Flavor Analysis - American Chemical Society

Chapter 13

γ- andδ-Thiolactones:An Interesting Class of SulfurContaining Flavor Compounds

Downloaded by NORTH CAROLINA STATE UNIV on September 28, 2012 | http://pubs.acs.org Publication Date: September 10, 1998 | doi: 10.1021/bk-1998-0705.ch013

Karl-Heinz Engel, Irmgard Roling, and Hans-Georg Schmarr Technische Universität München, Lehrstuhl für Allgemeine Lebensmitteltechnologie, A m Forum 2, D-85350 Freising-Weihenstephan, Germany th

This contribution is dedicated to Dr. Roy Teranishi on the occasion of his 75 birthday. γ-Thiolactones (5-alkyldihydro-2(3H)-thiophenones) and δ-thiolactones (6-alkyltetrahydro-2H-thiopyran-2-ones) were synthesized by reaction of the corresponding oxygen containing lactones with thiourea and hydrobromic acid. Analytical data (MS, 1H and C N M R ) , chromatographic behavior, G C separation of the enantiomers, and sensory properties of the newly described compounds are presented. The substitution of oxygen by sulfur in the lactone ring induces tropical fruit notes. Odor thresholds and sensory characteristics of the thiolactones vary with ring size and chain length. 13

Sulfur-containing compounds are known as outstanding contributors to pleasant notes as well as off-flavors of many foods (1,2). On one hand, they are formed during thermal processing of foods, such as roasting of coffee (3). On the other hand, they may be biogenetically derived and determine the aroma of natural systems, such as some tropical fruits (4,5). Another widely distributed class of substances known to have an impact on many flavors are aliphatic y- and 8-lactones (6). Nectarines (7), apricots (8), and peaches (9) are classic examples of fruits with a lactone-type aroma. The effect of a substitution of oxygen by sulfur on flavor quality and potency has been shown for many substances (70); one of the most impressive examples is the difference between oc-terpineol and l-p-menthene-8-thiol (77). The replacement of oxygen in the lactone ring by sulfur, resulting in y- and 8-thiolactones, has not yet been investigated. The short chain homologue, y-thiovalerolactone, has been identified after heating model systems with sulfur-containing amino acids (12,13); however, systematic data on this type of compound have not been described. In this contribution the analytical and sensory characterization of a series of y- and 8thiolactones is presented.

©1998 American Chemical Society In Flavor Analysis; Mussinan, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

141

142 Experimental


Synthesis. Experimental details of the synthesis and purification of the y- and 8thiolactones are described elsewhere (14). Capillary Gas Chromatography, (a) Carlo Erba Mega II 8575 (C.E. Instruments, Rodano, Italy) equipped with a flame ionization detector (FID) and a flame photometric detector (FPD, FPD 80, C.E. Instruments). Parallel detection was achieved by dividing the effluent of the column (DB-Wax, J&W, Folsom, C A ; 60 m x 0.32 mm i.d., 0.25 |im film thickness) via press-fit splitter and short pieces of deactivated fused silica capillaries to the two detectors. Split injection was performed at 220°C. Column temperature was programmed from 40°C (5 min hold) to 235°C (5 min hold) at 4°C/min. Hydrogen was used as carrier gas at a constant inlet pressure of 105 kPa. (b) Carlo Erba Fractovap 4160 equipped with FID and split injector; column: 30 m x 0.32 mm i.d. fused silica capillary column coated with a dimethylsiloxane stationary phase (PS-255, A B C R , Karlsruhe, Germany) providing a film thickness of 1.0 |jm. Column temperature: 40°C (10 min hold) to 300°C (5 min hold) at 4°C/min; carrier gas: hydrogen (inlet pressure 50 kPa). Separation of the enantiomers of the thiolactones was achieved on a fused silica column (30 m x 0.25 mm i.d.) coated with 25 % of heptakis-(2,3-di-0-methyl6-0-terf.-butyl dimethylsilyl)-P-cyclodextrin in SE 54 to provide a film thickness of 0.25 Jim. Gas chromatograph: Carlo Erba, Mega 5160 with a FID detector and split injection at 210°C. Hydrogen was used as carrier gas at a constant inlet pressure of 100 kPa. To separate the enantiomers of the thiolactones the oven temperature was programmed from 100°C (2 min hold) to 125°C (10 min hold) at a rate of 3 °C/min and then to 205°C at a rate of 1.5°C/min. Gas Chromatography-Mass Spectrometry ( G C - M S ) . Mass spectral data were acquired on an HP 5890 gas chromatograph coupled to an HP 5970 mass selective detector (Hewlett-Packard, Palo Alto, CA). The mass spectrometer interface temperature was set to 250°C, and the electron energy was 70 eV. The column used for GC-MS was a 50 m x 0.2 mm i.d. FFAP fused silica capillary with a film thickness of 0.33 jam (Hewlett-Packard). Split injection was performed at 230°C, and oven temperature was programmed from 70°C (5 min hold) to 235°C (10 min hold) at 5°C/min. Helium was used as carrier gas at a constant inlet pressure of 175 kPa. 1 3

