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Investigation of CO2 precursors in roasted coffee
Accepted Manuscript Rapid Communication$Analytical Methods$Nutritional and Clinical Methods Investigation of CO2 Precursors in Roasted coffee Xiuju Wang, Loong-Tak Lim PII: DOI: Reference:
S0308-8146(16)31487-X http://dx.doi.org/10.1016/j.foodchem.2016.09.095 FOCH 19875
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Food Chemistry
Received Date: Revised Date: Accepted Date:
11 April 2016 14 September 2016 14 September 2016
Please cite this article as: Wang, X., Lim, L-T., Investigation of CO2 Precursors in Roasted coffee, Food Chemistry (2016), doi: http://dx.doi.org/10.1016/j.foodchem.2016.09.095
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Investigation of CO2 Precursors in Roasted coffee XIUJU WANG, LOONG-TAK LIM* Department of Food Science, University of Guelph, Guelph, On, N1G 2W1, Canada
*Corresponding author. Tel: +1 (519) 824-4120 x 56586. Fax: +1 (519) 824-6631. E-mail: [email protected] 1
Abstract Two CO2 formation pathways (chlorgenic acid (CGA) degradation and Maillard reaction) during coffee roasting were investigated. CGA is shown not a major contributor to CO2 formation, as heating of this compound under typical roasting conditions did not release a large quantity of CO2. However, heating of a CGA moiety, caffeic acid, resulted in high yield of CO2 (>98%), suggesting that CGA hydrolysis could be the rate limiting step for CO2 formation from CGA. A large amount of CO2 was detected from glycine-sucrose model system under coffee roasting conditions, implying the importance of Maillard reactions in CO2 formation. Further studies on the heating of various components isolated from green coffee beans showed that CO2 was generated from various green coffee components, including water insoluble proteins and polysaccharides. Around 50% of CO2 was formed from thermal reactions of lower molecular weight compounds with this fraction representing ~25% by weight in green coffee.
Keywords Coffee roasting; CO2 precursor; Chlorogenic acid; Thermal degradation
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1. Introduction During the roasting of coffee beans, a large amount of CO2 is generated (~87% of gaseous compounds released) due to various thermal degradation reactions (Anderson et al., 2003; Baggenstoss, Poisson, Luethi, Perren, & Escher, 2007; Clarke & Macrae, 1987). After the roasting process, a considerable portion of CO2 remains trapped within the coffee matrices, which is continuously being released after cooling. Therefore, roasted coffee must be adequately degassed before packaging to prevent package swelling or bursting during storage. CO2 degassing from roasted coffee beans is a slow process, during which the coffee beans are susceptible to quality deterioration due to the loss of volatile aroma compounds and oxidation. Alternately, the roasted coffee may be packaged, without subjecting to degassing, in packages that are equipped with a vent valve. These active packages are capable of “sensing” the internal pressure and venting the headspace gas when it reaches a certain threshold value (Wang, 2014). The release of CO2 from roasted coffee is a highly complex process, involving factors such as time-temperature profile during roasting, degree of roast, coffee origins, ground particle size, storage conditions (temperature and humidity), and so on (Anderson et al., 2003; Geiger et al., 2005; Shimoni & Labuza, 2000; Wang & Lim, 2014b). As a result, the approach taken by the industry to mitigate the CO2 degassing phenomena has been largely empirical. Although CO2 degassing has been a challenge in the coffee industry for decades, there is still no clear understanding on how CO2 is being generated. By and large, the formation of CO2 in roasted coffee has been attributed to Maillard reactions (Anderson et al., 2003; Geiger et al., 2005; Shimoni & Labuza, 2000). Strecker degradation is believed to be the main pathway of CO2 formation, in which α-dicarbonyl compounds produced from the Maillard reactions react with amino acids, leading to the formation of CO2, α-aminoketones and 3
Strecker aldehydes (Berger, 2007; Rizzi, 2008). Studies with radioactive carbon have shown that over 80% of the CO2 liberated in the Maillard reaction originates from the amino acid, while less than 10% coming from uniformly labeled glucose (Stadtman, Chichester, & Mackinney, 1952). Maillard reaction is of utmost importance in determining coffee quality since it is the main contributor to the characteristic aroma and brown color of roasted coffee. Several studies adopted model systems involving selected amino acids and sugars to simulate reactions that produce aroma compounds under typical coffee roasting conditions (Baltes & Bochmann, 1987a, 1987b; Baltes & Mevissen, 1988; Kunert-Kirchhoff & Baltes, 1990; Wiiken & Baltes, 1990). However, there is a lack of information in the literature regarding the formation of CO2 in Maillard reaction model systems under the similar conditions. Some researchers believe that chlorogenic acid (CGA) can be a precursor of CO2 (Clarke & Vitztbum, 2001; Small & Horrel, 1993). Chlorogenic acid (CGA) is a trivial name for a family of phenolic compounds consisting of quinic acid esterified with a trans-cinnamic acid moiety (e.g., caffeic, ferulic and p-coumaric acid). Typical CGA content in green coffee ranges between 3 to 12 g/100 g (dry weight basis), with Robusta coffees generally being higher in CGA than the Arabicas (Campa et al., 2005; Farah et al., 2005; Trugo & Macrae, 1984). CGA has a melting temperature of about 207°C and is susceptible to thermal degradation under typical coffee roasting temperature at 220-240°C, producing CO2 gas (Sharma et al., 2002). The composition of CGA in coffee is dependent on the degree of roast; about 60% of CGA is lost for medium roast coffee, while most of the CGA is degraded in dark roast (Farah et al., 2005; Trugo & Macrae, 1984). On the basis that dark roast coffee tends to release a larger quantity of CO2, it is fair for researchers to hypothesize that CGA may be a precursor of CO2 (Clarke & Vitztbum, 2001; Small & Horrel, 1993). 4
In this study, experiments were conducted in an attempt to elucidate the CO2 precursors in roasted coffee. The CO2 formations from CGA degradation and Maillard reactions were investigated. In addition, the contribution of various green coffee fractions (water insoluble fraction, water soluble lower molecular fraction, and water soluble higher molecular fraction) to CO2 formation was evaluated.
2. Materials and methods Columbian (Excelso European Preparation) green coffee was donated by Mother Parkers Tea & Coffee Inc. (Mississauga, ON, Canada). Sodium carbonate, sodium hydroxide, tri-sodium citrate, concentrated sulfuric acid, DrieriteTM desiccant, Ascarite® II cartridges (sodium hydroxide coated silica gel), glycine, proline, glutamic acid, sucrose, CGA standard (purity > 99%, Acros Organics, Ottawa, Canada) and D(-)-quinic acid (purity > 98%, Acros Organics, Ottawa, Canada) were all purchased form Fisher Scientific International Inc. (Ottawa, ON, Canada). Caffeic acid was purchased from Sigma-Aldrich Canada Co. (Oakville, Ontario, Canada). Green coffee bean CGA extract (5-caffeoylquinic acid, purity > 98%) was purchased from Changsha Ya Ying Bio-Technology Co., Ltd. (Changsha, Hunan, China).
2.1 Coffee roasting To investigate the kinetics of CO2 formation, 25 g of Columbian green coffee beans were roasted at 230°C for various times (5, 10, 15, 20, 25, 30 min) in a 500 mL Pyrex® volumetric flask (Fig. 1) (Wang, 2014). The flask was loaded with green coffee beans, placed in the forced air oven of a gas chromatograph (Agilent 5890, Agilent Technologies, Santa Clara, USA), and the temperature was 5
increased to 230°C. The CO2 generated, including the evolved CO2 during roasting and the residual CO2 after roasting was determined. The roast loss and L* value were measured to determine the degree of roast as described in Wang & Lim (2014a).
2.1.1 Determination of evolved CO2 The amount of CO2 released from the coffee beans during roasting was determined using the experimental setup as shown in Fig. 1. Green coffee beans were heated in a 500 mL Pyrex® volumetric flask placed within a gas chromatograph (GC) oven. Drierite and Ascarite II cartridges, connected in series, were attached to the outlet port of the flask, while the inlet port was connected to the regulator of a N2 gas cylinder. During heating, the input port was shut, forcing the gaseous compounds produced to vent through the Drierite and Ascarite II cartridges, where water vapor and CO2 were being absorbed, respectively. The roasting was stopped by cooling the oven to 35°C and the remaining CO2 in the headspace of the flask was purged with N2 for 10 min through the same Drierite and Ascarite II cartridges. By measuring the weight increase of the Ascarite II cartridge, the amount of CO2 evolved during the heating treatment was calculated. To evaluate the efficiency of CO2 entrapment of the method, a known quantity of sodium bicarbonate was heated at 230°C for 20 min, and the amount of CO2 generated was determined. An average yield (measured value/theoretical value) of about 98% was obtained, indicating that the method is adequately robust for capturing CO2 (Wang, 2014).
2.1.2 Determination of residual CO2 Residual CO2 content in the coffee beans was determined by a gravimetric method (Anderson et 6
al., 2003; Shimoni & Labuza, 2000; Wang & Lim, 2014a, 2014b). About 17 g coffee samples were mixed with 50 mL of an alkali tri-sodium citrate solution (50 g/L tri-sodium citrate in 0.3 M NaOH) to extract the residual CO2 overnight at room temperature. The resulting solution was added to a three-neck distillation flask connected to a cold-water condenser. Sulfuric acid solution (5M) was added to release the CO2, which was purged with nitrogen gas to a Drierite desiccant column to remove the moisture. The dried gas was then passed through an Ascarite II column in which the CO2 was trapped. The weight gain of the Ascarite II column was recorded and converted to milligram CO2 per gram of roasted coffee.
