Formation of Sulfur-Containing Volatiles under Coffee Roasting

Chapter 22

Formation of Sulfur-Containing Volatiles under Coffee Roasting Conditions George P . Rizzi

Downloaded by IOWA STATE UNIV on February 22, 2017 | http://pubs.acs.org Publication Date: January 15, 2000 | doi: 10.1021/bk-2000-0754.ch022

Procter and Gamble Company, Miami Valley Laboratories, Cincinnati, OH 45253-8707 Experiments support a new hypothesis for the formation of key aroma components in roasted coffee. Furfuryl mercaptan and related compounds are readily formed by reactions of Maillardderived furfuryl alcohols and amides of S-containing amino acids at acid pH. Model reactions suggested that soluble coffee proteins may be the precursors of flavor significant sulfur volatiles found in roasted coffee.

Introduction Volatile sulfur compounds, especially furan derivatives like 2-furfurylthiol, i.e., furfuryl mercaptan (3a) have long been associated with coffee flavor chemistry. Furfuryl mercaptan is unique among the sulfur volatiles of roasted coffee since its vapor alone, in high dilution, can convey the unmistakable pleasant scent of roasted coffee. In the first detailed, quantitative analysis of roasted coffee aroma, Silwar et al separated the steam volatile components into six fractions by partition chromatography on silica gel. O f these, a single fraction exhibited "typical, pleasant roast coffee aroma" and was shown to contain eleven (2-furyl)methanethio derivatives including 3a in addition to six 3-furylthio compounds (1). More recently the organoleptic significance of 3a and 2-methyl-3~furanthioi in coffee products was confirmed by GC-olfactometry techniques (2). The purpose of our research was to gain a better understanding into the formation of 3a and related (2-furyl)methanethio compounds during coffee roasting.

Aroma Formation Hypothesis Extensive model studies have shown that 3a and similar compounds are readily formed by heating cysteine or methionine with sugars and/or furfural and its homologs (3) and it has been suggested that reactions of this kind are important for 210

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Parliment et al.; Caffeinated Beverages ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


211 coffee aroma formation. Model system studies generally indicate Strecker degradations of sulfur-containing amino acids and subsequent reactions of hydrogen sulfide or methanethiol with sugar-derived intermediates such as furfurals yield furfuryl mercaptan. However, the existing model system studies are open to some criticism. Current model systems do not produce true coffee aroma nor do they explain why similar foodstuffs , e.g., sesame seeds which contain flavor precursors similar to those found in coffee do not smell like coffee upon roasting (4). Also, most current model systems require cysteine as a reactant and there seems to be some question as to whether free cysteine actually exists in unroasted coffee beans (5). In view of these questions we began to consider non-Strecker flavor-forming reactions. We and others (6) propose an alternative model for coffee aroma formation in which soluble raw bean protein serves as the limiting source of sulfur for the formation of key aroma compounds. Bean protein provides constant, biogenetically determined numbers and locations of sulfhydryl and methylthio residues which conceivably can offer more control over aroma formation compared with reactions of free amino acids. Moreover, reducing sugars in coffee glycoprotein may be uniquely positioned to interact with sulftir-containing amino acid residues during roasting. We hypothesize that reactive intermediates formed during caramelization (or Maillard reaction) of ambient sugars, i.e., glucose, fructose or rhamnose will interact with pendant sulfur functional groups on bean proteins to produce coffee aroma.

Experimental Results and Discussion

General Procedures To test the new aroma formation hypothesis we allowed furfuryl alcohol (la) or 5-methylfurfuryl alcohol (lb) to react with various sulfur-containing amino acid derivatives in p H 4.0 aqueous acetate buffer solution at 100°C to simulate protein reactions in the initial stage of coffee bean roasting. Following the reactions, the mixtures were made basic by adding sodium hydroxide solution and subjected to atmospheric steam distillation extraction (SDE) with water/methylene chloride to isolate volatile products. Details have been reported elsewhere (7). After isolation, the SDE-volatile products were identified and quantified by sniflport G C and by GC/mass spectrometry techniques. Yields of individual volatile products are reported as G C area percentages of total volatiles area minus solvent peaks (% T V ) , Table I. Compounds la and lb were selected as reactants because they both are prevalent in roasted coffee and because of their predicted reactivity in acidic media (8). We suspect that la is being formed during roasting by autoxidation/decarboxylation of the prevalent hexose degradation product, 5hydroxymethylfurfural. Loss of the aldehyde carbon as carbon dioxide is also


