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Chemical Changes in the Components of Coffee Beans during Roasting
C H A P T E R
10
Chemical Changes in the Components of Coffee Beans during Roasting Feifei Wei1,2, Masaru Tanokura1 1Department
of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo, Japan; 2Japan Society for the Promotion of Science, Chiyoda-ku, Tokyo, Japan
List of Abbreviations
10.2 COMPONENTS IN ROASTED COFFEE BEANS
CQA Caffeoylquinic acid CQL Caffeoylquinic acid lactone diCQA Dicaffeoylquinic acid FQA Feruloylquinic acid FQL Feruloylquinic acid lactone GABA γ-Aminobutyrate HMF 5-Hydroxymethyl-2-furfural HMFA 5-Hydroxymethyl-2-furoic acid HPLC High-performance liquid chromatography kDa Kilo dalton NMR Nuclear magnetic resonance pCoQA p-Coumaroylquinic acid pCoQL p-Coumaroylquinic acid lactone
The general differences between the compositions of green and roasted coffee beans are shown in Table 10.13 and Figure 10.1.4 Although the compositions vary for coffee beans of different species, origins, roasting degrees, or analytical methods, the degradation of polysaccharides, oligosaccharides (especially sucrose), chlorogenic acids, and trigonelline is commonly observed. It can be considered that the formation of the characteristic aroma, flavor, taste, and color of the roasted coffee bean result first from the dramatically decreased green coffee bean components such as sucrose, free amino acids, chlorogenic acids, and trigonelline, as well as from the breakdown of polysaccharides and proteins.
10.1 INTRODUCTION Roasting is probably the most important factor in the development of the complex flavors that make coffee enjoyable. During the roasting process, the beans undergo many complex and poorly defined chemical reactions, leading to important physical changes and to the formation of the substances responsible for the sensory qualities of the beverage. The roasted coffee beans are composed of carbohydrates, protein fragments, low-molecular-weight acids, caffeine, trigonelline, lipids, many unknown molecules usually called melanoidins, and more than 900 volatile compounds mainly formed during the roasting process.1,2 This chapter describes the changes of components in the coffee beans and the main reactions that take place during roasting process.
Coffee in Health and Disease Prevention http://dx.doi.org/10.1016/B978-0-12-409517-5.00010-3
10.3 THE ROASTING DEGREES To produce high-quality coffee beans, the roasting degree is probably the single most important factor. The longer the coffee beans are held in the roaster and/ or the higher the roasting temperature, the darker the coffee beans. The roasting degrees can be measured by judging the bean’s color by the naked eye or a colorimeter, or by the weight loss of water after roasting. As shown in Table 10.2,3 the higher the degree of roasting, the lower the L value (for lightness in the CIELAB color space).
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10. CHEMICAL CHANGES DURING ROASTING
TABLE 10.1 Chemical Components of Green Coffee Beans and Roasted Coffee Beansa,3 Arabica
Robusta
Components
Green Coffee Bean
Roasted Coffee Bean
Green Coffee Bean
Roasted Coffee Bean
Polysaccharides
50.0–55.0
24.0–39.0
37.0–47.0
–
Oligosaccharides
6.0–8.0
0–3.5
5.0–7.0
0–3.5
Lipids
12.0–18.0
14.5–20.0
9.0–13.0
11.0–16.0
Free amino acids
2.0
0
2.0
0
Proteins
11.0–13.0
13.0–15.0
11.0–13.0
13.0–15.0
Chlorogenic acids
5.5–8.0
1.2–2.3
7.0–10.0
3.9–4.6
Caffeine
0.9–1.2
0–1.0
1.6–2.4
0–2.0
Trigonelline
1.0–1.2
0.5–1.0
0.6–0.8
0.3–0.6
Fatty acids
1.5–2.0
1.0–1.5
1.5–2.0
1.0–1.5
Minerals
3.0–4.2
3.5–4.5
4.0–4.5
4.6–5.6
Melanoidins
–
16.0–17.0
–
16.0–17.0
aPercentage
of the dry weights.
FIGURE 10.1 Roasting changes in the compositions of the arabica coffee bean extracts, as analyzed by nuclear magnetic resonance. The concentrations of roasted coffee bean extract components were compared to those in green coffee bean extract. Light gray indicates the concentrations in green coffee bean extract, whereas dark gray indicates those in roasted coffee bean extract. α-(1-3)-lAraf, α-(1-5)-l-Araf, β-(1-3)-d-Galp, β-(1-6)-d-Galp, and β-(1-4)-dManp indicate α-(1-3)-l-arabinofuranose, α-(1-5)-l-arabinofuranose, β-(1-3)-d-galactopyranose, β-(1-6)-d-galactopyranose and β-(1-4)-dmannopyranose, respectively.4
TABLE 10.2 Roasting Degrees of Coffee Beans3 Roasting Degree
Roasting Name
L Valuea
Weight Loss (% of Green Coffee Bean)
Light roasting degree
Light roast
30.2
10.0–14.0
Cinnamon roast
27.3
Medium roast
24.2
High roast
21.5
City roast
18.5
Fully city roast
16.8
French roast
15.5
Italian roast
14.2
Medium roasting degree
Dark roasting degree aThe
14.0–17.0
17.0–21.0
>21.0
L value ranges from 0 (black) to 100 (white).