2.2 Heating experiments with CGA 2.2.1 Heating of pure CGA To evaluate the extent by which CGA contribute to the formation of CO2, CGA extract (about 0.05 g) was heated in uncapped 15 mL silanised vials (Fisher scientific, Ottawa, ON, Canada) in the GC oven at 210, 230, 250, and 270 °C for up to 15 min (Casal, Oliveira, & Ferreira, 2000). The oven was increased to the target temperature and equilibrated for 5 min before the vials were inserted to begin the heating. At predetermined time interval, the vials were removed from the oven and cooled in a desiccator before measuring the weight loss. The char was dissolved in 3 mL of methanol, diluted to 250 folds with 20% aqueous methanol, and then analyzed in a high performance liquid chromatograph (HPLC) to determine the residual CGA content (Farah et al., 2005). The extent of CGA degradation was expressed as the percentage of degraded CGA to original CGA.
2.2.2 Fourier Transform Infrared (FTIR) spectroscopy analysis 7
CGA and its char samples were analyzed by a FTIR spectrometer (IR Prestige-21; Shimadzu Corp., Tokyo, Japan) equipped with an attenuated total reflection (ATR) accessory (Pike Technologies, Madison, WI, USA). Before analysis, samples were ground with pestle and mortar into fine powders. The powders were spread onto the ATR crystal and then clamped down with a press. Three measurements were taken for each sample. Sixty scans were collected and averaged for each measurement at 600 to 4000 cm-1 wavenumber at 4 cm-1 resolution.
2.2.3 Sugar, amino acid and CGA model system To investigate how CGA affects the formation of CO2 from the Maillard reactions, a model system comprised of sucrose and glycine, spiked with CGA, was adopted. Here, CGA (0.004 or 0.008 mol) was homogeneously blended with sucrose and glycine mixture (0.004 or 0.008 mol of each) and heated at 230 °C for 25 min using the same setup as shown in Fig. 1 (Wang, 2014). The CO2 evolved from the reactions was recorded as described in Section 2.1.1.
2.3 Isolation and roasting of green coffee fractions To further decipher how coffee bean components contribute to CO2 generation, various fractions of green coffee were isolated using a sequential fractionation procedure, adapted from De Maria et al. (1994). Green coffee beans were firstly grinded using Comil® Conical mill (Quadro Engineering Corp. Waterloo, Canada) and the particles with diameter of 1.7-2.4 mm were collected through sieving. The coffee ground was first defatted using hexane, followed by aqueous extraction (300 g defatted coffee) with 1 L of distilled water at 80 °C for 15 min with the aid of a magnetic stir bar. The extraction process was repeated four times with fresh hot distilled water to ensure complete 8
extraction. The soluble (Fraction A) and insoluble (Fraction D) fractions were separated by filtration, and then freeze-dried. Fraction A (20 g) was re-dissolved in 25 mL of distilled water (50°C), followed by the addition of 100 mL 80% ethanol. The mixture was shaken for 30 min and centrifuged at 100× g for 10 min. The supernatant was collected and set aside, while the residue was re-dissolved in water and extracted with 80% ethanol for two more times. The supernatants were combined to give Fraction B. The final residue obtained was freeze-dried to produce Fraction C. Fractions A, B, C, and D were subjected to heating separately at 230°C for 25 min in the volumetric flask (Fig. 1). The CO2 produced during heating was quantified. For Fraction D, part of CO2 formed during heating was trapped in the cellular structure, thus further analysis was conducted on the residual CO2 using the method in Section 2.1.2. The total CO2 produced for this fraction is the sum of residual CO2 and CO2 evolved during heating (Wang, 2014).
3. Results and discussion 3.1 CO2 generation during coffee roasting During the roasting of the coffee beans at 230°C, the amount of CO2 generated increased with increasing roast time (Fig 2). In accordance with the findings from a previous studies involving a commercial fluidized bed roaster (Wang & Lim, 2014a), the residual CO2 increased with increasing roast degree up to dark roast, beyond which a decrease in residual CO2 was observed. In contrast, the amount of CO2 evolved during roasting increased monotonously with the roast degree. Fig. 2 also indicates that the ratio of the evolved CO2 to residual CO2 increased from 0.1 to 8.7 as heating increased from 5 to 30 min, respectively, , implying that the CO2 trapping ability of the coffee beans decreased as roasting proceed implying that the amount of CO2 trapped in the beans decreased as the 9
roasting progressed. This observation can be attributed to the diffusion loss of the CO2 due to the extended exposure of the beans to high temperature during roasting, as well as the depletion of CO2 precursors in the beans.
3.2 CO2 generation from model system 3.2.1 CO2 generation from CGA To gain insight on the thermal degradation behavior of CGA, green coffee bean CGA extract was heated under typical coffee roasting temperatures (210-270°C) and its weight loss was measured. Qualitatively, as the CGA was heated, it melted followed by bubbling as heating continued, indicating the generation of considerable gases and vapors. These phenomena are in agreement with the CGA pyrolysis studies of Shama et al. (2000; 2002). The weight loss data of CGA at different temperatures are summarized in Fig. 3a, showing that the weight loss increased exponentially (R2 > 0.97) with increasing heating time. Moreover, at any given heating time, the extent of weight loss was greater at higher temperature. The residual chars were further analyzed by HPLC to determine the residual CGA. At 230°C and above, the majority of CGA was degraded after three min of heating. At 210°C, total degradation occurred at around six min (Fig. 3b). Fig. 3c shows a sigmoidal relationship between CGA degradation and weight loss. Initial heating resulted in 2-3% weight loss with negligible CGA degradation, probably due to the desorption of moisture from the polyphenol. Further heating resulted in concomitant degradation of CGA and weight loss up to about 9% weight loss, indicating the formation of volatile compounds as the CGA underwent thermal degradation. Beyond this point, further heating did not induce further CGA degradation, indicating that all CGA was destructed. However, the weight loss continued as heating progressed, implying that the degraded CGA were converted into volatile compounds. 10
To investigate the changes in CGA’s molecular structure during thermal degradation, the residual char was analyzed by FTIR. As shown in Fig. 4a, the bands at 1681 and 1639 cm-1wavenumbers can be attributed to the stretching of C=O in quinic acid and caffeic acid moiety, respectively (Lyman et al., 2003; Jiang et al., 2009; Wang, Fu, & Lim, 2011). The bands located at 1597-1614 cm-1 and 1448 cm-1 represent the C=C vibrations outside and inside the aromatic ring of caffeic acid, respectively. The FTIR spectra of CGA after being heated at 230°C are shown in Fig. 4b. As shown, there are considerable changes in the molecular structure of CGA during heating; the bands for both hydroxyl (3313 and 3464 cm-1) and carbonyl groups (1681 and 1639 cm-1) progressively became smaller with increasing heating time. At 6 min, the C=O absorbance bands for both caffeic acid and quinic acid moieties disappeared. Based on these results, we speculate that during the heating process, CGA was first hydrolyzed to quinic acid and caffeic acid. The carbonyl group in the caffeic acid was decarboxylated to form CO2 and phenolic volatiles, such as 4-vinylcatechol (Clarke & Vitztbum, 2001; Rizzi & Boekley, 1993). Due to the lacking of the C=C bond in quinic acid as compared to caffeic acid, the carbonyl group in the former is expected to be more stable than the latter (Clarke & Vitztbum, 2001). However, the disappearing of C=O band in quinic acid moiety upon extended heating indicated that decarboxylation occurred as well during the heating process (Moon & Shibamoto, 2010). The amounts of CO2 generated during the heating of CGA are shown in Fig. 5a. About 10 mg of CO2 was detected from one gram of CGA at 230 °C. In addition, the amount of CO2 generated is dependent on the heating temperature. More than double amount of CO2 was produced at 270 °C compared with 230 °C (Fig. 5b). These findings are consistent with those reported by Sharma et al. (2002) who observed that CO2 is the major gas produced during the pyrolysis of CGA. As the CGA 11
content in green coffee is 3-12% (dry weight basis) (Campa, Doulbeau, Dussert, Hamon, & Noirot, 2005; Farah, De Paulis, Trugo, & Martin, 2005; Trugo & Macrae, 1984), the amount of CO2 generated from CGA in one gram of green coffee could roughly be calculated as 0.3-1.2 mg. The calculated value is equal to only ~8% yield based on the proposed degradation pathway above (hydrylysis and decarboxylation of caffeic acid). These magnitudes are considerably lower than that determined from coffee roasting experiment (42 mg CO2 /g green beans; Fig. 2). This analysis suggests that CGA is one of the CO2 precursors in coffee, but not the principal one. Next, to evaluate the thermal stability of CGA degradation byproducts - caffeic and quinic acids, these components were heated at 230 °C and the amount of CO2 generated were determined. After 25 min of heating, around 0.24 g CO2 was detected from one gram of caffeic acid. Assuming the CO2 generated is derived from decarboxylation of the carbonyl group, the yield would be ~98%. This value is consistent with the findings of Rizzi and Boekley (1993), who observed that cinnamic acids with p-hydroxyl substituent were susceptible to decarboxylation degradation upon heating, producing CO2. On the other hand, no CO2 was detected from the heating of quinic acid under the same temperature-time conditions. This result suggests that quinic acid is thermally more stable, or the degradation reaction does not take the decarboxylation route under the experimental conditions used. The quinic acid may undergo dehydration to form quinide (quinic acid lactone) during heating by losing a water molecule (Clarke & Vitztbum, 2001; Farah et al., 2005; Sharma et al., 2002). Considering these findings altogether, the lower yield of CO2 from CGA degradation as compared to caffeic acid suggests that the hydrolysis of CGA is probably the limiting step for CO2 formation under typical roasting conditions.