N M R spectroscopy. *H and C N M R spectra were recorded at 22°C using a D R X 500 and an A C 200 spectrometer, respectively, from Bruker, Karlsruhe, Germany. H detected experiments were performed using an inverse broadband probehead, and C detected experiments were performed using a dual C / H probehead. DEPT and twodimensional double quantum filtered COSY and HMQC experiments were performed according to standard Bruker software. Chemical shifts were referenced to solvent signals ( H , 7.24 ppm; C , 77.0 ppm). !

1 3

1 3

l

l

l 3

Odor Thresholds. Odor thresholds in water were determined in Teflon squeeze bottles according to the procedure described by Guadagni and Buttery (15). Odor

In Flavor Analysis; Mussinan, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

143 thresholds in air were determined by gas chromatography-olfactometry according to the method reported by Schieberle (16).


Synthesis and Identity of Compounds The structures of the thiolactones synthesized in the course of this study are depicted in Figure 1. The reaction sequence starting from the corresponding oxygen containing y- and 8-lactones and proceeding via isothiouronium bromides as intermediates (17) is outlined in Figure 2. The method was applied according to the procedure described for the conversion of y-butyrolactone by Kharasch and Langford (18). The identity of the products was established by means of IR, MS as well as U and C N M R (14). In analogy to the typical ions m/z 85 and 99, known for y- and 8-lactones, the mass spectral patterns of the thio-compounds showed the corresponding fragments at m/z 101 and 115, respectively. As examples, the spectra of y- and 8-thiooctalactone are shown in Figure 3. H and C N M R spectroscopic data are summarized in Figure 4. The substitution of oxygen by sulfur in the lactone ring results in a marked upfield shift of the signals obtained for the proton as well as for the carbon in position 4 (position 5 for 8-lactones, respectively). The C NMR signal assigned to the carbon in position 1 is significantly shifted downfield in the sulfur containing compound. These differences are indicated in Figure 4 for the C6 and C i homologues. l

!

1 3

1 3

1 3

0

Gas chromatographic Data Kovats retention indices (KI) of aliphatic y- and 8-lactones and the synthesized thiocompounds on a polar and an apolar stationary phase are listed in Table I. These data demonstrate that the chromatographic behavior of the thiolactones is comparable to that of their oxygen containing counterparts.

Table I. Comparison of Retention Indices (KI) for y- and 8-Lactones and their Sulfur Containing Counterparts KIDB-Wax KIPS-255 O-lactones S-lactones O-lactones S-lactones 1094 1671 1697 0999 y-c 1312 1889 1909 1209 Y-C 1534 2122 2142 1426 Y-Cio 1754 2350 2382 1643 Y-C12 6

8

8-C 8-C

6

8

8-C10

8-C

12

1762 1941 2169 2404

1791 2004 2233 2473

1038 1234 1448 1663

1124 1344 1565 1789



144

R ethyl ( Y - S - C ) 6

butyl ( Y - S - C ) 8

y - thiolactones

hexyl ( Y - S - C ) I 0

octyl ( Y - S - C ) I 2

methyl (5 - S - C ) 6

propyl

(8-S-C )

pentyl

(8-S-C )

heptyl

(8-S-C )

8

i 0

8 - thiolactones 12

Figure 1. Structures of the synthesized y-thiolactones (5-alkyldihydro-2(3H)thiophenones) and 8-thiolactones (6-alkyltetrahydro-2H-thiopyran-2-ones)



Figure 2. Synthetic route to thiolactones



41

101

45

114 101 55

i

1

.1

3

87 89 158

Ji

l i b in

ll,.

