GC is Fine For Compounds in Wine
Producing Quality Wine Requires Careful Processing
Although wine has been produced in Turkey for thousands of years, it is only relatively recently that there have been concerted attempts to apply scientific methods in order to produce high-quality products. Wine is typically produced by a pressing process, but it possible to obtain some liquid from the macerated material without pressing (free-run juice); wines from this free-run juice typically differ from those made from a pressing process.
The Adana researchers used GC to examine the differences between wines made from free-run juice and those from pressing. The Öküzgözü grape variety was used, which can produce a fine red wine if carefully handled. As well as GC-MS, GC-olfactometry (GC-O) was employed, where human assessors smell the odour of products after they emerge from the GC.
Differences noted by GC between pressed and unpressed wines
Crushed grapes were macerated for 7 days in the presence of yeast in a stainless steel tank. Half the material was separated without pressing, while the other half was mechanically pressed at 2.0 atmospheres. Both liquids were further fermented by the malolactic method and were then clarified using gelatine and filtered. The wines were extracted with dichloromethane, the internal standard (4-nonanol) was added and the extract solutions were concentrated prior to GC.
GC was carried out using an Agilent 6890 instrument fitted with a J&W DB-Wax column. The temperature was kept at 40 °C for 10 min, taken to 160 °C at 3 °C/min, heated to 240 °C at 6 °C/min and then kept at 240 °C for more than 25 min. The output from the GC was divided into three: one part going to a flame ionisation detector (FID), one part to a mass detector (an Agilent 5973 MSD with electron ionisation) and one part to an olfactometry sniffing device (Gerstel ODP 2). Three experienced researchers sniffed the main odour compounds, while also determining how notable they were at various dilutions of the GC input. The compounds present in the wine were identified by their retention times and mass spectra, which were compared to library data.
58 aroma compounds at a total concentration of 147 mg/L were detected by GC in the unpressed wine and 59 such compounds at a total concentration of 183 mg/L in the pressed wine.
Isoamyl alcohol was the most abundant aroma compound in both wines. Esters, such as ethyl octanoate and isoamyl acetate, were particularly important in determining the odour of the wines, while the alcohol 2-phenylethanol gave a flowery note to both. The pressing process tended to increase the quantity of unpleasant compounds, such as 4-ethyl guaiacol and 4-vinyl guaiacol (both with a phenolic odour), while decreasing that of some of the pleasant fruity esters.
Sensory examination of the whole wines by a panel of nine experienced assessors concluded that the unpressed wine had a better flavour, while the pressed wine was more astringent and bitter, as might be expected from the GC results.
LC-MS/MS analysis the wines detected a number of anthocyanins, the overall concentration of these compounds being slightly higher in the pressed (334 mg/L) compared to the unpressed wine (298 mg/L). Anthocyanins are believed to possibly have health benefits and are major contributors towards the colour of red wines.
Light pressing recommended for Öküzgözü wines
Sensory examination showed that the upressed wine had a better flavour than the pressed wine. The combination of GC-MS and GC-O showed the chemical compounds that were responsible for the difference. In practice, it would not be economical to only produce only unpressed wine, so a minimal pressing was recommended by the authors.
Journal of Mass Spectrometry, Early View paper. Tetik et al. Screening of key odorants and anthocyanin compounds of cv. Okuzgozu (Vitis vinifera L.) red wines with a free run and pressed pomace using GC-MS-Olfactometry and LC-MS-MS.
Journal of Separation Science, 2006, 29, 2107-2125. Delahunty et al. Gas chromatography-olfactometry.
Critical Reviews in Food Science and Nutrition, 2015, 55, 202-218. González-Barreiro et al. Wine aroma compounds in grapes: A critical review.
Article by Ryan De Vooght-Johnson