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3.2.2 CO2 formation from Maillard reaction model compounds under coffee roasting conditions As shown in Fig. 5c, about 3.4 mg CO2 was generated from 0.004 mol of sucrose (equal to 1.369 g) at 230°C for 25 min. The amount of CO2 produced was doubled when 0.008 mol sucrose (equal to 2.738g) was heated. Considering the sucrose content in Arabica green coffee is 6-8.5% (Clarke and Vitztbum 2001), the theoretical amount of CO2 formed, due to the degradation of sucrose alone, would be 0.15-0.21 mg CO2/g green coffee. In comparison with the total CO2 generated for the green coffee beans (Fig. 2; about 35 mg CO2/g green coffee, 25 min heating at 230°C), it can be concluded that the contribution of sucrose alone to the formation of CO2 in coffee is minimal. On the other hand, Fig. 5c shows that the amounts of CO2 generated from glycine were higher than that of sucrose, but are still about two orders of magnitude lower than the CO2 generated from roasting of coffee beans. These findings show that the sucrose and amino acid alone are not the main precursors for the formation of CO2. However, when glycine and sucrose were mixed and heated, significantly larger amounts of CO2 were generated than those when the compounds were heated separately (Fig. 5d). For example, 172 mg CO2 was formed from the heating of 0.004 mol glycine-sucrose mixture, while only 12.4 mg CO2 was generated from separate heating of sucrose and glycine. Assuming that the CO2 is originated from the Strecker degradation reaction, the theoretical amount of CO2 formed would be 176 and 352 mg for 0.004 mol glycine-sucrose mixture and 0.008 mol glycine-sucrose mixture, respectively. These calculated values are in the similar orders of magnitude as the experimental data reported in Fig. 5d, implying that CO2 was mainly derived from the glycine structure by Strecker degradation. When mixture of 0.008 mol glycine and 0.004 mol sucrose was heated, 230.5 mg CO2 was generated, which is significantly lower than that generated from the mixture of 0.008 mol glycine 13
and 0.008 mol sucrose (330.1 mg) (Fig. 5d). This observation can be explained by the less amount of α-dicarbonyl compounds formed due to the reduced amount of sugar in the model system. During Strecker degradation, α-iminocarbonyls are produced due to nucleophilic addition of an amino group (from proteins and amino acids) to α-dicarbonyls from Maillard reactions (Rizzi, 2008). The α-iminocarbonyls then undergo decarboxylation, releasing CO2 gas. Considering that α-dicarbonyl compounds are the fission products of sugar from Maillard reaction, the decrease in sucrose content will decrease the available α-dicarbonyl reactants needed for producing the α-iminocarbonyl precursors, and therefore decreasing the amount of CO2 produced. In addition, Fig. 5d also shows that 214.7 mg CO2 was generated from the heating of 0.004 mol glycine and 0.008 mol sucrose mixture. This value is significantly higher than the theoretical value calculated (176 mg) from Strecker degradation pathway, suggesting that there might be other pathways, in addition to Strecker degradation, is available for the formation of CO2 from sugars in the presence of amino acid. The similar phenomenon was observed in the study by Cole (1967), where CO2 was detected in the glucose-amines system (Strecker degradation is impossible).
3.2.3 CO2 formation in glycine-sucrose-CGA model system To elucidate how CGA interacts with glycine and sucrose in affecting the Maillard and Strecker reactions, more experiments were conducted to determine the amount of CO2 generated from different combinations of glycine-sucrose-CGA mixture. As shown in Fig. 5e, the addition of CGA to the Maillard reaction model system caused a decrease in CO2 formation in the four mixtures investigated, suggesting that the CGA suppressed the Maillard reaction between the amino acid and sugar. The CGA’s suppression behavior on the formation of Maillard flavor compounds, such as pyrazines and Strecker aldehydes, was also observed by other researchers (Jiang et al., 2009; Jiang & 14
Peterson, 2010; Wang & Ho, 2013; Wang, 2000). Wang (2000) suggested that the underlining mechanism by which the hydroxycinnamic acids altered Maillard flavor generation was due to their radical scavenging capability, because the formation of some aroma compounds, such as pyrazines, is based on the radical pathway (Berger, 2007). In addition, these phenolic antioxidants may alter the reaction redox potential, which will influence the generation of sugar fragments (Yaylayan, 2003). In addition to redox reactions, Jiang et al. (2009, 2010) proposed that free hydroxycinnamic acids can undergo decarboxylation, exposing the vinyl group that participates in various pericyclic reactions with Maillard intermediates to produce phenolic-Maillard adducts. In summary, the above results indicate that instead of being a major precursor of CO2, CGA to some extent has the ability to suppress the formation of CO2 from Maillard reactions and Strecker degradation.
3.3 Fragmentation of green coffee components and their contribution to CO2 formation The generation of significant amount of CO2 through Maillard reactions under typical coffee roasting conditions implies that free amino acid and sugar are the key contributing precursors for the formation of CO2. However, typical free amino acid (0.2-0.8%) and sugar (6-9%) contents in green coffee beans are too low to account for the large amount of CO2 generated during roasting. Other components must have also contributed to the formation of CO2. To elucidate the roles of other green bean components, an isolation procedure was adopted to separate the green coffee components into Fractions A, B, C, and D. The contribution of each of these fractions to CO2 formation was investigated. The weight percentage values for each fraction are summarized in Fig. 6a, showing that 68.5% of the green bean is made up of water-insoluble (Fraction D) materials, mainly consisting of water 15
insoluble proteins and high molecular weight polysaccharides. The water-soluble Fraction A can be further fractionated into ethanol-insoluble (Fraction C; 6.3%) and ethanol-soluble (Fraction B; 25.2%). Fractions A, B, C and D obtained were subjected to heating at 230°C for 25 min, during which the CO2 evolved was determined (Fig. 6b). As shown, Fraction B contributed to 48% of the CO2 formed during the roasting process, while Fraction C accounted for 7%. On the other hand, the dominant mass Fraction D contributed to 22% of the CO2 generated. Using the same fractionation procedure, De Maria et al. (1994; 1996) determined that Fraction B is mainly consisting of small molecular weight compounds such as sucrose, CGA, amino acids and peptides, while Fraction C is mainly made up of water-soluble proteins and arabinogalactans. Accordingly, we can deduct that the majority of CO2 generated during roasting was originated from the water-soluble fraction, primarily from Maillard reactions and CGA degradation. The smaller but significant amount of CO2 from Fraction C could be originated from the Maillard reactions between monosaccharide from arabinogalactan degradation and ε-amino group of lysine and the α-amino groups of terminal amino acids in proteins (Berger, 2007). In comparison, CO2 formation from Fraction D can be attributed to the thermal degradation of cellulose. Here, compounds with carbonyl and carboxyl groups are formed through depolymerization and oxidation reactions (Shafizadeh & Bradbury, 1979;Brebu & Vasile, 2010). By comparing the total CO2 produced from Fractions A and D to the amount of CO2 generated during coffee roasting, an unaccounted quantity of about 24% of CO2 from coffee roasting was observed. The observation is indicative of the existence of component-interaction between these two fractions. The unaccounted CO2 fraction can also be attributable to the different reactions occurred during the roasting of coffee beans versus the extracted fractions. Specifically, the pressure buildup 16
within the whole beans that causes the beans to crack was absent during the heating of the fraction samples. Moreover, during roasting of beans, the interior of the beans is depleted of O2 due to rapid oxidation reactions, while the surface of the beans continued to be exposed to oxygen-rich hot air. These conditions are absent during the heating of Fractions B, C and D. In summary, although CGA is a major carbonyl containing compound in green coffee, its contribution to CO2 generation is very limited, while Maillard reactions is confirmed to be the main pathway. In addition to free amino acids and sugar, high molecular polysaccharide and protein are also involved in the CO2 generation. Lower molecular compounds which represent ~25% content in green coffee generate about 50% CO2 and thus are the major precursors of CO2.
4. Acknowledgement We gratefully acknowledge the financial and material supports from Mother Parkers Tea & Coffee Inc. and Natural Sciences and Engineering Research Council of Canada (NSERC).
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Food Reviews International, 24, 416–435. Rizzi, G. P., & Boekley, L. J. (1993). Flavor chemistry based on the thermally-induced decarboxylation of p-hydroxylcinnamic acids. In Food Flavors, Ingredients and Composition (pp. 663–670). Amsterdam, Netherlands: Elsevier Science Publishers. Shafizadeh, F., & Bradbury, A. G. W. (1979). Thermal degradation of cellulose in air and nitrogen at low temperatures. Journal of Applied Polymer Science, 23, 1431–1442. Sharma, R. K., Fisher, T. S., & Hajaligol, M. R. (2002). Effect of reaction conditions on pyrolysis of chlorogenic acid. Journal of Analytical and Applied Pyrolysis, 62(2), 281–296. Sharma, R. K., Hajaligol, M. R., Smith, P. A. M., Wooten, J. B., & Baliga, V. (2000). Characterization of char from pyrolysis of chlorogenic acid. Energy and Fuels, 14, 1083–1093. Shimoni, E., & Labuza, T. P. (2000). Degassing kinetics and sorption equilibrium of carbon dioxide in fresh roasted and ground coffee. Journal of Food Process Engineering, 23, 419–436. Small, L. E., & Horrel, R. S. (1993). High yield coffee technology. In 15th ASIC Colloquium (pp. 719–726). Montpellier: ASIC, Paris, France. Stadtman, F. H., Chichester, C. O., & Mackinney, G. (1952). Carbon dioxide production in the browning reaction. Journal of the American Chemical Society, 74, 3194–3196. Trugo, L. C., & Macrae, R. (1984). A study of the effect of roasting on the chlorogenic acid composition of coffee using HPLC. Food Chemistry, 15, 219–227. Wang, N., Fu, Y., & Lim, L.-T. (2011). Feasibility study on chemometric discrimination of roasted Arabica coffees by solvent extraction and Fourier transform infrared spectroscopy. Journal of Agricultural and Food Chemistry, 59(7), 3220–3226. Wang, X., & Lim, L.-T. (2014a). A kinetics and modeling study of coffee roasting under isothermal conditions. Food and Bioprocess Technology, 7, 621–632. Wang, X., & Lim, L.-T. (2014b). Effect of roasting conditions on carbon dioxide degassing behavior in coffee. Food Research International, 61, 144–151. Wang, X. (2014). Understanding the formation of CO2 and its degassing behaviours in coffee (Doctoral dissertation). University of Guelph. Wang, Y. (2000). Effects of naturally occurring phenolic compounds on the formation of Maillard Aroma. Rutgers University. Wang, Y., & Ho, C.-T. (2013). Effects of naturally occuring phenolic compounds in coffee on the formation of Maillard aromas. In C.-T. Ho, C. Mussinan, F. Shahidi, & E. T. Contis (Eds.), Nutrition, Functional and Sensory Properties of Foods (pp. 98–110). Cambridge: The Royal Society of Chemistry. Wiiken, C., & Baltes, W. (1990). Model reaction on roast aroma formation-formation of pyrrole-2, 5-diones by roasting of aspartic acid and asparagine. Z. Lebensm. Unters. Forsch., 191, 116–118. Yaylayan, V. A. (2003). Recent advances in the chemistry of Strecker degradation and amadori rearrangement: implications to aroma and color formation. Food Science and Technology Research, 9(1), 1–6.