40

60

80

40

60

80

100

100

ll, ,,. , 1 u^-.

^

^

120

140

160

120

140

160

Figure 3. Mass spectra of y-thiooctalactone (a) and 8-thiooctalactone (b)


. m

/

m

z

/

z


147

!

l 3

Figure 4. H and C N M R spectroscopic data of y-/8-hexa- and decathiolactones


148 The enantiomers of the y- and 8-thiolactones could be separated on heptakis(2,3-di-0-methyl-6-0-TBDMS)-p-cyclodextrin (Figure 5). The separation factors of the thiolactone enantiomers are lower than those obtained for the oxygen containing lactones on this modified cyclodextrin stationary phase (79). The 8-thiolactones show a minimum enantioseparation for the Cg homologue. The assignment of the order of elution by means of optically pure enantiomers is the subject of ongoing studies.


Sensory Evaluation Odor thresholds in air were determined by gas chromatography-olfactometry according to the method reported by Schieberle (16). As demonstrated in Figure 6, the odor thresholds (ng/1) in air vary with ring size and chain length. Within the series of synthesized compounds both, y- and 8-thiolactones, exhibit the lowest odor thresholds for the Cg and Cio homologues. Odor thresholds in water were determined in Teflon squeeze bottles according to the procedure described by Guadagni and Buttery (75). Table II shows that the substitution of oxygen by sulfur has different effects depending on the ring size. The odor threshold (ppb) in water for y-decathiolactone is increased compared to the oxygen containing counterpart; the threshold of the corresponding 8-lactone, however, is lowered drastically.

Table II. Impact of Oxygen and Sulfur on Odor Thresholds of C i - L a c tones in Water lactone odor threshold (H 0), [ppb] y-decalactone 11 y-thiodecalactone 47 0

2

a

a

8-decalactone 8-thiodecalactone Engel et al. (7)

100 6

a

These differences in the effects caused by the introduction of sulfur depending on the size of the lactone ring also become obvious in the sensory descriptions obtained for solutions (0.004%) of the thiolactones in water (Table HI). The y-thiolactones are characterized by mushroom-like odors (Q) or fatty, rancid notes (Cio). The 8thiolactones, on the other hand, still exhibit the pleasant lactone aroma, however, combined with a pronounced tropical fruit character. In accordance with the typical behavior of sulfur containing volatiles, the odor impressions of thiolactones in water at higher concentrations are rather unpleasant. Only after dilution are desirable notes perceived (Table IV).


149


(mV)

8-S-C6

8-S-C10

"T-S-C6 8-S-C8

9.6

y-S-C10

Y-S-C12 S - S ' C "

12

Y-S-C8

3.2

1

JUUI 12

24

36

60

48

(min)

Figure 5. Capillary GC separation of the enantiomers of y- and 8-thiolactones on heptakis-(2,3-di-0-methyl-6-0-TBDMS)-P-cyclodextrin odor threshold [ng/l in air]

y-thiolactones 8-thiolactones

11

12 13 carbon number

Figure 6. Odor thresholds (ng/l) in air determined for y- and 8-thiolactones


150 Table III. Sensory Properties of Thiolactones thiolactone odor quality sweet, sulfury, burnt Y-S-C 8-S-Q slightly fruity, petroleum, sulfury 3

6


Y-S-Q 8-S-C

mushroom homogenate, coconut, sweet, sulfury coconut, green, tropical fruit

8

Y-S-C 8-S-Cio 10

fruity, fatty, rancid tropical fruit, fresh

slightly fruity, soapy sweet, soapy, apricot determined in 0.004% solutions in water