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Table I. Formation o f Sulfur-containing Volatiles Amino acid or Derivative Cysteine S-Methylcysteine Homocysteine same @ 0,5 M Methionine Ethionine N-Acetylcysteine same same N-Acetyl methionine same same Glutathione same Glycylmethionine

Alcohol la la la la la la la lb 4 la lb 4 la la la

Reaction Time (h) 2.0 16.0 2.0 21.5 2.5 3.0 2.0 3.0 2.0 2.5 2.5 2.0 2.0 15.3 5.0

Sulfur Volatile 3a 3b 3a 3a 3b 3c 3a 3d 6a 3b 3e 6b 3a 3a 3b

% TV t t ND t 0.05 6.5 7.9 11 30 1.4 0.03 0.92 2.7 18 ND

Structure Proof a c c a a a b b a b b c a

NOTES: Equimolar amounts of reactants initially at 0.25 M in 0.1 M pH 4.0 acetate buffer refluxed under nitrogen atmosphere for (h) hours; TV = area % of total FID volatiles; t=trace ND=none detected; structure proof: (a) GC/MS comparison with authentic substance, (b) MS compares well with literature spectrum (no standard available), (c] retention time and sniffport aroma only. S O U R C E : Reprinted with permission from reference 7. Copyright 1995 Elsevier Science B . V .

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consistent with previously reported C-13 labelling experiments (9) in which un labelled l a was obtained as a product in Maillard reactions of aldehyde-labelled glucose. Reactions of amino acids and derivatives are shown in Figure 1.

Reactions of Free Amino Acids A t 100°C, l a and cysteine generated only traces of 3a whose odor dominated the smell of the reaction mixture. Significantly, cysteine plus furfural produced even less 3a (detectable only by sniffport/GC) and a reaction odor dominated by hydrogen sulfide. Reactions of homocysteine and la led only to traces of 3a at extended reaction times. For free amino acids, the low yields of 3a may have due to poor nucleophilicity of the sulfur atom at p H 4 where cysteine and homocysteine exist mainly in their zwitterionic and monoprotonated forms. S-alkyl amino acids were more effective for producing S-containing furan derivatives. Methionine plus la produced methyl furfuryl sulfide (3b) at 0.046% T V , a significant enhancement in S-volatile formation compared to unmethylated homocysteine. Similarly, ethionine plus l a generated ethyl furfuryl sulfide (3c) as 6.5% T V .



Figure 1. Formation of Sulfur-containing

Volatiles


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Reactions of N-Acetyl Amino Acids and Peptides Reactions of N-acetyl amino acids were investigated as simple models to simulate peptide/protein reactions. It was predicted that N-acetyl amino acids should be more prone to electrophilic alkylation at sulfur since as amides they are uncharged molecules in p H 4 solution. In fact, N-acetylcysteine (AcCys) and la produced more 3a (7.9% T V ) compared to free cysteine. Also, a similar reaction with 5-methylfurfuryl alcohol (lb) afforded 5-methylfurfuryl mercaptan (3d) as 11% T V . In addition, N-acetylmethionine (AcMet) with la and l b produced 3b (1.4% T V ) and 5-methylfurfuryl methyl sulfide (3e) (0.03% T V ) respectively. A s expected, the yield of 3b was higher with AcMet when compared with free methionine, but still strangely low compared with the AcCys results. At this stage more sophisticated experiments are needed to evaluate the meaning of reaction yields in terms of steric and electronic effects and/or complications due to unspecified side reactions. The cysteine tripeptide glutathione reacted with la to produce more 3a (2.7% T V ) than free cysteine suggesting that protein-bound cysteine could function as a direct precursor of 3a during food processing. The methionine dipeptide, glycyl-methionine, failed to generate 3b from la apparently due to its possessing a p K l greater than the 2.28 reported for free methionine. A higher p K l will lead to more net positive charge on the peptide at p H 4 and therefore reduced reactivity (nucleophilicity) at sulfur.