19
Valve
N2 Two-hole rubber stopper
Drierite column
Volumetric flask
Samples Ascarite II column
GC oven
Fig. 1. Setup for roasting and evolved CO2 determination
20
Very dark
Residual CO2 Residual CO2
40
9
Evolved EvolvedCO2 CO2
35
Very dark
Evolved/residualCO2 CO2 Evolved/residual
8 Very dark
7
30
6
25
5
Medium-dark
20
4
15
3 Light
10 5
10
2
Very light
1
0
Evolved/residual CO2 ratio
Residual or evolved CO2 amount (mg/g green beans)
45
0 5
10
15 20 Roasting time (min)
25
30
Fig. 2. CO2 formation (evolved+ residual) and the ratio of evolved to residual CO2 amount during coffee roasting under roasting temperature of 230 °C by using the apparatus as shown in Fig. 1. The degree of roast was determined by roast loss and L* value according to Wang & Lim (2014a).
21
210°C
230°C
250°C
270°C
1
a
Weight loss (Wt/W0)
0.98 0.96 0.94
y = -0.022ln(x) + 0.981 R² = 0.9756
0.92 0.9
y = -0.027ln(x) + 0.9675 R² = 0.987
0.88
y = -0.037ln(x) + 0.9645 R² = 0.9874
0.86
y = -0.043ln(x) + 0.9455 R² = 0.9799
0.84 0.82 1
210 °C
230 °C
250 °C
10 Time (min) 270 °C
210 °C
1.2
0.6 0.4 0.2 0 1
2
3
6
9
250 °C
270 °C
c CGA degradatio ratio (CGA residual/CGA original)
CGA degradation ratio (CGA residual/CGA original)
0.8
230 °C
1.2
b 1
0
100
1 0.8 0.6 0.4 0.2 0
15
0
Roasting time (min)
5
10 Weight loss (%)
15
20
Fig. 3. (a) Semi-log plot of weight loss kinetics data of CGA in roasting temperatures of 210, 230, 250, and 270 °C, with the data fitted with logarithmic equation; (b) Plots of CGA degradation versus roasting time at 210, 230, 250, and 270 °C; (c) Plot of CGA degradation versus weight loss.
22
a
1681 cm-1 1639 cm-1 1597-1614 cm-1
O HO
C
HC
HC
OH
1448 cm-1
OH
Caffeic acid H
OH
H
H
HO H
HO
H
H
H OOC
Quinic acid
H OH O H HOOC
Chlorogenic acid 3600
HO
3100
C
O
H
H
H
HO
H
H
HC
HC
OH
H OH OH
2600
2100
1600
1100
600
Wavenumber (cm-1)
b 3464 cm-1
1681 cm-1 1639 cm-1 1597-1614 cm-1
3313 cm-1
1448 cm-1
0 min 1 min 2 min 3 min 6 min 9 min 15 min 3600
3100
2600
2100
1600
1100
600
Wavenumber (cm-1)
Fig. 4. (a) FTIR spectra of pure CGA, caffeic acid and quinic acid, showing the wavenumbers of C=O stretching in quinic acid (1681 cm-1) and caffeic acid moieties (1639 cm-1); (b) FTIR spectra of CGA after being heated at temperature of 230°C for various times, showing the vanishing of C=O stretching in both quinic acid and caffeic acid moieties.
23
a
30
CO2 formed (mg)
CO2 formed (mg)
20 15 10
25 20 14.5
15 10
5
5
0
0 0
1 2 CGA amount (g) Sucrose
20.0 18.0 16.0 14.0 12.0 10.0 8.0 6.0 4.0 2.0 0.0
3
230 °C for 20 min
0 mol glycine
c
Glycine
400.0
17.5
Amount of CO2 formed (mg)
Amount of CO2 formed (mg)
30.7
30
y = 9.5791x + 0.6024 R² = 0.9993
25
b
35
8.9 6.3 3.4
270 °C for 20 min
0.004 mol glycine
d
0.008 mol glycine 330.1
350.0 300.0 230.5
214.7
250.0 171.9
200.0 150.0 100.0 50.0
3.4
6.3
0.0 0.004 0.008 Amount of sucrose (mol)
0.004 0.008 Amount of sugar or amino acid (mol) 400 0 mol CGA
0.004 mol CGA
0.008 mol CGA
e
Amount of CO2 formed (mg)
350 300 250 200 150 100 50 0 0.004 +0.004
0.004+0.008
0.008+0.004
0.008+0.008
Sucrose+ Glycine mixture (mol)
Fig. 5. (a) Amount of CO2 formation from CGA degradation at 230 °C; (b) Amount of CO2 formation from 1.5 g CGA degradation at 230 °C and 270°C; (c) Amount of CO2 formed from heating of sucrose and glycine under coffee roasting temperature of 230°C for 25 min; (d) Amount of CO2 formed from roasting of sucrose-glycine mixture under coffee roasting temperature of 230 °C for 25 min; (e) Amount of CO2 generation in glycine-sucrose-CGA model system. 24
a
6.30%
25.20%
Water insoluble (fraction D) 80% ethanol soluble (fraction B)
68.50%
80% ethanol insoluble (fraction C)
b
23.56%
21.50%
Water insoluble (fraction D) 80% ethanol soluble (fraction B) 80% ethanol insoluble (fraction C)
7.00%
Interaction between A and D
47.94%
Fig. 6. (a)The weight percentages of coffee fraction D, B and C, calculated as: the weight of fraction obtained/weight of green coffee beans used (dry weight)×100%; (b) The contribution of each fraction to the amount of CO2 formed during coffee roasting, calculated as: the amount of CO2 generated from the fraction/the amount of CO2 generated during coffee roasting ×100%. Fraction A=fraction B + fraction C. The interaction between A and D was calculated as the difference between amount of CO2 generated from coffee roasting and total amount of CO2 generated from heating of each fraction.
25
Highlights •
A method was developed to study the evolved CO2
•
CGA is shown not a major precursor of CO2
•
Mailard reaction is the main pathway for CO2 formation
•
About 50% of CO2 was from lower molecular weight compounds
26
S0308-8146(16)31487-X http://dx.doi.org/10.1016/j.foodchem.2016.09.095 FOCH 19875
To appear in:
Food Chemistry
Received Date: Revised Date: Accepted Date:
11 April 2016 14 September 2016 14 September 2016
Please cite this article as: Wang, X., Lim, L-T., Investigation of CO2 Precursors in Roasted coffee, Food Chemistry (2016), doi: http://dx.doi.org/10.1016/j.foodchem.2016.09.095
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of CO2 Precursors in Roasted coffee XIUJU WANG, LOONG-TAK LIM* Department of Food Science, University of Guelph, Guelph, On, N1G 2W1, Canada
*Corresponding author. Tel: +1 (519) 824-4120 x 56586. Fax: +1 (519) 824-6631. E-mail: [email protected] 1
Abstract Two CO2 formation pathways (chlorgenic acid (CGA) degradation and Maillard reaction) during coffee roasting were investigated. CGA is shown not a major contributor to CO2 formation, as heating of this compound under typical roasting conditions did not release a large quantity of CO2. However, heating of a CGA moiety, caffeic acid, resulted in high yield of CO2 (>98%), suggesting that CGA hydrolysis could be the rate limiting step for CO2 formation from CGA. A large amount of CO2 was detected from glycine-sucrose model system under coffee roasting conditions, implying the importance of Maillard reactions in CO2 formation. Further studies on the heating of various components isolated from green coffee beans showed that CO2 was generated from various green coffee components, including water insoluble proteins and polysaccharides. Around 50% of CO2 was formed from thermal reactions of lower molecular weight compounds with this fraction representing ~25% by weight in green coffee.
