Sulphur volatile

For analysis of free compounds, the headspace analysis with a multiphase fiber is even more interesting and less time-consuming. This approach can replace the majority of the quantification of free compounds measured by SPE techniques, considering also the possibility of the sampling automation using a GC-MS system which can be coupled to a statistical treatment of fragments abundance (Kinton et al., 2003 Cozzolino et al., 2006). Moreover, HS-SPME/GC-MS is a very effective and efficient method to analyze specific compounds present in trace levels at about ppt level, because they can be better and selectively enriched in the headspace. This method is employed nowadays to quantify some important and peculiar sensory compounds such as ethyl and vinylphenols, pyrazines, cork off-flavour substances (TCA, etc.) and other contaminants such as geosmine (Riu et al., 2002 Chatonnet et al., 2006) and, as shown below, sulphur volatiles. 

Common Fermentative Sulphur Volatiles (CFSV) in wine recent results in qualification and discrimination of different varietal wines at different storage time... 

The first group of sulphur compounds mentioned above can be defined as common fermentative sulphur volatiles (CFSV) and includes in our presentation ethylmercaptan (EtSH), dimethyl sulphide (DMS), diethyl sulphide (DES), dimethyl disulphide (DMDS), diethyl disulphide (DEDS), methyl thioacetate (MTA), ethyl thioacetate (ETA), 2-mercaptoethanol (ME), 2-(methylthio)-l-ethanol (MTE), 3-(methylthio)-l-propanol (MTP), 4-(methylthio)-l-butanol (MTB), benzothiazole (BT) and 5-(2-hydroxyethyl)-4-methylthiazole (HMT) (see Table 5.6). 

Table 5.6 Sensory thresholds and levels of common fermentative sulphur volatile compounds in wines from Trentino (Northern Italy) in the international grape varieties Chardonnay and Merlot and the red berries native grape varieties Marzemino and Teroldego. Samples were from the five vintages 1998 and 2001-2004 (Fedrizzi et al., 2007a).

Tropical Fruit Scenting Sulphur Volatiles (TFSV) in wine recent additional results in quantification and ranging in different varietal wines of Italy... 

The most important Tropical Fruit Scenting Sulphur Volatiles (TFSV) in wine are 3-mercaptohexan-l-ol (3-MH), 3-mercaptohexanol acetate (3-MFLA) and 4-methyl-4-mercaptopentan-2-one (4-MPP). The main problem for quantification of TFSV is their possible very low concentration which may be only a few ng/L. Therefore, only the use of selective isolation and pre-concentration steps allows this difficulty to be overcome. Another problem occurring in thiols analysis is their possible reactivity towards oxidation induced by the working conditions (Rapp et al., 1985) and/or by some components of the matrix in analysis (Murat et al., 2003 Blanchard et al., 2004). 

Fermentative Sulphur Volatile (CFSV) Compounds in Wine... 

Several analytical approaches are employed to quantify sulphur volatiles in wine among them, the headspace procedure, such as the Purge and Trap, and solid-phase microextraction methods, combined with GC coupled to different detectors, was shown to be quite effective (Mestres et al., 2000 Rauhut et al., 1998). Recent papers suggest that the HS-SPME technique, with an improved choice of fiber coating phases, would be one of the more promising approaches for the concurrent measurement of compounds with different boiling points (Mestres et al., 2002 Fang and Qian, 2005). 

Figure 5.12 Temperature effect in the HS-SPME adsorption of the wine CFSV on DVB/CAR/PDMS fiber. (Reprinted from Rapid Communications in Mass Spectrometry 21, Fedrizzi et al., Concurrent quantification of light and heavy sulphur volatiles in wine by headspace solid-phase microextraction coupled with gas chro-matography/mass spectrometry, p. 711, Copyright 2007, with permission from John Wiley Sons, Ltd)...

Recent studies performed on white and red compounds wines treated to eliminate all sulphur volatile compounds by Ag salts addition (Fedrizzi et al., 2007a 2007b), showed the matrix strongly influences the apparent partition between the liquid phase and the fiber coatings. Data of peak areas reported in Table 5.14 show an inversely proportional dependence between the ethanol concentration and the thiols signals, mostly for 3-MHA. 

Fedrizzi, B., Magno, F., Badocco, D., Nicolini, G. and Versini, G. (2007d) Aging effects and variety dependence on the content of sulphur volatiles in wine, J. Agric. Food Chem., 55, 10880-10887. 

Figure 5.4. The HS-SPME-GC/MS chromatograms recorded in SIM mode in the analysis of compounds reported in Table 5.5 (a) internal standards (33. d6-DMS, m/z 68 34. DPDS, m/z 108 35. MT, m/z 71 36. M ill, m/z 148), (b) analytes (37. EtSH, m/z 62 38. DMS, m/z 62 39. DES, m/z 75 40. MTA, m/z 90 41. DMDS, m/z 94 42. ETA, m/z 104 43. DEDS, m/z 122 44. ME, m/z 78 45. MTU, m/z 92 46. MTP, m/z 106 47. MTB, m/z 120 48. BT, m/z 135 49. IIMT, m/z 122).The SPME conditions are reported in Table 5.4. (Reprinted from Rapid Communications in Mass spectrometry 21, Fedrizzi et al., Concurrent quantification of light and heavy sulphur volatiles in wine by headspace sohd-phase microextraction coupled with gas chromatography/mass spectrometry, p. 710, Copyright 2007, with permission from John Whey Sons, Ltd.)...

Hop oil contains a series of thioesters (Table 13.7) the combined amount of which in steam-distilled hop oil usually exceeds 1000 ppm. The level of thioesters in the oil does not appear to be affected either by treatment of hops with elemental sulphur on the bine or by sulphur dioxide kilning [50]. Thioesters are formed in hops largely by the action of heat, so low levels will be introduced into beer by dry hopping. Few sulphur volatiles survive 60 min of wort boiling but after late addition of hops to the copper most of the sulphur compounds discussed above are present in the wort including the thioesters. During fermentation dimethyl trisulphide and some of the thioesters disappear but some sulphur volatiles survive into the finished beer S-methyl 2-methylbutanethiolate is the principal thioester to survive. This last ester and 5-methyl hexanethiolate, the thioester with the lowest taste threshold, are the major thioesters introduced into beer by dry hopping [50]. 

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