10.4 CHANGES OF CARBOHYDRATES Sucrose, which is the most abundant simple carbohydrate present in green coffee beans, acts as an aroma precursor during roasting, generating several classes of compounds, such as carboxylic acids, furans, and aldehydes, which will affect the flavor of coffees.5–7 Sucrose is found to be destroyed quickly at the early stage of roasting, as shown in Figure 10.2(A).8 At the same time, decreases in citric acid and malic acid and steady increases in lactic, acetic, and formic acids are also observed during coffee bean roasting, as shown in Figure 10.2(B).8 By means of model reactions, it has been confirmed that sucrose is the major source of the aliphatic acids (formic, acetic, glycolic, and lactic) produced during coffee roasting.6 Based on the results of these model reactions, a scheme for a sugar fragmentation pattern that would yield the principle aliphatic acids was derived and is shown in Figure 10.3.6 As can be seen in the figure, the glucose and fructose can interconvert via the 1,2-endiol as intermediate, and the fructose can also rearrange to give the 2,3-endiol. Because the thermal dehydration of these sugars to give 1-deoxyglucosone and 3-deoxyglucosone is well known, these four intermediate compounds can be regarded as acid precursors. As suggested by the 13C-labelled model reactions, the principal acids (formic, acetic, glycolic, and lactic) generated on roasting coffee are formed from carbohydrates. The principal precursor of these acids in green coffee beans is sucrose, although the polysaccharide fraction, particularly arabinogalactan, also makes a contribution. Furthermore, the acids are readily formed by thermal treatment of the pure carbohydrates, which suggests that secondary reactions with other components of the green bean do not play a key role in the acid formation during roasting.6
I. INTRODUCTORY AND GENERAL TEXT
10.5 Changes of Chlorogenic Acids
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been used successfully as a chemical index in ensuring adequate heat processing or for monitoring storage conditions for foods. Formation of HMF from carbohydrates has been found to depend on many factors, such as time, water activity, temperature, and the amount and type of catalyst and sugar used. The isotope studies revealed that the formation of HMF during coffee roasting also occurs through the cleavage of sucrose to monomeric carbohydrates and then to glyceraldehyde and methylglyoxal.9 In fact, HMF and 5-hydroxymethyl-2-furoic acid (HMFA) are both formed during the roasting of coffee and decomposed rather quickly upon further roasting, based on the observations by high-performance liquid chromatography (HPLC) (Figure 10.4).10 In summary, sucrose changes into aliphatic acids and aroma compounds during coffee bean roasting, as shown in Figure 10.5. On a dry-weight basis, almost half of green coffee beans are reported to be made of polysaccharides, which include cellulose, mannan, and arabinogalactan.11 In the green coffee beans, polysaccharides are retained in the coffee bean cell wall as part of the insoluble polysaccharide complex.12 The roasting process increases the solubility of both arabinogalactans and mannans from the bean by loosening the cell-wall structure as it swells and by depolymerization of the polysaccharides during the roasting process. The nuclear magnetic resonance (NMR) studies of coffee bean extracts in water revealed both the changes in solubility and the thermal stability of polysaccharides during the roasting process.4,8,13 As shown in Figure 10.2(C), the roasting process increases the solubility of both arabinogalactans and mannans from the beans by loosening the cell-wall structure as it swells and by depolymerization of the polysaccharides during the roasting process. The degradation of α-(1-3)-l-arabinofuranose, α-(1-5)-l-arabinofuranose, β-(1-3)-d-galactopyranose, and β-(1-6)-d-galactopyranose units from arabinogalactans after light roasting could be attributed to the higher thermal lability of arabinogalactans compared to that of the β-(1-4)-d-mannopyranose from mannan, which was less susceptible to degradation, even after a dark roast.14 The water-soluble polysaccharides that appear after roasting, which play an important role in the retention of volatile substances, contribute to the viscosity of the coffee brew and thus to the creamy sensation known as “body” in the mouth.15
FIGURE 10.2 Evolutions of (A) sucrose; (B) citric acid, malic
acid, lactic acid, acetic acid, and formic acid; and (C) β-(1-4)-dmannopyranose, α-(1-3)-l-arabinofuranose, α-(1-5)-l-arabinofuranose, β-(1-3)-d-galactopyranose, and β-(1-6)-d-galactopyranose in the extracts of coffee beans during the coffee bean roasting process observed by nuclear magnetic resonance signal intensity changes.8
5-Hydroxymethyl-2-furfural (HMF) is one of the major degradation products of carbohydrates and has been studied extensively as an indicator of heat damage. It has
10.5 CHANGES OF CHLOROGENIC ACIDS It has been reported that there are at least 30 chlorogenic acids in coffee beans, including caffeoylquinic acids (CQA), dicaffeoylquinic acids, feruloylquinic acids, and p-coumaroylquinic acids.16 During roasting, the levels of CQAs decreased considerably, as observed by NMR
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10. CHEMICAL CHANGES DURING ROASTING
FIGURE 10.3 Reaction scheme for acid formation from primary thermal degradation products of glucose.6
spectroscopy, whereas the levels of quinic acid and of γ-quinide and syllo-quinic acid increased during the roasting process (Figure 10.6(A) and (B)).8 The decomposition of chlorogenic acids could be used as an index of roasting degree. Quinic acid is a dominant acid in green coffee beans and is also a product of the degradation of chlorogenic acids (Figure 10.6(C)). During the roasting process, γ-quinide was formed as an internal ester of the quinic
acid; syllo-quinic acid, which is another isomeric product of quinic acid, was also produced under the prevalent conditions of roasting, which include a thermal atmosphere, low water content, and moderate acidity.8,17 In addition to the lactone of quinic acid, lactones of chlorogenic acids were also detected during the coffee bean roasting process. As shown in Table 10.3,18 feruloylquinic acid lactones, caffeoylquinic acid lactones, and p-coumaroylquinic acid
I. INTRODUCTORY AND GENERAL TEXT
10.7 Changes of Proteins and Free Amino Acids
FIGURE 10.4 Formation of 5-hydroxymethyl-2-furfural
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(HMF) and 5-hydroxymethyl-2-furoic acid (HMFA) during the roasting of coffee beans.10
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FIGURE 10.5 Changes of sucrose during the roasting of coffee beans. HMF, 5-hydroxymethyl-2-furfural; HMFA, 5-hydroxymethyl2-furoic acid.
lactones were formed during coffee bean roasting, and their formation was highly dependent on the degree of roasting. The optimum degree of roasting to achieve a maximum amount of lactones in coffee is a light medium roast, whereas darker roasts yield lower amounts.18 The other product of chlorogenic acids after roasting is cinnamic acids.19 The cinnamic acids released from degradation of chlorogenic acids may participate in the subsequent chemical reactions to form other flavor components.20
10.6 CHANGES OF TRIGONELLINE Trigonelline is a pyridine derivative known to contribute indirectly to the formation of desirable flavor products, including furans, pyrazine, alkyl-pyridines, and pyrroles, during coffee roasting. The importance of
trigonelline, not only as a precursor of flavor and aroma compounds but also as a beneficial nutritional factor, has been well documented in previous studies.21,22 Reports on the thermal degradation of trigonelline have revealed nicotinic acid and nicotinamide, as well as their O- and N-methyl derivatives, as reaction products when trigonelline is heated in a sealed tube.22 In fact, studies confirmed that N-methylpyridinium and nicotinic acid are the major nonvolatile products of trigonelline pyrolysis by both mass spectrometry as well as NMR spectroscopy.4,23 The time courses of trigonelline, N-methylpyridinium, and nicotinic acid were observed by NMR spectroscopy. As shown in Figure 10.7,8 trigonelline, which is present at high levels in green coffee beans, decreased continuously during the roasting process. N-methylpyridinium and nicotinic acid, two thermal decomposition products of trigonelline, increased continuously during the roasting, with the N-methylpyridinium as the major thermal product. The decrease in the N-methylpyridinium level after roasting for 7 min was probably attributable to further decomposition and/or interaction with other thermolytic products. Nicotinic acid, which is an important vitamin as well as the second major thermal degradation product of trigonelline, was positively correlated with the roasting degree. Trigonelline and its thermolytic products undoubtedly have direct and indirect effects on other physicochemical properties of a cup of coffee, such as flavor and aroma.
10.7 CHANGES OF PROTEINS AND FREE AMINO ACIDS The Maillard reaction is a chemical reaction between reducing carbohydrates and various amino acids, peptides, and proteins, which contain free amino groups.
