Effect of skin contact and pressure on the composition of Sauvignon Blanc must

Early white winemaking operations are known to affect the extraction of grape skin compounds into the juice fraction, which will dictate their concentration in the resulting wine. Grape skin contact and the amount of pressure applied during grape pressing affect the extraction of varietal aromas located in the skins. Compounds such as the polyphenols and glutathione, with antioxidant properties involved in juice oxidation processes and white wine stability, are also affected. The present study evaluates how grape skin contact and the amount of pressure applied during grape pressing affect the levels of S-(3-hexan-1-ol)cysteine (3MH-S-cys, a key grape-derived precursor to the volatile thiol 3-mercaptohexanol (3MH), which is reminiscent of passion fruit aroma); 2-methoxy-3-isobutylpyrazine (IBMP, with a capsicum-like descriptor); phenolic compounds; and glutathione in Sauvignon Blanc juice. The study was conducted using grapes obtained from commercial Marlborough (New Zealand) vineyards, using both commercial and laboratory grape-processing procedures. Immobilized metal ion chromatography was used to isolate the 3MH-S-cys precursor from the juices. The isolated precursor was then volatilized by trimethylsilylation and analyzed using gas chromatography/mass spectrometry (GC/MS). IBMP was analyzed by GC/MS after solvent extraction, and a high-performance liquid chromatography method was used for the quantification of phenolic compounds and glutathione. 3MH-S-cys levels were seen to increase in juice fractions obtained from a winery press operating at higher pressures. The increase was attributed to the cumulative effect of longer skin contact time and the amount of pressure applied. The highly water-soluble IBMP was less affected by the amount of pressure applied during commercial grape pressing. Additional information was generated by the specific assessment of skin contact and applied pressure during grape pressing in a laboratory trial. In this trial, a long (32 h) skin contact time resulted in a greater release of varietal aroma compounds, 3MH-S-cys, and IBMP into the juice, and the concentration was further raised by increasing the pressure applied during pressing. However, for both experiments, the extraction of the varietal aroma compounds was offset by a clear increase in the juice oxidative potential, seen by a decline in glutathione content, a natural grape antioxidant, and an increase in particular oxidizeable polyphenol compounds, which may cause the must or wine to brown and lead to a loss of varietal aromas.