Keywords Coffee roasting; CO2 precursor; Chlorogenic acid; Thermal degradation
2
1. Introduction During the roasting of coffee beans, a large amount of CO2 is generated (~87% of gaseous compounds released) due to various thermal degradation reactions (Anderson et al., 2003; Baggenstoss, Poisson, Luethi, Perren, & Escher, 2007; Clarke & Macrae, 1987). After the roasting process, a considerable portion of CO2 remains trapped within the coffee matrices, which is continuously being released after cooling. Therefore, roasted coffee must be adequately degassed before packaging to prevent package swelling or bursting during storage. CO2 degassing from roasted coffee beans is a slow process, during which the coffee beans are susceptible to quality deterioration due to the loss of volatile aroma compounds and oxidation. Alternately, the roasted coffee may be packaged, without subjecting to degassing, in packages that are equipped with a vent valve. These active packages are capable of “sensing” the internal pressure and venting the headspace gas when it reaches a certain threshold value (Wang, 2014). The release of CO2 from roasted coffee is a highly complex process, involving factors such as time-temperature profile during roasting, degree of roast, coffee origins, ground particle size, storage conditions (temperature and humidity), and so on (Anderson et al., 2003; Geiger et al., 2005; Shimoni & Labuza, 2000; Wang & Lim, 2014b). As a result, the approach taken by the industry to mitigate the CO2 degassing phenomena has been largely empirical. Although CO2 degassing has been a challenge in the coffee industry for decades, there is still no clear understanding on how CO2 is being generated. By and large, the formation of CO2 in roasted coffee has been attributed to Maillard reactions (Anderson et al., 2003; Geiger et al., 2005; Shimoni & Labuza, 2000). Strecker degradation is believed to be the main pathway of CO2 formation, in which α-dicarbonyl compounds produced from the Maillard reactions react with amino acids, leading to the formation of CO2, α-aminoketones and 3
Strecker aldehydes (Berger, 2007; Rizzi, 2008). Studies with radioactive carbon have shown that over 80% of the CO2 liberated in the Maillard reaction originates from the amino acid, while less than 10% coming from uniformly labeled glucose (Stadtman, Chichester, & Mackinney, 1952). Maillard reaction is of utmost importance in determining coffee quality since it is the main contributor to the characteristic aroma and brown color of roasted coffee. Several studies adopted model systems involving selected amino acids and sugars to simulate reactions that produce aroma compounds under typical coffee roasting conditions (Baltes & Bochmann, 1987a, 1987b; Baltes & Mevissen, 1988; Kunert-Kirchhoff & Baltes, 1990; Wiiken & Baltes, 1990). However, there is a lack of information in the literature regarding the formation of CO2 in Maillard reaction model systems under the similar conditions. Some researchers believe that chlorogenic acid (CGA) can be a precursor of CO2 (Clarke & Vitztbum, 2001; Small & Horrel, 1993). Chlorogenic acid (CGA) is a trivial name for a family of phenolic compounds consisting of quinic acid esterified with a trans-cinnamic acid moiety (e.g., caffeic, ferulic and p-coumaric acid). Typical CGA content in green coffee ranges between 3 to 12 g/100 g (dry weight basis), with Robusta coffees generally being higher in CGA than the Arabicas (Campa et al., 2005; Farah et al., 2005; Trugo & Macrae, 1984). CGA has a melting temperature of about 207°C and is susceptible to thermal degradation under typical coffee roasting temperature at 220-240°C, producing CO2 gas (Sharma et al., 2002). The composition of CGA in coffee is dependent on the degree of roast; about 60% of CGA is lost for medium roast coffee, while most of the CGA is degraded in dark roast (Farah et al., 2005; Trugo & Macrae, 1984). On the basis that dark roast coffee tends to release a larger quantity of CO2, it is fair for researchers to hypothesize that CGA may be a precursor of CO2 (Clarke & Vitztbum, 2001; Small & Horrel, 1993). 4
In this study, experiments were conducted in an attempt to elucidate the CO2 precursors in roasted coffee. The CO2 formations from CGA degradation and Maillard reactions were investigated. In addition, the contribution of various green coffee fractions (water insoluble fraction, water soluble lower molecular fraction, and water soluble higher molecular fraction) to CO2 formation was evaluated.
2. Materials and methods Columbian (Excelso European Preparation) green coffee was donated by Mother Parkers Tea & Coffee Inc. (Mississauga, ON, Canada). Sodium carbonate, sodium hydroxide, tri-sodium citrate, concentrated sulfuric acid, DrieriteTM desiccant, Ascarite® II cartridges (sodium hydroxide coated silica gel), glycine, proline, glutamic acid, sucrose, CGA standard (purity > 99%, Acros Organics, Ottawa, Canada) and D(-)-quinic acid (purity > 98%, Acros Organics, Ottawa, Canada) were all purchased form Fisher Scientific International Inc. (Ottawa, ON, Canada). Caffeic acid was purchased from Sigma-Aldrich Canada Co. (Oakville, Ontario, Canada). Green coffee bean CGA extract (5-caffeoylquinic acid, purity > 98%) was purchased from Changsha Ya Ying Bio-Technology Co., Ltd. (Changsha, Hunan, China).
2.1 Coffee roasting To investigate the kinetics of CO2 formation, 25 g of Columbian green coffee beans were roasted at 230°C for various times (5, 10, 15, 20, 25, 30 min) in a 500 mL Pyrex® volumetric flask (Fig. 1) (Wang, 2014). The flask was loaded with green coffee beans, placed in the forced air oven of a gas chromatograph (Agilent 5890, Agilent Technologies, Santa Clara, USA), and the temperature was 5
increased to 230°C. The CO2 generated, including the evolved CO2 during roasting and the residual CO2 after roasting was determined. The roast loss and L* value were measured to determine the degree of roast as described in Wang & Lim (2014a).
2.1.1 Determination of evolved CO2 The amount of CO2 released from the coffee beans during roasting was determined using the experimental setup as shown in Fig. 1. Green coffee beans were heated in a 500 mL Pyrex® volumetric flask placed within a gas chromatograph (GC) oven. Drierite and Ascarite II cartridges, connected in series, were attached to the outlet port of the flask, while the inlet port was connected to the regulator of a N2 gas cylinder. During heating, the input port was shut, forcing the gaseous compounds produced to vent through the Drierite and Ascarite II cartridges, where water vapor and CO2 were being absorbed, respectively. The roasting was stopped by cooling the oven to 35°C and the remaining CO2 in the headspace of the flask was purged with N2 for 10 min through the same Drierite and Ascarite II cartridges. By measuring the weight increase of the Ascarite II cartridge, the amount of CO2 evolved during the heating treatment was calculated. To evaluate the efficiency of CO2 entrapment of the method, a known quantity of sodium bicarbonate was heated at 230°C for 20 min, and the amount of CO2 generated was determined. An average yield (measured value/theoretical value) of about 98% was obtained, indicating that the method is adequately robust for capturing CO2 (Wang, 2014).
2.1.2 Determination of residual CO2 Residual CO2 content in the coffee beans was determined by a gravimetric method (Anderson et 6
al., 2003; Shimoni & Labuza, 2000; Wang & Lim, 2014a, 2014b). About 17 g coffee samples were mixed with 50 mL of an alkali tri-sodium citrate solution (50 g/L tri-sodium citrate in 0.3 M NaOH) to extract the residual CO2 overnight at room temperature. The resulting solution was added to a three-neck distillation flask connected to a cold-water condenser. Sulfuric acid solution (5M) was added to release the CO2, which was purged with nitrogen gas to a Drierite desiccant column to remove the moisture. The dried gas was then passed through an Ascarite II column in which the CO2 was trapped. The weight gain of the Ascarite II column was recorded and converted to milligram CO2 per gram of roasted coffee.
2.2 Heating experiments with CGA 2.2.1 Heating of pure CGA To evaluate the extent by which CGA contribute to the formation of CO2, CGA extract (about 0.05 g) was heated in uncapped 15 mL silanised vials (Fisher scientific, Ottawa, ON, Canada) in the GC oven at 210, 230, 250, and 270 °C for up to 15 min (Casal, Oliveira, & Ferreira, 2000). The oven was increased to the target temperature and equilibrated for 5 min before the vials were inserted to begin the heating. At predetermined time interval, the vials were removed from the oven and cooled in a desiccator before measuring the weight loss. The char was dissolved in 3 mL of methanol, diluted to 250 folds with 20% aqueous methanol, and then analyzed in a high performance liquid chromatograph (HPLC) to determine the residual CGA content (Farah et al., 2005). The extent of CGA degradation was expressed as the percentage of degraded CGA to original CGA.
2.2.2 Fourier Transform Infrared (FTIR) spectroscopy analysis 7
CGA and its char samples were analyzed by a FTIR spectrometer (IR Prestige-21; Shimadzu Corp., Tokyo, Japan) equipped with an attenuated total reflection (ATR) accessory (Pike Technologies, Madison, WI, USA). Before analysis, samples were ground with pestle and mortar into fine powders. The powders were spread onto the ATR crystal and then clamped down with a press. Three measurements were taken for each sample. Sixty scans were collected and averaged for each measurement at 600 to 4000 cm-1 wavenumber at 4 cm-1 resolution.
2.2.3 Sugar, amino acid and CGA model system To investigate how CGA affects the formation of CO2 from the Maillard reactions, a model system comprised of sucrose and glycine, spiked with CGA, was adopted. Here, CGA (0.004 or 0.008 mol) was homogeneously blended with sucrose and glycine mixture (0.004 or 0.008 mol of each) and heated at 230 °C for 25 min using the same setup as shown in Fig. 1 (Wang, 2014). The CO2 evolved from the reactions was recorded as described in Section 2.1.1.