I. INTRODUCTORY AND GENERAL TEXT
88
10. CHEMICAL CHANGES DURING ROASTING
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FIGURE 10.6 Evolutions of coffee components during the roasting process. (A) 5-caffeoylquinic acid, 4-caffeoylquinic acid, 3-caffeoylquinic acid; (B) quinic acid, γ-quinide, syllo-quinic acid in the extracts of coffee beans during the coffee bean roasting process observed by nuclear magnetic resonance signal intensity changes. The integral value of the signal due to caffeine was set to a constant of 100; (C) evolution of 5-caffeoylquinic acid during coffee bean roasting.8
It is chiefly responsible for the development of unique aromas and tastes during the thermal processing of foods. Although the Maillard reaction was discovered more than a century ago, its products and pathways are still being studied. The composition of coffee proteins has also been shown to be profoundly changed by the roasting of the green coffee bean.24 The major proteins present in green arabica coffee infusions had molecular weights of 58 and 38 kDa.12 From roasted coffees, only a defined band of <14 kDa and a diffuse band of >200 kDa were observed.12,25 This means roasting leads to protein denaturation with degradation. At the same time, the green coffee bean protein subunits are integrated into the polymeric structure of melanoidins formed during roasting.26 The melanoidins are defined as brown, highmolecular-weight products containing nitrogen and are end products of the Maillard reaction. They are a heterogeneous group of food polymers with a structural diversity that has hindered a definitive chemical definition. Polysaccharides and proteins have a key role in the formation of the most abundant population of coffee brew melanoidins.27 However, recent studies show that coffee melanoidins incorporate a number of compounds in addition to polysaccharides and proteins, including chlorogenic acids and amino acid/protein fragments. In arabica-roasted coffee infusions, strong associations were observed between phenolics and brown compounds and the polymeric material. The different chemical compositions of melanoidin populations isolated from roasted coffee brews strongly suggest that there are several pathways through which coffee melanoidins are formed.28–30 On the other hand, the free amino acid content appears to be rather low, in the range of 0.3–0.6% on a dry weight basis in green coffee beans,31 or 0.1–0.7 mM in the green coffee bean extracts analyzed by NMR.13 There is no information available to suggest that these free amino acids are either required or sufficient for the generation of the coffee aroma. The time courses of some amino acids have been monitored by NMR spectroscopy. As shown in Figure 10.8, free amino acids are decreased during roasting. The four main free amino acids, l-glutamic acid, l-alanine, l-asparagine, and γ-aminobutyrate, in the green coffee beans disappeared after roasting.8
10.8 FORMATION OF AROMA COMPONENTS Green coffee beans lack the color and characteristic aroma of roasted coffee, both of which are formed during the roasting process. Coffee oil, which comprises about 10% of the roasted beans, carries most of the coffee aroma.2 The aroma is made up of a complex mixture of volatile compounds. The principal classes of aroma compounds in roasted ground coffee are listed in Table 10.4.2
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10.8 Formation of Aroma Components
TABLE 10.3 Chlorogenic Acid Lactone Content in Green and Roasted Coffee Beansa,18
Arabica (Brazil)
Robusta (Uganda)
Roasting Degree
3-CaffeoylQuinic Acid Lactone
4-CaffeoylQuinic Acid Lactone
3-FeruloylQuinic Acid Lactone
4-FeruloylQuinic Acid Lactone
3,4-diCaffeoylQuinic Acid Lactone
3-pCoumaroylQuinic Acid Lactone
4-pCoumaroylQuinic Acid Lactone
Green
Ndb
Nd
Nd
Nd
Nd
Nd
Nd
Verylight
71.6 ± 8.7
25.6 ± 1.6
6.5 ± 1.3
1.9 ± 0.2
2.8 ± 0.2
2.1 ± 0.3
0.5 ± 0.0
Light
160.2 ± 2.3
92.1 ± 2.9
14.1 ± 1.2
7.0 ± 0.4
4.1 ± 0.4
4.6 ± 0.4
5.3 ± 0.2
Lightmedium
248.5 ± 9.4
115.3 ± 1.0
28.3 ± 2.0
13.4 ± 0.7
6.6 ± 1.2
7.5 ± 0.4
7.8 ± 0.2
Darkmedium
146.6 ± 9.8
87.7 ± 2.1
29.8 ± 1.6
9.6 ± 0.4
2.1 ± 1.1
7.3 ± 0.7
6.6 ± 0.4
Dark
95.7 ± 4.7
53.1 ± 3.5
21.2 ± 1.9
7.8 ± 0.3
1.0 ± 0.2
4.4 ± 0.2
4.0 ± 1.6
Verydark
59.6 ± 10.0
24.8 ± 2.6
15.8 ± 6.0
6.2 ± 0.4
0.7 ± 0.06
2.3 ± 0.2
3.8 ± 0.6
Green
Nd
Nd
4.0 ± 0.0
9.1 ± 0.8
Verylight
57.7 ± 4.4
25.8 ± 1.4
6.4 ± 0.4
16.4 ± 1.7
Light
198.8 ± 4.4
110.0 ± 2.1
18.4 ± 0.7
23.6 ± 1.0
Lightmedium
253.6 ± 4.3
138.8 ± 2.0
31.1 ± 1.3
25.4 ± 1.0
Darkmedium
165.4 ± 10.6
125.9 ± 8.8
16.6 ± 0.4
10.0 ± 1.2
Dark
87.6 ± 4.8
45.0 ± 2.0
8.4 ± 1.3
4.3 ± 1.4
Verydark
54.4 ± 6.7
25.9 ± 2.9
5.1 ± 0.3
0.8 ± 0.2
aResults bNd,
are shown as the means of roasting in duplicates and extractions in triplicates ± standard deviation, expressed in mg/100 g of coffee beans dry weight. not detected.
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FIGURE 10.7 Nuclear magnetic resonance signal intensity changes of trigonelline, N-methylpyridinium, and nicotinic acid in the extracts of coffee beans during the coffee bean roasting process are shown. The integral value of the signal due to caffeine was set to a constant of 100.8
Some dilution experiments revealed that the aroma of coffee brew is mainly caused by some alkylpyrazines, furanones, and phenols, and by 2-furfurylthiol, methional, and 3-mercapto-3-methylbutyl formate.32 The higher impact of methional and formate and the lower aroma activity of 4-vinylguaiacol were in contrast to
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FIGURE 10.8 Evolutions of free amino acids during the roasting process. Nuclear magnetic resonance signal intensity changes of l-glutamic acid, l-alanine, l-asparagine and γ-aminobutyric acid (GABA) in the extracts of coffee beans during the coffee bean roasting process are shown. The integral value of the signal due to caffeine was set to a constant of 100.8
the results previously obtained for ground coffee of the same provenance and roast degree. The mechanisms of formation of coffee aroma are extremely complex. There is clearly a wide range of
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90
10. CHEMICAL CHANGES DURING ROASTING
TABLE 10.4 Classes of Volatile Compounds Identified in Roasted Coffee Beans2 Sulfur compounds Thiols Hydrogen sulfide Thiophenes (esters, aldehydes, ketones) Thiazoles (alkyl, alcoxy, and acetal derivatives) Pyrazines Pyrazine itself Thiol and furfuryl derivatives Alkyl derivatives (primarily methyl and dimethyl) Pyridines Methyl, ethyl, acetyl, and vinyl derivatives Pyrroles Alkyl, acyl, and furfuryl derivatives Oxazoles Furans Aldehydes, ketones, esters, alcohols, acids, thiols, sulfides and in combination with pyrazines and pyrroles Aldehydes and ketones Aliphatic and aromatic species Phenols
interactions between all the routes involved in the Maillard reaction, caramelization, Strecker degradation, and the breakdown of sulfur amino acids, hydroxy-amino acids, proline and hydroxyproline, trigonelline, quinic acid moiety, carotenoids, and minor lipids.2,33 Future research efforts will probably focus on the elucidation of mechanistic and kinetic parameters for the formation and deterioration of key aroma compounds.
10.9 TORREFACTO ROASTING Torrefacto is a roasting process in which sugar is added to robusta coffees in order to brown the coffee brew and mask negative flavors. This roasting technique is used in several countries of southern Europe and South America, where some segments of the population prefer espresso coffee with a high amount of foam, a dark brown color, a very intense aroma, and a strong taste, with a tendency toward bitterness.34 To obtain Torrefacto coffee, the coffee beans are roasted in the normal way but, at the end of the process, sucrose is added at no more than 15% by weight. The temperature of roasting is always above 200 °C, and the sucrose changes to caramel, forming a burnt film around the coffee beans.
