Chapter 11
Chemical and Sensory Properties of Thiolactones K.-H. Engel, A. Schellenberg, and H.-G. Schmarr
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Technische Universität München, Lehrstuhl für Allgemeine Lebensmitteltechnologie, Am Forum 2, D-85350 Freising, Germany Model experiments were performed to determine the recoveries of γ-/δ-thiolactones by isolation from aqueous solutions and the stabilities of these volatiles upon heat treatment at different pH -values. Using simultaneous distillation-extraction, thiolactones exhibited higher recoveries than the corresponding lactones. Upon refluxing at neutral or alkaline conditions and subsequent liquid-liquid extraction, thiolactones turned out to be unstable. Using octakis-(2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl)-γcyclodextrin as stationary phase for capillary gas chromatography, a baseline separation for γ- as well as δthiolactones could be achieved. Enantioselective enzyme -catalyzed hydrolysis of δ-thiooctalactone using Porcine Pancreas lipase proceeded enantioselectively, resulting in (R)δ-thiooctalactone and (S)-5-mercaptooctanoic acid of high optical purity. GC-olfactometry was applied to describe odor properties of thiolactone enantiomers and derivatives thereof.
Introduction Flavor and aroma compounds belong to a diverse spectrum of chemical substances (I). Suifur-containing volatiles are among the most prominent and important representatives (2). They are often characterized by a combination of low threshold and pronounced odor properties (3). Accordingly, many of the socalled "character impact compounds" of foods are sulfur-containing substances. They decisively influence thermally produced flavors (4) and they also play outstanding roles in the patterns of biosynthesized aroma constituents of several fruits, such as passion fruit (5), blackcurrant (6) and orange (7). In many cases the substitution of the hydroxyl group by a mercapto group
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© 2001 American Chemical Society
In Aroma Active Compounds in Foods; Takeoka, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
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results in drastic changes of the sensory properties of a compound (3). A n impressive example for this effect is the difference in odor quality and threshold between ot-terpineol and p-menthene-8-thiol (8).
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In recent studies y- and 8-thiolactones have been synthesized in order to investigate the effect of a replacement of oxygen in the ring of aliphatic lactones by sulfur on the sensory properties. The thiolactones turned out to have attractive sensory properties; especially 8-thiolactones exhibit a combination of the warm lactone character and spicy, tropical fruit notes (9, 10). In this contribution additional data on the analytical and sensorial characterization of y- and 8-thiolactones are presented. The following aspects will be covered: (i) chemical stability and recovery, (ii) capillary gas chromatographic enantiodifferentiation, (iii) enzyme-catalyzed kinetic resolution of the enantiomers, and (iv) odor properties of the thiolactones and derivatives thereof.
Experimental Stability and Recovery Two model mixtures were prepared containing 20 mg each of y- and 8lactones and the corresponding thiolactones, respectively. Undecan-2-one (20 mg) was used as internal standard. The substances were dissolved in 5 ml nhexane and filled up to 20 ml with ethanol. From this stock solution a dilution in ethanol (200 ng/|il) was prepared and used for the experiments. 1 ml of the diluted model mixture and 9 ml buffer (citrate/HCl for p H 3.5, phosphate for p H 7.0 and glycine/NaOH for p H 9.0) were subjected to the following isolation procedures: (i) micro simultaneous distillation-extraction (11) for 20 min using wpentane/diethyl ether (1:1, v:v) as solvent, (ii) extraction with diethyl ether after stirring for 20 min at room temperature, and (iii) extraction with diethyl ether after refluxing for 20 min. The extracts were concentrated to 1 ml using a Vigreux column and analyzed by capillary gas chromatography.
Capillary Gas Chromatographic Enantioseparation Separation of the enantiomers was achieved on a fused silica column (30 m x 0.25 mm i.d.) coated in the laboratory with 50% octakis-(2,3-di-
