 3-5 minutes a strong increase in several ion intensities. Particularly interesting is mass 37 which corresponds to a water cluster of protonated water, ( Η Ο Ή ) · ( Η 0 ) , and reflects the drying process of the beans. Once the beans approach 100°C they start to eliminate water. After the initial sharp increase of water signal, the moisture content in the HS increases steadily but slowly, until 18 minutes. A decrease of the water signal was then observed with a concomitant strong increase at other masses, such as 61 (acetic acid), 73 (butanal, isobutanal, pentane, butanone), 75 (propanoic acid, not shown here), 81 (pyrazine, main fragment from furfuryl alcohol [M+H-H 0] ) and 87 (2-, & 3- methylbutanal, 2,3-butanedione). They proceed through a maximum at 19 minutes, and decrease by nearly one order of magnitude within the next two minutes. +

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118 Table I: Tentative chemical assignment of PTR-MS HS profile of Robusta green coffee beans (Figure 3), based on refs. (13,21,22). The classification of odor intensities + (weak), ++ (strong), +++ (very strong) are from GC-olfactometric evaluations of green bean extracts in ref. (13).


Mass +l(V 33 37 45 47 51 59 61 65 73 75 87 89 97 99 101 103 107 113 115 121 127 141 143

Compound Methanol (H 0 )(H 0) Acetaldehyde Ethanol Methanol(H 0) Acetone Propanol Ethanol(H 0) Isobutanal Butanone Propanoic acid Isobutanol 3-Methyl-2-buten-l-ol 3-Methyl butanal Isobutanoic acid 2-, & 3-Methylbutanol 2E,4E-Hexadienal 2E-Hexenal Hexanal 3-Methyl butanoic acid Benzaldehyde 2E-Heptenal Heptanal Phenylacetaldehyde l-Octen-3-one 2-Nonenal Nonanal

Odor Intensity

+

3

2

2

Propanal

2

Butanal Butanol 2E-Butenoic acid 2,3-Butanedione Pentanol 2-Pentanol

2,3 Pentanedione + +++

2E-Octenal

+++ + +++ ++

At a few masses we observe formation of clusters between V O C H + and H2O. Besides, ( H 3 0 ) ( H 2 0 ) which has a rather high intensity, due to the strong H 3 0 ion signal, the other cluster peaks are quite low, showing intensities of only about 1% of the intensity of the non-clustered protonated parent peak. +

+

(*) The indicated masses correspond to the protonated molecular masses as they are detected by P T R - M S .

From the literature it is known that the acetic acid is most abundant at medium roast level (24). Another interesting time-intensity pattern is seen at mass 33 (methanol). After reaching a maximum after about 9 minutes, the HS density of



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methanol steadily decreases. Methanol is found in high concentration in the HS of green coffee (see Figure 3), and evaporates off the beans. The change in ion intensity with time shown in Figure 4 clearly documents the two phases of roasting. A t the end of the experiment (33 minutes), the beans reached a very dark roast state. To investigate the effect of the roast-gas temperature on the roast process, we roasted coffee beans at three different temperatures (180, 185, 190°C), but under otherwise identical conditions. In Figure 5, we see that an increase by only 10°C shifts the maximum (medium roast level) from 30 minutes to 19 minutes! Clearly, small differences in temperature have a major effect on the roast process. We also observe a sharpening of the ion-intensity profiles around the maximum. Considering that roast-times of 6-10 minutes are common pratice in industry, we begin to understand how delicate the job of an operator is. On-line monitoring of roast gas could be critical to ensure stable and reproducible roast quality. This is particularly the case when the temperature and the moisture content of the roaster air fluctuates during a day and from day to day or the moisture level of the beans varies from batch to batch.

Figure 4: Ion-intensity profiles for seven masses, monitored during the roasting of 40 gram Arabica (Columbia) coffee beans at 190°C Tentative chemical assignments are also included.


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M75: methyl acetate M81: pyridine M87: 2-methylbutanal 3-methylbutanal 2,3-butanedione

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time [min] Figure 5: Ion-intensity profiles for volatile compounds during the roasting of Arabica (Columbia) coffee beans at 180°C and J90°C The first roasting setup was designed to monitor an averaged HS over a large number of beans (batch roasting). Yet, considering that the "elementary units" of coffee roasting are the individual beans, it is crucial to also investigate the progress at the individual bean level, rather than observing statistically averaged phenomena. For this we have designed a second setup, in which only 6 beans were roasted and a smaller stream of gas was used to sweep the HS gas (see Experimental: Single Bean


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Robusta and Arabica beans were roasted at 185°C and 195°C and ion intensity profiles of 60 masses were simultaneously monitored, with a time resolution of 40 seconds. Figure 6 shows nine ion-intensity traces for the roasting of Arabica at 185°C. As in the case of the batch roasting, we obeserved a sharp increase of the methanol signal over more than two orders of magnitude, shortly after the beans were heated. Superimposed on this smooth curve, smaller peaks can be seen. These peaks are also seen at other masses, coincident in time with the methanol peaks. The most intense appears at mass 73. At exactly the same time these peaks appeared, we could hear popping sounds, indicating explosions of single beans due to high internal gas pressures. During the roasting process, large amounts of C0 are formedfroma variety of reactions. Much of this C0 is entrapped inside the cellular structure of the beans. The internal pressure that is built up within these closed cavities can reach 25 bar before the pressure is released by cracking. The sounds of these poppings can be clearly heard during the roasting process. At each of these individual popping, C0 is released into the HS, together with volatile compounds that have been accumulated in these cavities. As can be seenfromFigure 6, the intensities of some masses, such as 75, 80, 81 or 111 (not shown here) do not seem to be affected by these poppings. This suggests two possible explanations. Either, the physical properties of the compounds may determine whether they are released during popping or whether they remain bound to cellular structures. Or compounds are generated/released in physically distinct areas within the cell structure that are differently affected by the poppings. Roasting).