Usually, coffee beans roasted with sugar are of low quality and the defects of the beans are masked by the sugar. This manner of roasting was invented to prevent coffee from losing essential oils and to slow the staling of coffee by preventing oxygen from making contact with the beans.5 When roasting coffee with sugar, pyrazines, pyridines, and furans are formed in greater quantity than in natural roasted coffee; these compounds result in the roasty, burnt, and caramel qualities that are the distinctive sensory characteristics of Torrefacto roasted coffee.
References 1. Charles-Bernard M, Kraehenbuehl K, Rytz A, Roberts DD. Interactions between volatile and nonvolatile coffee compo nents. 1. Screening of nonvolatile components. J Agric Food Chem 2005;53:4417–25. 2. Buffo RA, Cardelli-Freire C. Coffee flavour: an overview. Flavour Fragrance J 2004;19:99–104. 3. Clarke RJ, Macrae R. Coffee. London: Elsevier Applied Science; 1985. 4. Wei F, Furihata K, Hu F, Miyakawa T, Tanokura M. Two- dimensional (1)H-(13)C nuclear magnetic resonance (NMR)-based comprehensive analysis of roasted coffee bean extract. J Agric Food Chem 2011;59:9065–73. 5. Sanz C, Maeztu L, Zapelena MJ, Bello J, Cid C. Profiles of volatile compounds and sensory analysis of three blends of coffee: influence of different proportions of arabica and robusta and influence of roasting coffee with sugar. J Sci Food Agric 2002;82:840–7. 6. Ginz M, Balzer HH, Bradbury AGW, Maier HG. Formation of aliphatic acids by carbohydrate degradation during roasting of coffee. Eur Food Res Technol 2000;211:404–10. 7. Nakabayashi T. Chemical studies on quality of coffee. 7. Formation of organic-acids from sucrose by roast. J Jpn Soc Food Sci Technol – Nippon Shokuhin Kagaku Kogaku Kaishi 1978;25:257–61. 8. Wei F, Furihata K, Koda M, Hu F, Miyakawa T, Tanokura M. Roasting process of coffee beans as studied by nuclear magnetic resonance: time course of changes in composition. J Agric Food Chem 2012;60:1005–12. 9. Locas CP, Yaylayan VA. Isotope labeling studies on the formation of 5-(hydroxymethyl)-2-furaldehyde (HMF) from sucrose by pyrolysis-GC/MS. J Agric Food Chem 2008;56:6717–23. 10. Murkovic M, Bornik MA. Formation of 5-hydroxymethyl-2- furfural (HMF) and 5-hydroxymethyl-2-furoic acid during roasting of coffee. Mol Nutr Food Res 2007;51:390–4. 11. Bradbury AGW, Halliday DJ. Chemical structures of green coffee bean polysaccharides. J Agric Food Chem 1990;38:389–92. 12. Nunes FM, Coimbra MA. Chemical characterization of the high molecular weight material extracted with hot water from green and roasted arabica coffee. J Agric Food Chem 2001;49:1773–82. 13. Wei F, Furihata K, Hu F, Miyakawa T, Tanokura M. Complex mixture analysis of organic compounds in green coffee bean extract by two-dimensional NMR spectroscopy. Magn Reson Chem 2010;48:857–65. 14. Redgwell RJ, Trovato V, Curti D, Fischer M. Effect of roasting on degradation and structural features of polysaccharides in arabica coffee beans. Carbohydr Res 2002;337:421–31. 15. Chapter 9: The cup. In: Illy A, Viani R, editors. Espresso coffee: the chemistry of quality. San Diego: Academic Press; 1995. p. 187. 16. Clifford MN, Johnston KL, Knight S, Kuhnert N. Hierarchical scheme for LC-MSn identification of chlorogenic acids. J Agric Food Chem 2003;51:2900–11.
I. INTRODUCTORY AND GENERAL TEXT
References
17. Scholz-Böttcher, Ernst L, Maier HG. New stereoisomers of quinic acid and their lactones. Liebigs Ann Chem 1991:1029–36. 18. Farah A, de Paulis T, Trugo LC, Martin PR. Effect of roasting on the formation of chlorogenic acid lactones in coffee. J Agric Food Chem 2005;53:1505–13. 19. Clifford MN. Chlorogenic acids and other cinnamates – nature, occurrence, dietary burden, absorption and metabolism. J Sci Food Agric 2000;80:1033–43. 20. Vitzthum OG. Thirty years of coffee chemistry research. In: Teranishi, editor. Flavor chemistry: 30 years of progress. New York: Kluwer Academic/Plenum Publishers; 1999. pp. 117–33. 21. Taguchi H, Sakaguchi M, Shimabayashi Y. Trigonelline content in coffee beans and the thermal-conversion of trigonelline into nicotinic-acid during the roasting of coffee beans. Agric Biol Chem 1985;49:3467–71. 22. Viani R, Horman I. Thermal-behavior of trigonelline. J Food Sci 1974;39:1216–7. 23. Stadler RH, Varga N, Hau J, Vera FA, Welti DH. Alkylpyridiniums. 1. Formation in model systems via thermal degradation of trigonelline. J Agric Food Chem 2002;50:1192–9. 24. Feldman JR, Ryder WS, Kung JT. Importance of nonvolatile compounds to flavor of coffee. J Agric Food Chem 1969;17:733. 25. Nunes FM, Coimbra MA. Chemical characterization of the highmolecular-weight material extracted with hot water from green and roasted robusta coffees as affected by the degree of roast. J Agric Food Chem 2002;50:7046–52.
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26. Montavon P, Mauron AF, Duruz E. Changes in green coffee protein profiles during roasting. J Agric Food Chem 2003;51: 2335–43. 27. Bekedam EK, De Laat MPFC, Schols HA, Van Boekel MAJS, Smit G. Arabinogalactan proteins are incorporated in negatively charged coffee brew melanoidins. J Agric Food Chem 2007;55: 761–8. 28. Nunes FM, Coimbra MA. Role of hydroxycinnamates in coffee melanoidin formation. Phytochem Rev 2010;9:171–85. 29. Silvan JM, Morales FJ, Saura-Calixto F. Conceptual study on maillardized dietary fiber in coffee. J Agric Food Chem 2010;58:12244–9. 30. Gniechwitz D, Reichardt N, Meiss E, Ralph J, Steinhart H, Blaut M. Characterization and fermentability of an ethanol soluble high molecular weight coffee fraction. J Agric Food Chem 2008;56:5960–9. 31. Arnold U, Ludiwg E, Kuhn R, Moschwitzer U. Analysis of free amino-acids in green coffee beans .1. Determination of amino-acids after precolumn derivatization using 9-fluorenylmethylchloroformate. Z Lebensm Unters Forsch 1994;199:22–5. 32. Semmelroch P, Grosch W. Studies on character impact odorants of coffee brews. J Agric Food Chem 1996;44:537–43. 33. Illy A, Viani R, editors. Espresso coffee: the chemistry of quality. San Diego: Academic Press; 1995. 34. Andueza S, De Pena MP, Cid C. Chemical and sensorial characteristics of espresso coffee as affected by grinding and torrefacto roast. J Agric Food Chem 2003;51:7034–9.
