Formation Pathways for Primary Roasted Coffee Aroma Compounds

Chapter 16

Formation Pathways for Primary Roasted Coffee Aroma Compounds 1

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 31, 2015 | http://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch016

Wilhardi Holscher and Hans Steinhart 1

Corporate Research and Development, Jacobs-Suchard, Weser-Ems-Strasse 3-5, D-2800 Bremen 44, Germany Institute of Biochemistry and Food Chemistry, University of Hamburg, Grindelallee 117, D-2000 Hamburg 13, Germany

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The majority of the more than 800 volatiles which have been identified in roasted coffee, are formed by Maillard reactions. However, recent sensory specific investigations showed that only a small number of these contribute to the flavor complex of roasted coffee and that many odorants with a strong flavor impact are generated by formation pathways besides Maillard reaction. This chapter gives an overview of aroma compounds of roasted coffee and their corresponding formation pathways during roasting. Apparently certain pathways are specific for roasted coffee flavor formation.

Generally speaking, the composition of flavors generated by thermal treatment or fermentation is extremely complex eg beer, meat, tobacco or coffee. Recent compilations of volatile compounds in roasted coffee list more than 800 compounds of all chemical types (1, 2). We expect a large number could still be found. This is one reason why the knowledge of coffee aroma chemistry is still incomplete after more than 70 years of systematic research in this field. Identification of unknown coffee volatiles without studying their sensory impact makes sense only for academic purposes, as it does not fit the current requirement of aroma research which focuses more and more on the character impact compounds. The first attempts to investigate the aroma relevance of roasted coffee volatiles have been recently made by application of semi-quantitative GC/sniffing assessments carried out as aroma dilution analyses (3-5). The results indicated that only a comparatively small number of components, between 60 and 80, contribute to the aroma character of roasted coffee. Many of these chemicals arise from Maillard reactions directly or indirectly. However, a number of other aroma impact compounds are generated by different kinds of formation pathways.

0097-6156/94/0543-0206$06.00/0 © 1994 American Chemical Society

In Thermally Generated Flavors; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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Experimental Isolation of volatiles of medium roasted Colombian coffee and identification of the character impact compounds was carried out as described in detail earlier (3, 5, 6): Volatiles were collected by means of high vacuum distillation and simultaneous distillation/extraction with a mixture of diethylether and n-pentane. After preseparation by means of column chromatography on silica gel, preparative H P L C and G C , the aroma concentrates were investigated by capillary G C , G C - M S , and simultaneous GC/sniffmg. Identification was carried out by G C retention and spectroscopic data compared with authentic reference compounds. Results and Discussion Over the last decade, GC-effluent sniffing has proved to be a powerful tool for the investigation of relevant aroma compounds in various kinds of foods and therefore was applied in the characterization of roasted coffee odorants. Table I lists the character impact compounds identified in roasted Colombian coffee by means of combined GC-MS/sniffing. Sotolone and abhexone (see below) were identified by Blank et al. (4). A further 17 aroma notes detectable at the sniffing port have not yet been identified but appear to be of lower sensory importance. The overall number of aroma relevant compounds is therefore few considering that more than 1000 volatiles may occur in roasted coffee. Furthermore, the results indicate that not a single compound exhibits the typical roasted coffee-like aroma impression but the aroma consists of a complex mixture of compounds possessing many different odor qualities. Table I also gives an overview of their predominant formation pathway and precursors during the roasting of the green coffee beans. Breakdown Products of Lipid Oxidation. Sniffing indicated that compounds such as hexanal, l-octen-3-one, 2(cis)-nonenal, 2(tr)-nonenal, 2,6(tr,cis)-nonadienal, 2,4(tr,tr)-nonadienal or 2,4(tr,tr)-decadienal contribute to roasted coffee flavor to a limited extent, although the odor notes were weak. Their presence is not surprising considering that green coffee beans which are a seed contain lipids and proteins to support germination. Total lipids amount to about 13 % in Arabica coffee (7), and of this over half is linoleic acid. The formation of various aldehydes and ketones by autoxidation of unsaturated fatty acids via breakdown of hydroperoxide intermediates is well established in the literature (8). 2(tr)-Nonenal is related to "woody" or "cardboard-like" off flavor notes in coffee beverages (9). 2(tr)-Nonenal exhibits synergistic effects and may influence the sour taste perception without changing the pH. The carbonyl compounds mentioned above were identified in the staling of roasted coffee in oxygen-containing atmospheres. Stale notes in roasted coffee could be correlated with the generation of hexanal after 7 weeks storage in air (10, 19). Whether hexanal arises from autoxidation of the lipid complex or from volatile roasting products is not yet clear. Pyrazines. 2-Methoxy-3-isopropyl pyrazine (III, Figure 1) and 2-methoxy-3-isobutyl pyrazine which possess strong vegetable-like odors are present in green coffee (11) and contribute to the final coffee aroma impression after roasting. As