2


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m61: acetic acid m69: furan m73: butanal isobutanal pentane butanone

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-X-

m83: methyl furan

1.E+00

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time [min] Figure 6: Ion-intensity profiles for volatile compounds during the roasting of six Arabica beans at 185°C. The experimental conditions were chosen such that emissions from single beans could be observed.


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Conclusions Volatile organic compounds, emitted during the roasting of coffee beans, were monitored on-line by Proton-Transfer-Reaction Mass-Spectrometry (PTR-MS). In a first series of experiments 40 grams of green coffee beans were roasted at 180°C, 185°C and 190°C. Monitoring the time-intensity profiles for a large number of masses, the different stages of the roasting process and their transitions could be observed in great detail. In a second series of experiments, we roasted just six beans to investigate phenomena occurring at the single bean level. We observed, in coincidence with popping sound from the beans, sharp bursts of some volatiles. These on-line observations of the coffee roasting process, via ion-intensity profiles of volatile organic compounds, have given us a direct insight into the complex chemical transformations occurring within the beans. Furthermore, they demonstrate the potential of PTR-MS for on-line process monitoring and eventually pave the way for a rational control of fast industrial processes, via off-gas analysis.

Acknowledgements: We would like to thank A . Hansel for fruitful discussions. This project was supported by the "Fonds zur Fôrderung der wissenschaftlichen Forschung" under project Ρ 12022.

References 1.

Nijssen, L . M.; Visscher, C. Α.; Maarse, H . ; Willemsens L . C. In Volatile Compounds in Food; 7 edition, 1996, T N O Nutrition and Food Research Institute. Semmelroch, P.; Grosch, W. J. Agric. Food Chem. 1996, 44, 537-543. Semmelroch, P.; Grosch, W. Lebensm.-Wiss. u.-Technol. 1995, 28, 310. Czerny, M.; Mayer, F.; Grosch W. J. Agric. Food Chem. 1999, 47, 695. Linforth, R. S. T.; Ingham, Κ. E ; Taylor, A . J. In Flavour Science: Recent Developments; Taylor, A. J.; Mottram, D. S., Eds.; Special Publication No. 197, Royal Society of Chemistry, 1996, Thomas Graham House, Cambridge. Zimmermann, R.; Heger, H . J.; Yeretzian, C.; Nagel, H.; Boesl, U . Rapid Comm Mass Spectrom. 1996, 10, 1975-1979. Zimmermann, R.; Dorfner, R.; Kettrup, Α.; Yeretzian, C. 18 International Scientific Colloqium on Coffee; Helsinki, Finland (ASIC 18), August 2-6, 1999; in press. Lindinger, W.; Hansel, Α.; Jordan, A . Int. J. Mass Spectrom. Ion Processes 1998, 173, 191-241. Lindinger, W.; Hansel, Α.; Jordan, A . Chemical Society Review 1998, 27, 347354. th

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123 10. Hansel, Α.; Jordan, Α.; Holzinger, R.; Prazeller, P.; Vogel, W.; Lindinger, W. Int. J. Mass Spectrom. Ion Processes 1995, 149/150, 609-619. 11. Merrit, C,; Robertson, D. H . ; McAdoo, D. J. 4 International Colloquim on the Chemistry of Coffee, Amsterdam, Netherland (ASIC4), 2-6 June 1969, 144-148. 12. Vitzthum, O. G.; Werkhoff, P.; Ablanque, E . 7 International Colloqium on the Chemistry of Coffee, Hamburg, Germany (ASIC7), 9-14 June, 1976, 115-123. 13. Holscher, W.; Steinhart, H . In Food Flavors: Generation, Analysis and Process Influence; Charalambous, C., Ed.; Elsevier Science 1995; 785-803. 14. Grosch, W. 18 International Scientific Colloqium on Coffee; Helsinki, Finland (ASIC18), August 2-6, 1999; in press. 15. Czerny, M.; Grosch, W. J. Agric. Food Chem. submitted. 16. Grob, K . ; Artho, Α.; Biedermann, M,; Caramashi, A . J. AOAC Intern 1992, 75, 283. 17. Spadone, J. C.; Takeoka, G.; Liardon, R. J. Agric. Food Chem. 1990, 38, 226. 18. Holscher, W.; Bade-Wegner, H . ; Bendig, I.; Wolkenhauser, P.; Vitzthum, O. G . 16 International Scientific Colloquim on Coffee; Kyoto, Japan (ASIC16), 9-14 April, 1995, 174-182. 19. Cantergiani, E.; H.Brevard, H . ; Amadò, R.; Krebs, Y . ; Feria-Morales, Α.; Yeretzian, C. 18 International Scientific Colloquim on Coffee; Helsinki, Finland (ASIC18), August 2-6, 1999; in press. 20. Becker, R.; Döhla, B.; Nitz, S.; Vitzthum, O. G. 12 International Scientific Colloqium on Coffee; Montreux, Switzerland (ASIC12); 29 June - 3 July 1987, 203-215. 21. Rhoades, J. W. J. Agric. Food Chem. 1960, 8, 136-141. 22. Rodriguez, D. B . ; Frank, Η. Α.; Yamamoto, H . Y. J. Sci. Food Agric. 1969, 20 15-17. 23. MacDonald R.; Fall. R. Atmos. Environ. 1993, 27A, 1709-1713. 24. Espresso Coffee, The Chemistry of Quality; Illy, Α.; Viani, R., Eds.; Academic Press Ltd., London, 1995. th

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On-Line Monitoring of Coffee Roasting by Proton-Transfer-Reaction

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