I. INTRODUCTORY AND GENERAL TEXT
10
Chemical Changes in the Components of Coffee Beans during Roasting Feifei Wei1,2, Masaru Tanokura1 1Department
of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo, Japan; 2Japan Society for the Promotion of Science, Chiyoda-ku, Tokyo, Japan
List of Abbreviations
10.2 COMPONENTS IN ROASTED COFFEE BEANS
CQA Caffeoylquinic acid CQL Caffeoylquinic acid lactone diCQA Dicaffeoylquinic acid FQA Feruloylquinic acid FQL Feruloylquinic acid lactone GABA γ-Aminobutyrate HMF 5-Hydroxymethyl-2-furfural HMFA 5-Hydroxymethyl-2-furoic acid HPLC High-performance liquid chromatography kDa Kilo dalton NMR Nuclear magnetic resonance pCoQA p-Coumaroylquinic acid pCoQL p-Coumaroylquinic acid lactone
The general differences between the compositions of green and roasted coffee beans are shown in Table 10.13 and Figure 10.1.4 Although the compositions vary for coffee beans of different species, origins, roasting degrees, or analytical methods, the degradation of polysaccharides, oligosaccharides (especially sucrose), chlorogenic acids, and trigonelline is commonly observed. It can be considered that the formation of the characteristic aroma, flavor, taste, and color of the roasted coffee bean result first from the dramatically decreased green coffee bean components such as sucrose, free amino acids, chlorogenic acids, and trigonelline, as well as from the breakdown of polysaccharides and proteins.
10.1 INTRODUCTION Roasting is probably the most important factor in the development of the complex flavors that make coffee enjoyable. During the roasting process, the beans undergo many complex and poorly defined chemical reactions, leading to important physical changes and to the formation of the substances responsible for the sensory qualities of the beverage. The roasted coffee beans are composed of carbohydrates, protein fragments, low-molecular-weight acids, caffeine, trigonelline, lipids, many unknown molecules usually called melanoidins, and more than 900 volatile compounds mainly formed during the roasting process.1,2 This chapter describes the changes of components in the coffee beans and the main reactions that take place during roasting process.
Coffee in Health and Disease Prevention http://dx.doi.org/10.1016/B978-0-12-409517-5.00010-3
10.3 THE ROASTING DEGREES To produce high-quality coffee beans, the roasting degree is probably the single most important factor. The longer the coffee beans are held in the roaster and/ or the higher the roasting temperature, the darker the coffee beans. The roasting degrees can be measured by judging the bean’s color by the naked eye or a colorimeter, or by the weight loss of water after roasting. As shown in Table 10.2,3 the higher the degree of roasting, the lower the L value (for lightness in the CIELAB color space).
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10. CHEMICAL CHANGES DURING ROASTING
TABLE 10.1 Chemical Components of Green Coffee Beans and Roasted Coffee Beansa,3 Arabica
Robusta
Components
Green Coffee Bean
Roasted Coffee Bean
Green Coffee Bean
Roasted Coffee Bean
Polysaccharides
50.0–55.0
24.0–39.0
37.0–47.0
–
Oligosaccharides
6.0–8.0
0–3.5
5.0–7.0
0–3.5
Lipids
12.0–18.0
14.5–20.0
9.0–13.0
11.0–16.0
Free amino acids
2.0
0
2.0
0
Proteins
11.0–13.0
13.0–15.0
11.0–13.0
13.0–15.0
Chlorogenic acids
5.5–8.0
1.2–2.3
7.0–10.0
3.9–4.6
Caffeine
0.9–1.2
0–1.0
1.6–2.4
0–2.0
Trigonelline
1.0–1.2
0.5–1.0
0.6–0.8
0.3–0.6
Fatty acids
1.5–2.0
1.0–1.5
1.5–2.0
1.0–1.5
Minerals
3.0–4.2
3.5–4.5
4.0–4.5
4.6–5.6
Melanoidins
–
16.0–17.0
–
16.0–17.0
aPercentage
of the dry weights.
FIGURE 10.1 Roasting changes in the compositions of the arabica coffee bean extracts, as analyzed by nuclear magnetic resonance. The concentrations of roasted coffee bean extract components were compared to those in green coffee bean extract. Light gray indicates the concentrations in green coffee bean extract, whereas dark gray indicates those in roasted coffee bean extract. α-(1-3)-lAraf, α-(1-5)-l-Araf, β-(1-3)-d-Galp, β-(1-6)-d-Galp, and β-(1-4)-dManp indicate α-(1-3)-l-arabinofuranose, α-(1-5)-l-arabinofuranose, β-(1-3)-d-galactopyranose, β-(1-6)-d-galactopyranose and β-(1-4)-dmannopyranose, respectively.4
TABLE 10.2 Roasting Degrees of Coffee Beans3 Roasting Degree
Roasting Name
L Valuea
Weight Loss (% of Green Coffee Bean)
Light roasting degree
Light roast
30.2
10.0–14.0
Cinnamon roast
27.3
Medium roast
24.2
High roast
21.5
City roast
18.5
Fully city roast
16.8
French roast
15.5
Italian roast
14.2
Medium roasting degree
Dark roasting degree aThe
14.0–17.0
17.0–21.0
>21.0
L value ranges from 0 (black) to 100 (white).