Reactions W i t h 3-MethyI-2-butene-1 -ol (4) The above results with furfuryl alcohols suggested that other unsaturated alcohols present in coffee might also function as precursors of important sulfur volatiles. In particular, 3-methyl-2-butene-l-ol (prenyl alcohol) (4) was recently described as a reasonable precursor of 3-methyl-2-butene-l-thiol (prenyl mercaptan) (6a) (10). In our system, 4 reacted with AcCys to form 6a, a compound known to exist in roasted coffee volatiles, in surprisingly high relative yield (30% T V ) . The reason for such a high yield is best explained by less by-product formation compared with furfuryl alcohol reactions. Compound 4 also reacted with AcMet to generate the homolog, methyl prenyl sulfide (6b, 0.92% T V ) . Methyl prenyl sulfide has not yet been reported in coffee, but it has been observed as a component of hop oil volatiles (11).

Reaction Mechanism Formation of sulfur-containing volatiles is explained by an acid-catalyzed


215 alkylation of amino acid sulfur atoms, followed by hydrolysis (7), Figure 1. Products formed during acid-catalyzed polymerization of furfuryl alcohol have been rationalized by invoking furfuryl cations similar to 2 (8). In a similar fashion, unsaturated alcohols like 4 can, via dehydration, procède via cationic intermediates with structure 5. In general, unsaturated alcohols may form electrophilic species in acidic solution which in turn can react with ambient nucleophilic sites, i.e., amino acid sulfur atoms. Initial products of sulfur alkylation will be sulfonium salts which finally may undergo hydrolysis to form the observed sulfur volatiles 3 and 6.


References

1.

Silwar, R.;Kamperschröer, H.;Tressl, R. Chem. Mikrobiol. Technol. Lebensm. 1987, 10, 176. 2. Blank, I.;Sen, A.;Grosch, W. Ζ. Lebensm. Unters. Forsch. 1992, 195, 239. 3. Holscher, W . ; Steinhart, H . In Thermally Generated Flavors, Maillard, Microwave and Extrusion Processes; Parliment, T. H . ; Morello, M. J.; McGorrin, R. J., Eds., A C S Symposium Series 543; American Chemical Society: Washington, D C , 1994, pp 206-217. 4. Nakamura, S.;Nishimura, O.;Masuda, H.;Mihara, S. In 11 Intl. Congr. Essential Oils and Flavours; Bhattacharyya, S. C.;Sen, N.;Sethi, K . L., Eds., Oxford & I B H Ltd., New Delhi, 1989; pp 73-87. 5. Arnold, U.; Eberhard, L . ; Kühn, R.; Möschwitzer Ζ. Lebensm. Unters. Forsch. 1994, 199, 22. 6. Ludwig, E.; Raczek, Ν . N.; Kurzrock, T. 14 Coll. Scient. Int. sur le Café, Kyoto, 1995, p 359. 7. Rizzi, G . P. In Food Flavors: Generation, Analysis and Process Influence; Charalambous, G . , Ed.; Elsevier Science Β. V.: New York, N Y , 1995, pp289302. 8. Gonzalez, R.; Martinez, R.; Ortiz, P. Makromol.. Chem. Rapid Commun. 1992, 13, 517. 9. Tressl, R.; Kersten, E . ; Rewicki, D . J. Agric. Food Chem. 1993, 41, 2278. 10. Holscher, W.;Vitzthum, O.;Steinhart, H. J. Agric. Food Chem. 1992, 40, 655. 11. Moir M.; Gallacher, I. M.; Seaton, J. C.; Suggett, A. Chem. Ind. (London) 1980, 624. th

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Formation of Sulfur-Containing Volatiles under Coffee Roasting

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