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Table I. Odor description, intensity and predominant formation pathway of aroma relevant volatiles in roasted Colombian coffee

Compound

Odor Description

Intensity

Pathway/ Precursor

Methanethiol Methylsulfide 2-Methylpropanal 2-Methylbutanal 3-Methylbutanal 2,3-Butanedione 2,3-Pentanedione n-Hexanal 3-Methyl-2-buten- 1-thiol l-Octen-3-one 2-Methyl-3-furanthiol 2-Ethyl pyrazine 2-Ethyl-3-methyl pyrazine 2,3,5-Trimethyl pyrazine 2-Furanmethanethiol 2-Methoxy-3-isopropyl pyrazine Acetic acid Methional 2-Ethyl-3,5-dimethyl pyrazine 2-Furfurylmethylsulfide 2(cis)-Nonenal 3-Mercapto-3-methylbutyl formate 2-Methoxy-3-isobutyl pyrazine 2(tr)-Nonenal Linalool 2,6(tr ,cis)-Nonadienal 5-Methyl-6,7-dihydro-5H-cyclopenta(b)pyrazine 2-Phenylacetaldehyde 3-Mercapto-3-methyl butanol 2-Methyl butyric acid 3-Methyl butyric acid 2,4(tr,tr)-Nonadienal 2,4(tr,tr)-Decadienal β-Damascenone Cyclotene Guaiacol 2,6-Dimethylphenol 2-Phenylethanol

putrid sulfury pungent, fruity fermented, fruity pungent, fruity buttery buttery nutty, green foxy, skunky mushroom-like meat-like roasty roasty roasty, musty roasty, coffee-like peasy vinegar-like cooked-potato-like roasty, musty garlic-like metallic, tallowy catty paprika-like tallowy, fatty flowery cucumber-like

+ + + + + ++ + + ++ + +++ + + ++ +++ + + +++ ++ + + +++ +++ + + +

M,S M,S M,S M,S M,S M M L Ρ L Τ M M M M G M M,S M M L Ρ G L G L

peanutty honey-like broth-like fermented, sweaty footsweat-like geranium-like fried, oily fruity, tea-like spicy sweet, phenolic phenolic honey, beer-like

+ + + + +++ + + +++ + ++ + +

M,Ca M,S Ρ M,S M,S L L C Ca PA PA M,S


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Table I. (continued)

Compound

Odor Description

Intensity

Pathway/ Precursor

Phenol 4-Ethylguaiacol 2,5-Dimethyl-4-hydroxy-3-(2H)-furanone m-Cresol 3-Ethylphenol 4-Vinylguaiacol Sotolone* 3,4-Dimethylphenol Abhexone* 3-Methylindole

phenolic clove-like

+ +

PA PA

caramel-like phenolic phenolic clove-like spicy phenolic spicy faecal

++ + + + ++ +++ + ++ +

M PA PA PA M PA M M

Compounds listed according to their retention on D B - W A X ; a: according to reference (4); + : weak; + + : intense; + + + : very intense; M : Maillard Products; S: Strecker Degradation; L : Lipid Oxidation; G : Green Coffee Volatiles; Τ: Thiamine Degradation; P: Prenylalcohol; C : Carotinoid Degradation; C a : Caramelization; PA: Phenolic Acid Degradation.

opposed to the more than 80 pyrazines identified as roasted coffee constituents, the methoxy alkyl pyrazines are not formed by Maillard reaction but are generated within the coffee plant tissue. One speculative mechanism was proposed by Murray et al. (12) and is depicted in Figure 1. Amidation of valine or leucine, in the case of the isobutyl derivative, gives the intermediate product I, which undergoes a condensation with glyoxal to form compound II. Finally the methoxy alkyl pyra zines are generated via methylation of the hydroxy group. This special pathway may be the reason why III and 2-methoxy-3-isobutyl pyrazine are detectable in other raw plant material such as peas and paprika. Excessive amounts of III were the reason for a "peasy" off flavor note in certain batches of Ruandian coffee (12). Numerous pyrazines derived from Maillard reactions occur in roasted coffee in remarkable amounts and form about 14 % of the overall volatile content (14). Their formation pathway is shown in Figure 2. α-Aminoketones (IV) with various alkyl groups (generated by Strecker degradation, see Figure 3) can undergo condensation to form pyrazines. However, the majority show only negligible sen sory potency. According to the results of semi-quantitative sniffing assessment, only 2,3,5-trimethyl pyrazine (V), 2-ethyl-3,5-dimethyl pyrazine (VI) and some pyrazine-like aroma notes of unknown chemical structure seem to have an in fluence on coffee aroma. The aroma note "roasty/musty/flowery" typically formed by pyrazines, occurred several times during sniffing and the effect of pyrazines on the overall aroma presumably must be evaluated additively.



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THERMALLY GENERATED FLAVORS

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I

H j N ^ O

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Formation Pathways for Primary Roasted Coffee Aroma Compounds

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