10.4 CHANGES OF CARBOHYDRATES Sucrose, which is the most abundant simple carbohydrate present in green coffee beans, acts as an aroma precursor during roasting, generating several classes of compounds, such as carboxylic acids, furans, and aldehydes, which will affect the flavor of coffees.5–7 Sucrose is found to be destroyed quickly at the early stage of roasting, as shown in Figure 10.2(A).8 At the same time, decreases in citric acid and malic acid and steady increases in lactic, acetic, and formic acids are also observed during coffee bean roasting, as shown in Figure 10.2(B).8 By means of model reactions, it has been confirmed that sucrose is the major source of the aliphatic acids (formic, acetic, glycolic, and lactic) produced during coffee roasting.6 Based on the results of these model reactions, a scheme for a sugar fragmentation pattern that would yield the principle aliphatic acids was derived and is shown in Figure 10.3.6 As can be seen in the figure, the glucose and fructose can interconvert via the 1,2-endiol as intermediate, and the fructose can also rearrange to give the 2,3-endiol. Because the thermal dehydration of these sugars to give 1-deoxyglucosone and 3-deoxyglucosone is well known, these four intermediate compounds can be regarded as acid precursors. As suggested by the 13C-labelled model reactions, the principal acids (formic, acetic, glycolic, and lactic) generated on roasting coffee are formed from carbohydrates. The principal precursor of these acids in green coffee beans is sucrose, although the polysaccharide fraction, particularly arabinogalactan, also makes a contribution. Furthermore, the acids are readily formed by thermal treatment of the pure carbohydrates, which suggests that secondary reactions with other components of the green bean do not play a key role in the acid formation during roasting.6
I. INTRODUCTORY AND GENERAL TEXT
10.5 Changes of Chlorogenic Acids
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been used successfully as a chemical index in ensuring adequate heat processing or for monitoring storage conditions for foods. Formation of HMF from carbohydrates has been found to depend on many factors, such as time, water activity, temperature, and the amount and type of catalyst and sugar used. The isotope studies revealed that the formation of HMF during coffee roasting also occurs through the cleavage of sucrose to monomeric carbohydrates and then to glyceraldehyde and methylglyoxal.9 In fact, HMF and 5-hydroxymethyl-2-furoic acid (HMFA) are both formed during the roasting of coffee and decomposed rather quickly upon further roasting, based on the observations by high-performance liquid chromatography (HPLC) (Figure 10.4).10 In summary, sucrose changes into aliphatic acids and aroma compounds during coffee bean roasting, as shown in Figure 10.5. On a dry-weight basis, almost half of green coffee beans are reported to be made of polysaccharides, which include cellulose, mannan, and arabinogalactan.11 In the green coffee beans, polysaccharides are retained in the coffee bean cell wall as part of the insoluble polysaccharide complex.12 The roasting process increases the solubility of both arabinogalactans and mannans from the bean by loosening the cell-wall structure as it swells and by depolymerization of the polysaccharides during the roasting process. The nuclear magnetic resonance (NMR) studies of coffee bean extracts in water revealed both the changes in solubility and the thermal stability of polysaccharides during the roasting process.4,8,13 As shown in Figure 10.2(C), the roasting process increases the solubility of both arabinogalactans and mannans from the beans by loosening the cell-wall structure as it swells and by depolymerization of the polysaccharides during the roasting process. The degradation of α-(1-3)-l-arabinofuranose, α-(1-5)-l-arabinofuranose, β-(1-3)-d-galactopyranose, and β-(1-6)-d-galactopyranose units from arabinogalactans after light roasting could be attributed to the higher thermal lability of arabinogalactans compared to that of the β-(1-4)-d-mannopyranose from mannan, which was less susceptible to degradation, even after a dark roast.14 The water-soluble polysaccharides that appear after roasting, which play an important role in the retention of volatile substances, contribute to the viscosity of the coffee brew and thus to the creamy sensation known as “body” in the mouth.15
FIGURE 10.2 Evolutions of (A) sucrose; (B) citric acid, malic
acid, lactic acid, acetic acid, and formic acid; and (C) β-(1-4)-dmannopyranose, α-(1-3)-l-arabinofuranose, α-(1-5)-l-arabinofuranose, β-(1-3)-d-galactopyranose, and β-(1-6)-d-galactopyranose in the extracts of coffee beans during the coffee bean roasting process observed by nuclear magnetic resonance signal intensity changes.8
5-Hydroxymethyl-2-furfural (HMF) is one of the major degradation products of carbohydrates and has been studied extensively as an indicator of heat damage. It has
10.5 CHANGES OF CHLOROGENIC ACIDS It has been reported that there are at least 30 chlorogenic acids in coffee beans, including caffeoylquinic acids (CQA), dicaffeoylquinic acids, feruloylquinic acids, and p-coumaroylquinic acids.16 During roasting, the levels of CQAs decreased considerably, as observed by NMR
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10. CHEMICAL CHANGES DURING ROASTING
FIGURE 10.3 Reaction scheme for acid formation from primary thermal degradation products of glucose.6
spectroscopy, whereas the levels of quinic acid and of γ-quinide and syllo-quinic acid increased during the roasting process (Figure 10.6(A) and (B)).8 The decomposition of chlorogenic acids could be used as an index of roasting degree. Quinic acid is a dominant acid in green coffee beans and is also a product of the degradation of chlorogenic acids (Figure 10.6(C)). During the roasting process, γ-quinide was formed as an internal ester of the quinic
acid; syllo-quinic acid, which is another isomeric product of quinic acid, was also produced under the prevalent conditions of roasting, which include a thermal atmosphere, low water content, and moderate acidity.8,17 In addition to the lactone of quinic acid, lactones of chlorogenic acids were also detected during the coffee bean roasting process. As shown in Table 10.3,18 feruloylquinic acid lactones, caffeoylquinic acid lactones, and p-coumaroylquinic acid
I. INTRODUCTORY AND GENERAL TEXT
10.7 Changes of Proteins and Free Amino Acids
FIGURE 10.4 Formation of 5-hydroxymethyl-2-furfural
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(HMF) and 5-hydroxymethyl-2-furoic acid (HMFA) during the roasting of coffee beans.10
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FIGURE 10.5 Changes of sucrose during the roasting of coffee beans. HMF, 5-hydroxymethyl-2-furfural; HMFA, 5-hydroxymethyl2-furoic acid.
lactones were formed during coffee bean roasting, and their formation was highly dependent on the degree of roasting. The optimum degree of roasting to achieve a maximum amount of lactones in coffee is a light medium roast, whereas darker roasts yield lower amounts.18 The other product of chlorogenic acids after roasting is cinnamic acids.19 The cinnamic acids released from degradation of chlorogenic acids may participate in the subsequent chemical reactions to form other flavor components.20
10.6 CHANGES OF TRIGONELLINE Trigonelline is a pyridine derivative known to contribute indirectly to the formation of desirable flavor products, including furans, pyrazine, alkyl-pyridines, and pyrroles, during coffee roasting. The importance of
trigonelline, not only as a precursor of flavor and aroma compounds but also as a beneficial nutritional factor, has been well documented in previous studies.21,22 Reports on the thermal degradation of trigonelline have revealed nicotinic acid and nicotinamide, as well as their O- and N-methyl derivatives, as reaction products when trigonelline is heated in a sealed tube.22 In fact, studies confirmed that N-methylpyridinium and nicotinic acid are the major nonvolatile products of trigonelline pyrolysis by both mass spectrometry as well as NMR spectroscopy.4,23 The time courses of trigonelline, N-methylpyridinium, and nicotinic acid were observed by NMR spectroscopy. As shown in Figure 10.7,8 trigonelline, which is present at high levels in green coffee beans, decreased continuously during the roasting process. N-methylpyridinium and nicotinic acid, two thermal decomposition products of trigonelline, increased continuously during the roasting, with the N-methylpyridinium as the major thermal product. The decrease in the N-methylpyridinium level after roasting for 7 min was probably attributable to further decomposition and/or interaction with other thermolytic products. Nicotinic acid, which is an important vitamin as well as the second major thermal degradation product of trigonelline, was positively correlated with the roasting degree. Trigonelline and its thermolytic products undoubtedly have direct and indirect effects on other physicochemical properties of a cup of coffee, such as flavor and aroma.
10.7 CHANGES OF PROTEINS AND FREE AMINO ACIDS The Maillard reaction is a chemical reaction between reducing carbohydrates and various amino acids, peptides, and proteins, which contain free amino groups.