2.3 Isolation and roasting of green coffee fractions To further decipher how coffee bean components contribute to CO2 generation, various fractions of green coffee were isolated using a sequential fractionation procedure, adapted from De Maria et al. (1994). Green coffee beans were firstly grinded using Comil® Conical mill (Quadro Engineering Corp. Waterloo, Canada) and the particles with diameter of 1.7-2.4 mm were collected through sieving. The coffee ground was first defatted using hexane, followed by aqueous extraction (300 g defatted coffee) with 1 L of distilled water at 80 °C for 15 min with the aid of a magnetic stir bar. The extraction process was repeated four times with fresh hot distilled water to ensure complete 8
extraction. The soluble (Fraction A) and insoluble (Fraction D) fractions were separated by filtration, and then freeze-dried. Fraction A (20 g) was re-dissolved in 25 mL of distilled water (50°C), followed by the addition of 100 mL 80% ethanol. The mixture was shaken for 30 min and centrifuged at 100× g for 10 min. The supernatant was collected and set aside, while the residue was re-dissolved in water and extracted with 80% ethanol for two more times. The supernatants were combined to give Fraction B. The final residue obtained was freeze-dried to produce Fraction C. Fractions A, B, C, and D were subjected to heating separately at 230°C for 25 min in the volumetric flask (Fig. 1). The CO2 produced during heating was quantified. For Fraction D, part of CO2 formed during heating was trapped in the cellular structure, thus further analysis was conducted on the residual CO2 using the method in Section 2.1.2. The total CO2 produced for this fraction is the sum of residual CO2 and CO2 evolved during heating (Wang, 2014).
3. Results and discussion 3.1 CO2 generation during coffee roasting During the roasting of the coffee beans at 230°C, the amount of CO2 generated increased with increasing roast time (Fig 2). In accordance with the findings from a previous studies involving a commercial fluidized bed roaster (Wang & Lim, 2014a), the residual CO2 increased with increasing roast degree up to dark roast, beyond which a decrease in residual CO2 was observed. In contrast, the amount of CO2 evolved during roasting increased monotonously with the roast degree. Fig. 2 also indicates that the ratio of the evolved CO2 to residual CO2 increased from 0.1 to 8.7 as heating increased from 5 to 30 min, respectively, , implying that the CO2 trapping ability of the coffee beans decreased as roasting proceed implying that the amount of CO2 trapped in the beans decreased as the 9
roasting progressed. This observation can be attributed to the diffusion loss of the CO2 due to the extended exposure of the beans to high temperature during roasting, as well as the depletion of CO2 precursors in the beans.
3.2 CO2 generation from model system 3.2.1 CO2 generation from CGA To gain insight on the thermal degradation behavior of CGA, green coffee bean CGA extract was heated under typical coffee roasting temperatures (210-270°C) and its weight loss was measured. Qualitatively, as the CGA was heated, it melted followed by bubbling as heating continued, indicating the generation of considerable gases and vapors. These phenomena are in agreement with the CGA pyrolysis studies of Shama et al. (2000; 2002). The weight loss data of CGA at different temperatures are summarized in Fig. 3a, showing that the weight loss increased exponentially (R2 > 0.97) with increasing heating time. Moreover, at any given heating time, the extent of weight loss was greater at higher temperature. The residual chars were further analyzed by HPLC to determine the residual CGA. At 230°C and above, the majority of CGA was degraded after three min of heating. At 210°C, total degradation occurred at around six min (Fig. 3b). Fig. 3c shows a sigmoidal relationship between CGA degradation and weight loss. Initial heating resulted in 2-3% weight loss with negligible CGA degradation, probably due to the desorption of moisture from the polyphenol. Further heating resulted in concomitant degradation of CGA and weight loss up to about 9% weight loss, indicating the formation of volatile compounds as the CGA underwent thermal degradation. Beyond this point, further heating did not induce further CGA degradation, indicating that all CGA was destructed. However, the weight loss continued as heating progressed, implying that the degraded CGA were converted into volatile compounds. 10
To investigate the changes in CGA’s molecular structure during thermal degradation, the residual char was analyzed by FTIR. As shown in Fig. 4a, the bands at 1681 and 1639 cm-1wavenumbers can be attributed to the stretching of C=O in quinic acid and caffeic acid moiety, respectively (Lyman et al., 2003; Jiang et al., 2009; Wang, Fu, & Lim, 2011). The bands located at 1597-1614 cm-1 and 1448 cm-1 represent the C=C vibrations outside and inside the aromatic ring of caffeic acid, respectively. The FTIR spectra of CGA after being heated at 230°C are shown in Fig. 4b. As shown, there are considerable changes in the molecular structure of CGA during heating; the bands for both hydroxyl (3313 and 3464 cm-1) and carbonyl groups (1681 and 1639 cm-1) progressively became smaller with increasing heating time. At 6 min, the C=O absorbance bands for both caffeic acid and quinic acid moieties disappeared. Based on these results, we speculate that during the heating process, CGA was first hydrolyzed to quinic acid and caffeic acid. The carbonyl group in the caffeic acid was decarboxylated to form CO2 and phenolic volatiles, such as 4-vinylcatechol (Clarke & Vitztbum, 2001; Rizzi & Boekley, 1993). Due to the lacking of the C=C bond in quinic acid as compared to caffeic acid, the carbonyl group in the former is expected to be more stable than the latter (Clarke & Vitztbum, 2001). However, the disappearing of C=O band in quinic acid moiety upon extended heating indicated that decarboxylation occurred as well during the heating process (Moon & Shibamoto, 2010). The amounts of CO2 generated during the heating of CGA are shown in Fig. 5a. About 10 mg of CO2 was detected from one gram of CGA at 230 °C. In addition, the amount of CO2 generated is dependent on the heating temperature. More than double amount of CO2 was produced at 270 °C compared with 230 °C (Fig. 5b). These findings are consistent with those reported by Sharma et al. (2002) who observed that CO2 is the major gas produced during the pyrolysis of CGA. As the CGA 11
content in green coffee is 3-12% (dry weight basis) (Campa, Doulbeau, Dussert, Hamon, & Noirot, 2005; Farah, De Paulis, Trugo, & Martin, 2005; Trugo & Macrae, 1984), the amount of CO2 generated from CGA in one gram of green coffee could roughly be calculated as 0.3-1.2 mg. The calculated value is equal to only ~8% yield based on the proposed degradation pathway above (hydrylysis and decarboxylation of caffeic acid). These magnitudes are considerably lower than that determined from coffee roasting experiment (42 mg CO2 /g green beans; Fig. 2). This analysis suggests that CGA is one of the CO2 precursors in coffee, but not the principal one. Next, to evaluate the thermal stability of CGA degradation byproducts - caffeic and quinic acids, these components were heated at 230 °C and the amount of CO2 generated were determined. After 25 min of heating, around 0.24 g CO2 was detected from one gram of caffeic acid. Assuming the CO2 generated is derived from decarboxylation of the carbonyl group, the yield would be ~98%. This value is consistent with the findings of Rizzi and Boekley (1993), who observed that cinnamic acids with p-hydroxyl substituent were susceptible to decarboxylation degradation upon heating, producing CO2. On the other hand, no CO2 was detected from the heating of quinic acid under the same temperature-time conditions. This result suggests that quinic acid is thermally more stable, or the degradation reaction does not take the decarboxylation route under the experimental conditions used. The quinic acid may undergo dehydration to form quinide (quinic acid lactone) during heating by losing a water molecule (Clarke & Vitztbum, 2001; Farah et al., 2005; Sharma et al., 2002). Considering these findings altogether, the lower yield of CO2 from CGA degradation as compared to caffeic acid suggests that the hydrolysis of CGA is probably the limiting step for CO2 formation under typical roasting conditions.
12
3.2.2 CO2 formation from Maillard reaction model compounds under coffee roasting conditions As shown in Fig. 5c, about 3.4 mg CO2 was generated from 0.004 mol of sucrose (equal to 1.369 g) at 230°C for 25 min. The amount of CO2 produced was doubled when 0.008 mol sucrose (equal to 2.738g) was heated. Considering the sucrose content in Arabica green coffee is 6-8.5% (Clarke and Vitztbum 2001), the theoretical amount of CO2 formed, due to the degradation of sucrose alone, would be 0.15-0.21 mg CO2/g green coffee. In comparison with the total CO2 generated for the green coffee beans (Fig. 2; about 35 mg CO2/g green coffee, 25 min heating at 230°C), it can be concluded that the contribution of sucrose alone to the formation of CO2 in coffee is minimal. On the other hand, Fig. 5c shows that the amounts of CO2 generated from glycine were higher than that of sucrose, but are still about two orders of magnitude lower than the CO2 generated from roasting of coffee beans. These findings show that the sucrose and amino acid alone are not the main precursors for the formation of CO2. However, when glycine and sucrose were mixed and heated, significantly larger amounts of CO2 were generated than those when the compounds were heated separately (Fig. 5d). For example, 172 mg CO2 was formed from the heating of 0.004 mol glycine-sucrose mixture, while only 12.4 mg CO2 was generated from separate heating of sucrose and glycine. Assuming that the CO2 is originated from the Strecker degradation reaction, the theoretical amount of CO2 formed would be 176 and 352 mg for 0.004 mol glycine-sucrose mixture and 0.008 mol glycine-sucrose mixture, respectively. These calculated values are in the similar orders of magnitude as the experimental data reported in Fig. 5d, implying that CO2 was mainly derived from the glycine structure by Strecker degradation. When mixture of 0.008 mol glycine and 0.004 mol sucrose was heated, 230.5 mg CO2 was generated, which is significantly lower than that generated from the mixture of 0.008 mol glycine 13
and 0.008 mol sucrose (330.1 mg) (Fig. 5d). This observation can be explained by the less amount of α-dicarbonyl compounds formed due to the reduced amount of sugar in the model system. During Strecker degradation, α-iminocarbonyls are produced due to nucleophilic addition of an amino group (from proteins and amino acids) to α-dicarbonyls from Maillard reactions (Rizzi, 2008). The α-iminocarbonyls then undergo decarboxylation, releasing CO2 gas. Considering that α-dicarbonyl compounds are the fission products of sugar from Maillard reaction, the decrease in sucrose content will decrease the available α-dicarbonyl reactants needed for producing the α-iminocarbonyl precursors, and therefore decreasing the amount of CO2 produced. In addition, Fig. 5d also shows that 214.7 mg CO2 was generated from the heating of 0.004 mol glycine and 0.008 mol sucrose mixture. This value is significantly higher than the theoretical value calculated (176 mg) from Strecker degradation pathway, suggesting that there might be other pathways, in addition to Strecker degradation, is available for the formation of CO2 from sugars in the presence of amino acid. The similar phenomenon was observed in the study by Cole (1967), where CO2 was detected in the glucose-amines system (Strecker degradation is impossible).