Y-S-C,

2

8-S-C12 a

Table IV. Odor Profiles of Thiolactones in Water concentration [ppm] y-thiodecalactone 8-thiodecalactone 400 leek, cabbage sweet, rancid 100 sulfury, leek rancid, soapy 40 sweet, fatty, rancid tropical fruit, spicy 4 cucumber, fatty tropical fruit, sweet 1 green, cucumber, melon sweet, fruity

Conclusion y- and 8-Thiolactones constitute an interesting class of sulfur containing flavor compounds with promising sensory characteristics. Especially the tropical fruit notes observed for 8-thiolactones are very attractive. For a final evaluation of the sensory properties of the thiolactones, aroma profiles at various concentrations and the characterization of the resolved enantiomers will be required. The next step will be the search for these newly described sensorially potent compounds in natural systems. Tropical fruits, many of them known for both lactone and sulfur metabolism, might be potential sources for these flavor components. Acknowledgments The authors thank R.G. Buttery (Western Regional Research Center, U.S. Department of Agriculture) and P. Schieberle and T. Hofmann (Institut fur Lebensmittelchemie, Technische Universitat Miinchen) for determining odor thresholds.


151

Literature Cited


1. 2. 3. 4. 5. 6. 7. 8.

Boelens, H . M . Perfumer

1993, 18, 29-39 Mussinan, C.J.; Keelan, M.E., Eds.; ACS Symposium Series 564; American Chemical Society: Washington, DC, 1994 Flament, I. Food Rev. Int. 1989, 5, 317-414 Engel, K.-H.; Tressl, R. J. Agric. Food Chem. 1991, 39, 2249-2252 Fischer, N . dragoco report (flavoring information service) 1996, 41, 137-147 Maga, J.A. CRC Crit. Rev. Food Sci. Nutr. 1976, 1-56 Engel, K.-H.; Flath, R.A.; Buttery, R.G.; Mon, T.R.; Ramming, D.W.; Teranishi, R. J. Agric. Food Chem. 1988, 36, 549-553 Takeoka, G.R.; Flath, R.A.; Mon, R.T.; Teranishi, R.; Güntert,M.J. Agric. Food & Flavorist

Sulfur Compounds in Foods;

Chem. 1990, 38, 471-477

9. 10. 11 12. 13.

14. 15. 16. 17. 18. 19.

Horvat, R.J.; Chapman Jr., G.W.; Robertson, J.A.; Meredith, F.I.; Scorza, R.; Callahan, A . M . ; Morgens, P. J. Agric. Food Chem. 1990, 38, 234-237 Riechstoffe und Geruchssinn. Die molekulare Welt der Düfte; Ohloff, G., Ed.;Springer Verlag: Berlin, Heidelberg, 1990 Demole, E.; Enggist, P.; Ohloff, G. Hel. Chim. Acta 1982, 65, 1785-1794 Mussinan, C.J.; Katz, I. J. Agric. Food Chem. 1973, 21, 43-45 Shu, C.-K.; Ho, C.-T. In Thermal Generation of Aromas; Parliment, T.H.; McGorrin, R.J.; Ho, C.T., Eds.; ACS Symposium Series 409; American Chemical Society: Washington, DC, 1989, pp. 229-241 Roling, I.; Schmarr, H.-G.; Eisenreich, W.; Engel, K.-H. J. Agric. Food Chem. 1998, 2, 668-672 Guadagni, D.G.; Buttery, R.G. J. Food Sci. 1978, 43, 1346-1347 Schieberle, P. J. Agric. Food Chem. 1991, 39, 1141-1144 Frank R.L.; Smith, P.V. J. Am. Chem. Soc. 1946, 68, 2103-2104 Kharasch,N.;Langford R.B. J. Org. Chem. 1963, 28, 1901-1903 Maas, B.; Dietrich, A.; Mosandl, A. J. High Resol. Chromatogr. 1994, 17, 109115


Flavor Analysis - American Chemical Society

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