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10. CHEMICAL CHANGES DURING ROASTING
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FIGURE 10.6 Evolutions of coffee components during the roasting process. (A) 5-caffeoylquinic acid, 4-caffeoylquinic acid, 3-caffeoylquinic acid; (B) quinic acid, γ-quinide, syllo-quinic acid in the extracts of coffee beans during the coffee bean roasting process observed by nuclear magnetic resonance signal intensity changes. The integral value of the signal due to caffeine was set to a constant of 100; (C) evolution of 5-caffeoylquinic acid during coffee bean roasting.8
It is chiefly responsible for the development of unique aromas and tastes during the thermal processing of foods. Although the Maillard reaction was discovered more than a century ago, its products and pathways are still being studied. The composition of coffee proteins has also been shown to be profoundly changed by the roasting of the green coffee bean.24 The major proteins present in green arabica coffee infusions had molecular weights of 58 and 38 kDa.12 From roasted coffees, only a defined band of <14 kDa and a diffuse band of >200 kDa were observed.12,25 This means roasting leads to protein denaturation with degradation. At the same time, the green coffee bean protein subunits are integrated into the polymeric structure of melanoidins formed during roasting.26 The melanoidins are defined as brown, highmolecular-weight products containing nitrogen and are end products of the Maillard reaction. They are a heterogeneous group of food polymers with a structural diversity that has hindered a definitive chemical definition. Polysaccharides and proteins have a key role in the formation of the most abundant population of coffee brew melanoidins.27 However, recent studies show that coffee melanoidins incorporate a number of compounds in addition to polysaccharides and proteins, including chlorogenic acids and amino acid/protein fragments. In arabica-roasted coffee infusions, strong associations were observed between phenolics and brown compounds and the polymeric material. The different chemical compositions of melanoidin populations isolated from roasted coffee brews strongly suggest that there are several pathways through which coffee melanoidins are formed.28–30 On the other hand, the free amino acid content appears to be rather low, in the range of 0.3–0.6% on a dry weight basis in green coffee beans,31 or 0.1–0.7 mM in the green coffee bean extracts analyzed by NMR.13 There is no information available to suggest that these free amino acids are either required or sufficient for the generation of the coffee aroma. The time courses of some amino acids have been monitored by NMR spectroscopy. As shown in Figure 10.8, free amino acids are decreased during roasting. The four main free amino acids, l-glutamic acid, l-alanine, l-asparagine, and γ-aminobutyrate, in the green coffee beans disappeared after roasting.8
10.8 FORMATION OF AROMA COMPONENTS Green coffee beans lack the color and characteristic aroma of roasted coffee, both of which are formed during the roasting process. Coffee oil, which comprises about 10% of the roasted beans, carries most of the coffee aroma.2 The aroma is made up of a complex mixture of volatile compounds. The principal classes of aroma compounds in roasted ground coffee are listed in Table 10.4.2
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10.8 Formation of Aroma Components
TABLE 10.3 Chlorogenic Acid Lactone Content in Green and Roasted Coffee Beansa,18
Arabica (Brazil)
Robusta (Uganda)
Roasting Degree
3-CaffeoylQuinic Acid Lactone
4-CaffeoylQuinic Acid Lactone
3-FeruloylQuinic Acid Lactone
4-FeruloylQuinic Acid Lactone
3,4-diCaffeoylQuinic Acid Lactone
3-pCoumaroylQuinic Acid Lactone
4-pCoumaroylQuinic Acid Lactone
Green
Ndb
Nd
Nd
Nd
Nd
Nd
Nd
Verylight
71.6 ± 8.7
25.6 ± 1.6
6.5 ± 1.3
1.9 ± 0.2
2.8 ± 0.2
2.1 ± 0.3
0.5 ± 0.0
Light
160.2 ± 2.3
92.1 ± 2.9
14.1 ± 1.2
7.0 ± 0.4
4.1 ± 0.4
4.6 ± 0.4
5.3 ± 0.2
Lightmedium
248.5 ± 9.4
115.3 ± 1.0
28.3 ± 2.0
13.4 ± 0.7
6.6 ± 1.2
7.5 ± 0.4
7.8 ± 0.2
Darkmedium
146.6 ± 9.8
87.7 ± 2.1
29.8 ± 1.6
9.6 ± 0.4
2.1 ± 1.1
7.3 ± 0.7
6.6 ± 0.4
Dark
95.7 ± 4.7
53.1 ± 3.5
21.2 ± 1.9
7.8 ± 0.3
1.0 ± 0.2
4.4 ± 0.2
4.0 ± 1.6
Verydark
59.6 ± 10.0
24.8 ± 2.6
15.8 ± 6.0
6.2 ± 0.4
0.7 ± 0.06
2.3 ± 0.2
3.8 ± 0.6
Green
Nd
Nd
4.0 ± 0.0
9.1 ± 0.8
Verylight
57.7 ± 4.4
25.8 ± 1.4
6.4 ± 0.4
16.4 ± 1.7
Light
198.8 ± 4.4
110.0 ± 2.1
18.4 ± 0.7
23.6 ± 1.0
Lightmedium
253.6 ± 4.3
138.8 ± 2.0
31.1 ± 1.3
25.4 ± 1.0
Darkmedium
165.4 ± 10.6
125.9 ± 8.8
16.6 ± 0.4
10.0 ± 1.2
Dark
87.6 ± 4.8
45.0 ± 2.0
8.4 ± 1.3
4.3 ± 1.4
Verydark
54.4 ± 6.7
25.9 ± 2.9
5.1 ± 0.3
0.8 ± 0.2
aResults bNd,
are shown as the means of roasting in duplicates and extractions in triplicates ± standard deviation, expressed in mg/100 g of coffee beans dry weight. not detected.
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FIGURE 10.7 Nuclear magnetic resonance signal intensity changes of trigonelline, N-methylpyridinium, and nicotinic acid in the extracts of coffee beans during the coffee bean roasting process are shown. The integral value of the signal due to caffeine was set to a constant of 100.8
Some dilution experiments revealed that the aroma of coffee brew is mainly caused by some alkylpyrazines, furanones, and phenols, and by 2-furfurylthiol, methional, and 3-mercapto-3-methylbutyl formate.32 The higher impact of methional and formate and the lower aroma activity of 4-vinylguaiacol were in contrast to
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FIGURE 10.8 Evolutions of free amino acids during the roasting process. Nuclear magnetic resonance signal intensity changes of l-glutamic acid, l-alanine, l-asparagine and γ-aminobutyric acid (GABA) in the extracts of coffee beans during the coffee bean roasting process are shown. The integral value of the signal due to caffeine was set to a constant of 100.8
the results previously obtained for ground coffee of the same provenance and roast degree. The mechanisms of formation of coffee aroma are extremely complex. There is clearly a wide range of
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90
10. CHEMICAL CHANGES DURING ROASTING
TABLE 10.4 Classes of Volatile Compounds Identified in Roasted Coffee Beans2 Sulfur compounds Thiols Hydrogen sulfide Thiophenes (esters, aldehydes, ketones) Thiazoles (alkyl, alcoxy, and acetal derivatives) Pyrazines Pyrazine itself Thiol and furfuryl derivatives Alkyl derivatives (primarily methyl and dimethyl) Pyridines Methyl, ethyl, acetyl, and vinyl derivatives Pyrroles Alkyl, acyl, and furfuryl derivatives Oxazoles Furans Aldehydes, ketones, esters, alcohols, acids, thiols, sulfides and in combination with pyrazines and pyrroles Aldehydes and ketones Aliphatic and aromatic species Phenols
interactions between all the routes involved in the Maillard reaction, caramelization, Strecker degradation, and the breakdown of sulfur amino acids, hydroxy-amino acids, proline and hydroxyproline, trigonelline, quinic acid moiety, carotenoids, and minor lipids.2,33 Future research efforts will probably focus on the elucidation of mechanistic and kinetic parameters for the formation and deterioration of key aroma compounds.
10.9 TORREFACTO ROASTING Torrefacto is a roasting process in which sugar is added to robusta coffees in order to brown the coffee brew and mask negative flavors. This roasting technique is used in several countries of southern Europe and South America, where some segments of the population prefer espresso coffee with a high amount of foam, a dark brown color, a very intense aroma, and a strong taste, with a tendency toward bitterness.34 To obtain Torrefacto coffee, the coffee beans are roasted in the normal way but, at the end of the process, sucrose is added at no more than 15% by weight. The temperature of roasting is always above 200 °C, and the sucrose changes to caramel, forming a burnt film around the coffee beans.