3.2.3 CO2 formation in glycine-sucrose-CGA model system To elucidate how CGA interacts with glycine and sucrose in affecting the Maillard and Strecker reactions, more experiments were conducted to determine the amount of CO2 generated from different combinations of glycine-sucrose-CGA mixture. As shown in Fig. 5e, the addition of CGA to the Maillard reaction model system caused a decrease in CO2 formation in the four mixtures investigated, suggesting that the CGA suppressed the Maillard reaction between the amino acid and sugar. The CGA’s suppression behavior on the formation of Maillard flavor compounds, such as pyrazines and Strecker aldehydes, was also observed by other researchers (Jiang et al., 2009; Jiang & 14
Peterson, 2010; Wang & Ho, 2013; Wang, 2000). Wang (2000) suggested that the underlining mechanism by which the hydroxycinnamic acids altered Maillard flavor generation was due to their radical scavenging capability, because the formation of some aroma compounds, such as pyrazines, is based on the radical pathway (Berger, 2007). In addition, these phenolic antioxidants may alter the reaction redox potential, which will influence the generation of sugar fragments (Yaylayan, 2003). In addition to redox reactions, Jiang et al. (2009, 2010) proposed that free hydroxycinnamic acids can undergo decarboxylation, exposing the vinyl group that participates in various pericyclic reactions with Maillard intermediates to produce phenolic-Maillard adducts. In summary, the above results indicate that instead of being a major precursor of CO2, CGA to some extent has the ability to suppress the formation of CO2 from Maillard reactions and Strecker degradation.
3.3 Fragmentation of green coffee components and their contribution to CO2 formation The generation of significant amount of CO2 through Maillard reactions under typical coffee roasting conditions implies that free amino acid and sugar are the key contributing precursors for the formation of CO2. However, typical free amino acid (0.2-0.8%) and sugar (6-9%) contents in green coffee beans are too low to account for the large amount of CO2 generated during roasting. Other components must have also contributed to the formation of CO2. To elucidate the roles of other green bean components, an isolation procedure was adopted to separate the green coffee components into Fractions A, B, C, and D. The contribution of each of these fractions to CO2 formation was investigated. The weight percentage values for each fraction are summarized in Fig. 6a, showing that 68.5% of the green bean is made up of water-insoluble (Fraction D) materials, mainly consisting of water 15
insoluble proteins and high molecular weight polysaccharides. The water-soluble Fraction A can be further fractionated into ethanol-insoluble (Fraction C; 6.3%) and ethanol-soluble (Fraction B; 25.2%). Fractions A, B, C and D obtained were subjected to heating at 230°C for 25 min, during which the CO2 evolved was determined (Fig. 6b). As shown, Fraction B contributed to 48% of the CO2 formed during the roasting process, while Fraction C accounted for 7%. On the other hand, the dominant mass Fraction D contributed to 22% of the CO2 generated. Using the same fractionation procedure, De Maria et al. (1994; 1996) determined that Fraction B is mainly consisting of small molecular weight compounds such as sucrose, CGA, amino acids and peptides, while Fraction C is mainly made up of water-soluble proteins and arabinogalactans. Accordingly, we can deduct that the majority of CO2 generated during roasting was originated from the water-soluble fraction, primarily from Maillard reactions and CGA degradation. The smaller but significant amount of CO2 from Fraction C could be originated from the Maillard reactions between monosaccharide from arabinogalactan degradation and ε-amino group of lysine and the α-amino groups of terminal amino acids in proteins (Berger, 2007). In comparison, CO2 formation from Fraction D can be attributed to the thermal degradation of cellulose. Here, compounds with carbonyl and carboxyl groups are formed through depolymerization and oxidation reactions (Shafizadeh & Bradbury, 1979;Brebu & Vasile, 2010). By comparing the total CO2 produced from Fractions A and D to the amount of CO2 generated during coffee roasting, an unaccounted quantity of about 24% of CO2 from coffee roasting was observed. The observation is indicative of the existence of component-interaction between these two fractions. The unaccounted CO2 fraction can also be attributable to the different reactions occurred during the roasting of coffee beans versus the extracted fractions. Specifically, the pressure buildup 16
within the whole beans that causes the beans to crack was absent during the heating of the fraction samples. Moreover, during roasting of beans, the interior of the beans is depleted of O2 due to rapid oxidation reactions, while the surface of the beans continued to be exposed to oxygen-rich hot air. These conditions are absent during the heating of Fractions B, C and D. In summary, although CGA is a major carbonyl containing compound in green coffee, its contribution to CO2 generation is very limited, while Maillard reactions is confirmed to be the main pathway. In addition to free amino acids and sugar, high molecular polysaccharide and protein are also involved in the CO2 generation. Lower molecular compounds which represent ~25% content in green coffee generate about 50% CO2 and thus are the major precursors of CO2.
4. Acknowledgement We gratefully acknowledge the financial and material supports from Mother Parkers Tea & Coffee Inc. and Natural Sciences and Engineering Research Council of Canada (NSERC).
Reference Anderson, B. A., Shimoni, E., Liardon, R., & Labuza, T. P. (2003). The diffusion kinetics of carbon dioxide in fresh roasted and ground coffee. Journal of Food Engineering, 59, 71–78. Baggenstoss, J., Poisson, L., Luethi, R., Perren, R., & Escher, F. (2007). Influence of water quench cooling on degassing and aroma stability of roasted coffee. Journal of Agricultural and Food Chemistry, 55, 6685–6691. Baltes, W., & Bochmann, G. (1987a). Model reactions on roast aroma formation-mass spectrometric identification of pyridines from the reaction of serine and threonine with sucrose under the conditions of coffee roasting. Z. Lebensm. Unters. Forsch., 185(1), 5–9. Baltes, W., & Bochmann, G. (1987b). Model reactions on roast aroma formation-reaction of serine and threonine with sucrose under the conditons of coffee roasting and identification of new coffee aroma compounds. Journal of Agricultural and Food Chemistry, 35(3), 340–346. 17
Baltes, W., & Mevissen, L. (1988). Model reactions on roast aroma formation - volatile reaction products from the reaction of phenylalanine with glucose during cooking and roasting. Z. Lebensm. Unters. Forsch., 187, 209–214. Berger, R. G. (2007). Flavours and Fragrances: Chemistry, Bioprocessing and Sustainability. New York: Springer. Brebu, M., & Vasile, C. (2010). Thermal degradation of lignin- a review. Cellulose Chemistry and Technology, 44(9), 353–363. Campa, C., Doulbeau, S., Dussert, S., Hamon, S., & Noirot, M. (2005). Qualitative relationship between caffeine and chlorogenic acid contents among wild Coffea species. Food Chemistry, 93, 135–139. Casal, S., Oliveira, M. B., & Ferreira, M. A. (2000). HPLC/diode-array applied to the thermal degradation of trigonelline, nicotinic acid and caffeine in coffee. Food Chemistry, 68, 481–485. Clarke, R. J., & Macrae, R. (1987). Coffee-Volumen 2: Technology. Elsevier Science Publishers Ltd. New York: Elsevier Applied Science. Clarke, R. J., & Vitztbum, O. J. (2001). Coffee: Recent Development. Oxford: Blackwell Science. Cole, S. J. (1967). The Maillard reaction in food products carbon dioxide production. Journal of Food Science, 32(3), 245–250. De Maria, C. A. B., Trugo, L. C., Aquino Neto, F. R., Moreira, R. F. A., & Alviano, C. S. (1996). Composition of green coffee water-soluble fractions and identification of volatiles formed during roasting. Food Chemistry, 55(3), 203–207. De Maria, C. A. B., Trugo, L. C., Moreira, R. F. A., & Werneck, C. C. (1994). Composition of green coffee fractions and their contribution to the volatile profile formed during roasting. Food Chemistry, 50(2), 141–145. Farah, A., De Paulis, T., Trugo, L. C., & Martin, P. R. (2005). Effect of roasting on the formation of chlorogenic acid lactones in coffee. Journal of Agricultural and Food Chemistry, 53, 1505–1513. Geiger, R., Perren, R., Kuenzli, R., & Escher, F. (2005). Carbon dioxide evolution and moisture evaporation during roasting of coffee beans. Journal of Food Scinece Food Engineering and Physical Properties, 70(2), 124–130. Jiang, D., Chiaro, C., Maddali, P., Prabhu, K. S., & Peterson, D. G. (2009). Identification of hydroxycinnamic acid-Maillard reaction products in low-moisture baking model systems. Journal of Agricultural and Food Chemistry, 57, 9932–9943. Jiang, D., & Peterson, D. G. (2010). Role of hydroxycinnamic acids in food flavor: a brief overview. Phytochemistry Reviews, 9, 187–193. Kunert-Kirchhoff, J., & Baltes, W. (1990). Model reactions on roast aroma formation-specific products of phenylalanine after cooking L-Phenylalanine with D-glucose in a laboratory autoclave. Z. Lebensm. Unters. Forsch., 190, 9–13. Lyman, D. J., Benck, R., Dell, S., Merle, S., & Murray-Wijelath, J. (2003). FTIR-ATR analysis of brewed coffee: effect of roasting conditions. Journal of Agricultural and Food Chemistry, 51(11), 3268–3272. Moon, J.-K., & Shibamoto, T. (2010). Formation of volatile chemicals from thermal degradation of less volatile coffee components: quinic acid, caffeic acid, and chlorogenic acid. Journal of Agricultural and Food Chemistry, 58(9), 5465–5470. Rizzi, G. P. (2008). The Strecker degradation of amino acids: newer avenues for flavor formation. 18
