Direct accurate analysis of cysteinylated and glutathionylated precursors of 4-mercapto-4-methyl-2-pentanone and 3-mercaptohexan-1-ol in must by ultrahigh performance liquid chromatography coupled to mass spectrometry

A direct method for the quantitative determination in grapes of four known precursors of the varietal aromas of the thiol character of wine has been optimized and validated. A small volume of centrifuged and filtered must is directly injected in the ultrahigh liquid performance chromatography coupled to mass spectrometry system (UHPLC-MS-MS). Must sugars and other early eluting polar compounds are diverted to the waste, not entering the ion source. Cysteinyl (CYS) and glutathionyl (GLU) precursors from 3-mercaptohexanol (3MH) and 4-mercapto-4-methylpentanone (4M4MP) are separated in 5 min rendering narrow peaks (W1/2<9s). No system performance degradation has been noticed in series of more than 200 injections. A standard addition procedure using the consecutive injection of a spiked sample made it possible to satisfactorily correct for matrix effects, with recoveries very close to 100% in all cases. Intermediate reproducibility was satisfactory (RSD between 1 and 10%), except for the glutathione-4-mercapto-4-methyl-2-pentanone (GLUMP) precursor, for which this value is around 20%. The limits of detection in real-must were below 1 μg L(-1) for the precursors of the 4M4MP and for cysteine-3-mercaptohexan-1-ol (CYSMH) and glutathione-3-mercaptohexan-1-ol (GLUMH), 2 and 7 μg L(-1), respectively, what is enough for the determination of these precursors in musts of any variety.

Amino acids and volatile compounds in wines from Cabernet Sauvignon and Tempranillo varieties subjected to malolactic fermentation in barrels

The aim of the present paper is to compare the behaviour of industrial lactic bacteria and indigenous bacteria of the cellar when malolactic fermentation was carried out in barrels. The effects of these bacteria on the concentration of metabolised amino acids during malolactic fermentation and on the composition of volatile compounds both before and after malolactic fermentation are studied. The experiment was performed with wines of the Tempranillo and Cabernet Sauvignon varieties. An analysis has been made of the easily extractable volatile compounds of the wood and the compounds from the grapes, and the action of the yeasts during the alcoholic fermentation. Acetoin and diacetyl decreased during the malolactic fermentation in barrels and the concentrations of furfural and its derivatives were up to 100 times higher in wines not subjected to malolactic fermentation. Most of the volatile phenols increased during the malolactic fermentation in wines of the Tempranillo variety, while only guaiacol ( p < 0.05) and t-isoeugenol increased in the Cabernet Sauvignon wines. The decrease in amino acids during the malolactic fermentation depends much more on the variety than on the bacterial strain which carries out the malolactic fermentation.

Clues about the role of methional as character impact odorant of some oxidized wines

A young white wine that had undergone spontaneous oxidation and showed a strong off-flavor reminiscent of cooked vegetables was demixed by salting out to obtain an ethanolic extract that retained the off-flavor of the wine, as was demonstrated by sensory analysis. This extract, together with a second one obtained from a non-oxidized wine sample, were chromatographed into a reversed-phase HPLC column using a water/ethanol gradient. The column effluents were collected in 14 different fractions that were evaluated for smell. Four fractions were found to differ between oxidized and non-oxidized wine, but only one showed a clear cooked vegetables off-flavor and, when added to a non-oxidized wine, made it significantly more similar to the oxidized wine. The fraction was analyzed by GC-MS-olfactometry, which made it possible to identify methional as the single important odorant of the fraction. The odor threshold of methional in a synthetic wine was found to be 0.5 microg L(-)(1). The analytical determination of methional through GC-FPD in several oxidized and non-oxidized wines showed that, in the former, methional can reach more than 200 Odor Units, whereas in the non-oxidized samples, it was not possible to detect methional. The methional concentration was found to increase in wines spiked with both methionol or methionine, which suggests that it can be formed from direct peroxidation of methionol or via Strecker degradation of methionine mediated, probably, by o-quinones formed during wine oxidation.

Semipreparative reversed-phase liquid chromatographic fractionation of aroma extracts from wine and other alcoholic beverages

The suitability of reversed-phase HPLC for the semi-preparative fractionation of aroma extracts from wine and other alcoholic beverages has been explored. Aroma extracts are separated in a 250x10-mm Kromasil-C18 column using a water-ethanol gradient system as mobile phase. It has been demonstrated that the chromatographic separation does not induce any chemical change in the sample components. The maximum volume that can be injected without altering efficiency is as high as 2 ml if ethanolic extracts are injected, and slightly less in the case of less polar extracts. Aroma extracts are injected directly without the need of any pretreatment. As major compounds elute first, it is possible to fractionate all the volatiles contained in a 1-1.5-1 sample without peak distortion or mass overload problems. The usefulness of the method has been demonstrated by fractionating an extract from a Chardonnay wine to get 15 fractions that showed different aromas. The GC analysis with olfactometric and MS detection of those fractions has allowed us to identify more than 70 aroma compounds and to signal some of them as potential key aromas of Chardonnay wine.