Usually, coffee beans roasted with sugar are of low quality and the defects of the beans are masked by the sugar. This manner of roasting was invented to prevent coffee from losing essential oils and to slow the staling of coffee by preventing oxygen from making contact with the beans.5 When roasting coffee with sugar, pyrazines, pyridines, and furans are formed in greater quantity than in natural roasted coffee; these compounds result in the roasty, burnt, and caramel qualities that are the distinctive sensory characteristics of Torrefacto roasted coffee.
References 1. Charles-Bernard M, Kraehenbuehl K, Rytz A, Roberts DD. Interactions between volatile and nonvolatile coffee compo nents. 1. Screening of nonvolatile components. J Agric Food Chem 2005;53:4417–25. 2. Buffo RA, Cardelli-Freire C. Coffee flavour: an overview. Flavour Fragrance J 2004;19:99–104. 3. Clarke RJ, Macrae R. Coffee. London: Elsevier Applied Science; 1985. 4. Wei F, Furihata K, Hu F, Miyakawa T, Tanokura M. Two- dimensional (1)H-(13)C nuclear magnetic resonance (NMR)-based comprehensive analysis of roasted coffee bean extract. J Agric Food Chem 2011;59:9065–73. 5. Sanz C, Maeztu L, Zapelena MJ, Bello J, Cid C. Profiles of volatile compounds and sensory analysis of three blends of coffee: influence of different proportions of arabica and robusta and influence of roasting coffee with sugar. J Sci Food Agric 2002;82:840–7. 6. Ginz M, Balzer HH, Bradbury AGW, Maier HG. Formation of aliphatic acids by carbohydrate degradation during roasting of coffee. Eur Food Res Technol 2000;211:404–10. 7. Nakabayashi T. Chemical studies on quality of coffee. 7. Formation of organic-acids from sucrose by roast. J Jpn Soc Food Sci Technol – Nippon Shokuhin Kagaku Kogaku Kaishi 1978;25:257–61. 8. Wei F, Furihata K, Koda M, Hu F, Miyakawa T, Tanokura M. Roasting process of coffee beans as studied by nuclear magnetic resonance: time course of changes in composition. J Agric Food Chem 2012;60:1005–12. 9. Locas CP, Yaylayan VA. Isotope labeling studies on the formation of 5-(hydroxymethyl)-2-furaldehyde (HMF) from sucrose by pyrolysis-GC/MS. J Agric Food Chem 2008;56:6717–23. 10. Murkovic M, Bornik MA. Formation of 5-hydroxymethyl-2- furfural (HMF) and 5-hydroxymethyl-2-furoic acid during roasting of coffee. Mol Nutr Food Res 2007;51:390–4. 11. Bradbury AGW, Halliday DJ. Chemical structures of green coffee bean polysaccharides. J Agric Food Chem 1990;38:389–92. 12. Nunes FM, Coimbra MA. Chemical characterization of the high molecular weight material extracted with hot water from green and roasted arabica coffee. J Agric Food Chem 2001;49:1773–82. 13. Wei F, Furihata K, Hu F, Miyakawa T, Tanokura M. Complex mixture analysis of organic compounds in green coffee bean extract by two-dimensional NMR spectroscopy. Magn Reson Chem 2010;48:857–65. 14. Redgwell RJ, Trovato V, Curti D, Fischer M. Effect of roasting on degradation and structural features of polysaccharides in arabica coffee beans. Carbohydr Res 2002;337:421–31. 15. Chapter 9: The cup. In: Illy A, Viani R, editors. Espresso coffee: the chemistry of quality. San Diego: Academic Press; 1995. p. 187. 16. Clifford MN, Johnston KL, Knight S, Kuhnert N. Hierarchical scheme for LC-MSn identification of chlorogenic acids. J Agric Food Chem 2003;51:2900–11.
I. INTRODUCTORY AND GENERAL TEXT
References
17. Scholz-Böttcher, Ernst L, Maier HG. New stereoisomers of quinic acid and their lactones. Liebigs Ann Chem 1991:1029–36. 18. Farah A, de Paulis T, Trugo LC, Martin PR. Effect of roasting on the formation of chlorogenic acid lactones in coffee. J Agric Food Chem 2005;53:1505–13. 19. Clifford MN. Chlorogenic acids and other cinnamates – nature, occurrence, dietary burden, absorption and metabolism. J Sci Food Agric 2000;80:1033–43. 20. Vitzthum OG. Thirty years of coffee chemistry research. In: Teranishi, editor. Flavor chemistry: 30 years of progress. New York: Kluwer Academic/Plenum Publishers; 1999. pp. 117–33. 21. Taguchi H, Sakaguchi M, Shimabayashi Y. Trigonelline content in coffee beans and the thermal-conversion of trigonelline into nicotinic-acid during the roasting of coffee beans. Agric Biol Chem 1985;49:3467–71. 22. Viani R, Horman I. Thermal-behavior of trigonelline. J Food Sci 1974;39:1216–7. 23. Stadler RH, Varga N, Hau J, Vera FA, Welti DH. Alkylpyridiniums. 1. Formation in model systems via thermal degradation of trigonelline. J Agric Food Chem 2002;50:1192–9. 24. Feldman JR, Ryder WS, Kung JT. Importance of nonvolatile compounds to flavor of coffee. J Agric Food Chem 1969;17:733. 25. Nunes FM, Coimbra MA. Chemical characterization of the highmolecular-weight material extracted with hot water from green and roasted robusta coffees as affected by the degree of roast. J Agric Food Chem 2002;50:7046–52.
91
26. Montavon P, Mauron AF, Duruz E. Changes in green coffee protein profiles during roasting. J Agric Food Chem 2003;51: 2335–43. 27. Bekedam EK, De Laat MPFC, Schols HA, Van Boekel MAJS, Smit G. Arabinogalactan proteins are incorporated in negatively charged coffee brew melanoidins. J Agric Food Chem 2007;55: 761–8. 28. Nunes FM, Coimbra MA. Role of hydroxycinnamates in coffee melanoidin formation. Phytochem Rev 2010;9:171–85. 29. Silvan JM, Morales FJ, Saura-Calixto F. Conceptual study on maillardized dietary fiber in coffee. J Agric Food Chem 2010;58:12244–9. 30. Gniechwitz D, Reichardt N, Meiss E, Ralph J, Steinhart H, Blaut M. Characterization and fermentability of an ethanol soluble high molecular weight coffee fraction. J Agric Food Chem 2008;56:5960–9. 31. Arnold U, Ludiwg E, Kuhn R, Moschwitzer U. Analysis of free amino-acids in green coffee beans .1. Determination of amino-acids after precolumn derivatization using 9-fluorenylmethylchloroformate. Z Lebensm Unters Forsch 1994;199:22–5. 32. Semmelroch P, Grosch W. Studies on character impact odorants of coffee brews. J Agric Food Chem 1996;44:537–43. 33. Illy A, Viani R, editors. Espresso coffee: the chemistry of quality. San Diego: Academic Press; 1995. 34. Andueza S, De Pena MP, Cid C. Chemical and sensorial characteristics of espresso coffee as affected by grinding and torrefacto roast. J Agric Food Chem 2003;51:7034–9.
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