Food Reviews International, 24, 416–435. Rizzi, G. P., & Boekley, L. J. (1993). Flavor chemistry based on the thermally-induced decarboxylation of p-hydroxylcinnamic acids. In Food Flavors, Ingredients and Composition (pp. 663–670). Amsterdam, Netherlands: Elsevier Science Publishers. Shafizadeh, F., & Bradbury, A. G. W. (1979). Thermal degradation of cellulose in air and nitrogen at low temperatures. Journal of Applied Polymer Science, 23, 1431–1442. Sharma, R. K., Fisher, T. S., & Hajaligol, M. R. (2002). Effect of reaction conditions on pyrolysis of chlorogenic acid. Journal of Analytical and Applied Pyrolysis, 62(2), 281–296. Sharma, R. K., Hajaligol, M. R., Smith, P. A. M., Wooten, J. B., & Baliga, V. (2000). Characterization of char from pyrolysis of chlorogenic acid. Energy and Fuels, 14, 1083–1093. Shimoni, E., & Labuza, T. P. (2000). Degassing kinetics and sorption equilibrium of carbon dioxide in fresh roasted and ground coffee. Journal of Food Process Engineering, 23, 419–436. Small, L. E., & Horrel, R. S. (1993). High yield coffee technology. In 15th ASIC Colloquium (pp. 719–726). Montpellier: ASIC, Paris, France. Stadtman, F. H., Chichester, C. O., & Mackinney, G. (1952). Carbon dioxide production in the browning reaction. Journal of the American Chemical Society, 74, 3194–3196. Trugo, L. C., & Macrae, R. (1984). A study of the effect of roasting on the chlorogenic acid composition of coffee using HPLC. Food Chemistry, 15, 219–227. Wang, N., Fu, Y., & Lim, L.-T. (2011). Feasibility study on chemometric discrimination of roasted Arabica coffees by solvent extraction and Fourier transform infrared spectroscopy. Journal of Agricultural and Food Chemistry, 59(7), 3220–3226. Wang, X., & Lim, L.-T. (2014a). A kinetics and modeling study of coffee roasting under isothermal conditions. Food and Bioprocess Technology, 7, 621–632. Wang, X., & Lim, L.-T. (2014b). Effect of roasting conditions on carbon dioxide degassing behavior in coffee. Food Research International, 61, 144–151. Wang, X. (2014). Understanding the formation of CO2 and its degassing behaviours in coffee (Doctoral dissertation). University of Guelph. Wang, Y. (2000). Effects of naturally occurring phenolic compounds on the formation of Maillard Aroma. Rutgers University. Wang, Y., & Ho, C.-T. (2013). Effects of naturally occuring phenolic compounds in coffee on the formation of Maillard aromas. In C.-T. Ho, C. Mussinan, F. Shahidi, & E. T. Contis (Eds.), Nutrition, Functional and Sensory Properties of Foods (pp. 98–110). Cambridge: The Royal Society of Chemistry. Wiiken, C., & Baltes, W. (1990). Model reaction on roast aroma formation-formation of pyrrole-2, 5-diones by roasting of aspartic acid and asparagine. Z. Lebensm. Unters. Forsch., 191, 116–118. Yaylayan, V. A. (2003). Recent advances in the chemistry of Strecker degradation and amadori rearrangement: implications to aroma and color formation. Food Science and Technology Research, 9(1), 1–6.
19
Valve
N2 Two-hole rubber stopper
Drierite column
Volumetric flask
Samples Ascarite II column
GC oven
Fig. 1. Setup for roasting and evolved CO2 determination
20
Very dark
Residual CO2 Residual CO2
40
9
Evolved EvolvedCO2 CO2
35
Very dark
Evolved/residualCO2 CO2 Evolved/residual
8 Very dark
7
30
6
25
5
Medium-dark
20
4
15
3 Light
10 5
10
2
Very light
1
0
Evolved/residual CO2 ratio
Residual or evolved CO2 amount (mg/g green beans)
45
0 5
10
15 20 Roasting time (min)
25
30
Fig. 2. CO2 formation (evolved+ residual) and the ratio of evolved to residual CO2 amount during coffee roasting under roasting temperature of 230 °C by using the apparatus as shown in Fig. 1. The degree of roast was determined by roast loss and L* value according to Wang & Lim (2014a).
21
210°C
230°C
250°C
270°C
1
a
Weight loss (Wt/W0)
0.98 0.96 0.94
y = -0.022ln(x) + 0.981 R² = 0.9756
0.92 0.9
y = -0.027ln(x) + 0.9675 R² = 0.987
0.88
y = -0.037ln(x) + 0.9645 R² = 0.9874
0.86
y = -0.043ln(x) + 0.9455 R² = 0.9799
0.84 0.82 1
210 °C
230 °C
250 °C
10 Time (min) 270 °C
210 °C
1.2
0.6 0.4 0.2 0 1
2
3
6
9
250 °C
270 °C
c CGA degradatio ratio (CGA residual/CGA original)
CGA degradation ratio (CGA residual/CGA original)
0.8
230 °C
1.2
b 1
0
100
1 0.8 0.6 0.4 0.2 0
15
0
Roasting time (min)
5
10 Weight loss (%)
15
20
Fig. 3. (a) Semi-log plot of weight loss kinetics data of CGA in roasting temperatures of 210, 230, 250, and 270 °C, with the data fitted with logarithmic equation; (b) Plots of CGA degradation versus roasting time at 210, 230, 250, and 270 °C; (c) Plot of CGA degradation versus weight loss.
22
a
1681 cm-1 1639 cm-1 1597-1614 cm-1
O HO
C
HC
HC
OH
1448 cm-1
OH
Caffeic acid H
OH
H
H
HO H
HO
H
H
H OOC
Quinic acid
H OH O H HOOC
Chlorogenic acid 3600
HO
3100
C
O
H
H
H
HO
H
H
HC
HC
OH
H OH OH
2600
2100
1600
1100
600
Wavenumber (cm-1)
b 3464 cm-1
1681 cm-1 1639 cm-1 1597-1614 cm-1
3313 cm-1
1448 cm-1
0 min 1 min 2 min 3 min 6 min 9 min 15 min 3600
3100
2600
2100
1600
1100
600
Wavenumber (cm-1)
Fig. 4. (a) FTIR spectra of pure CGA, caffeic acid and quinic acid, showing the wavenumbers of C=O stretching in quinic acid (1681 cm-1) and caffeic acid moieties (1639 cm-1); (b) FTIR spectra of CGA after being heated at temperature of 230°C for various times, showing the vanishing of C=O stretching in both quinic acid and caffeic acid moieties.
23
a
30
CO2 formed (mg)
CO2 formed (mg)
20 15 10
25 20 14.5
15 10
5
5
0
0 0
1 2 CGA amount (g) Sucrose
20.0 18.0 16.0 14.0 12.0 10.0 8.0 6.0 4.0 2.0 0.0
3
230 °C for 20 min
0 mol glycine
c
Glycine
400.0
17.5
Amount of CO2 formed (mg)
Amount of CO2 formed (mg)
30.7
30
y = 9.5791x + 0.6024 R² = 0.9993
25
b
35
8.9 6.3 3.4
270 °C for 20 min
0.004 mol glycine
d
0.008 mol glycine 330.1
350.0 300.0 230.5
214.7
250.0 171.9
200.0 150.0 100.0 50.0
3.4
6.3
0.0 0.004 0.008 Amount of sucrose (mol)
0.004 0.008 Amount of sugar or amino acid (mol) 400 0 mol CGA
0.004 mol CGA
0.008 mol CGA
e
Amount of CO2 formed (mg)
350 300 250 200 150 100 50 0 0.004 +0.004
0.004+0.008
0.008+0.004
0.008+0.008
Sucrose+ Glycine mixture (mol)
Fig. 5. (a) Amount of CO2 formation from CGA degradation at 230 °C; (b) Amount of CO2 formation from 1.5 g CGA degradation at 230 °C and 270°C; (c) Amount of CO2 formed from heating of sucrose and glycine under coffee roasting temperature of 230°C for 25 min; (d) Amount of CO2 formed from roasting of sucrose-glycine mixture under coffee roasting temperature of 230 °C for 25 min; (e) Amount of CO2 generation in glycine-sucrose-CGA model system. 24
a
6.30%
25.20%
Water insoluble (fraction D) 80% ethanol soluble (fraction B)
68.50%
80% ethanol insoluble (fraction C)
b
23.56%
21.50%
Water insoluble (fraction D) 80% ethanol soluble (fraction B) 80% ethanol insoluble (fraction C)
7.00%
Interaction between A and D
47.94%
Fig. 6. (a)The weight percentages of coffee fraction D, B and C, calculated as: the weight of fraction obtained/weight of green coffee beans used (dry weight)×100%; (b) The contribution of each fraction to the amount of CO2 formed during coffee roasting, calculated as: the amount of CO2 generated from the fraction/the amount of CO2 generated during coffee roasting ×100%. Fraction A=fraction B + fraction C. The interaction between A and D was calculated as the difference between amount of CO2 generated from coffee roasting and total amount of CO2 generated from heating of each fraction.
25
Highlights •
A method was developed to study the evolved CO2
•
CGA is shown not a major precursor of CO2
•
Mailard reaction is the main pathway for CO2 formation
•
About 50% of CO2 was from lower molecular weight compounds
26