The effect of oxygen on the levels of glutathione, an important antioxidant, in must and wine was studied using a novel LC-MSMS method for the analysis of reduced and oxidized glutathione. This study found that the storage of grape juice at high SO2 and ascorbic acid levels at -20 degrees C did not lead to a decrease in reduced glutathione levels. The effect of varying the oxygen levels in South African white grape juices, which included a reductive treatment (less than 0.3 mg/L dissolved O2 added), a control treatment (between 1.0 and 1.5 mg/L dissolved O2 added), and an oxidative treatment (3.5-4 mg/ L dissolved O2 added, without SO2) on reduced glutathione levels in the juice and resulting wine was also investigated. A custom build press was used to press whole bunches of two different Sauvignon Blanc and Colombard grapes. Alcoholic fermentation and oxygen additions to the must led to lower reduced glutathione levels in the wine. Reduced glutathione levels were only significantly higher in the wine made from reductive juice that had the highest initial reduced glutathione levels in the grapes.

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ABSTRACT:The development and accumulation of secondary metabolites in grapes determine wine color, taste, and aroma. This study aimed to investigate the effect of leaf removal before flowering, a practice recently introduced to reduce cluster compactness and Botrytis rot, on anthocyanin, tannin, and methoxypyrazine concentrations in 'Merlot' grapes and wines. Leaf removal before flowering was compared with leaf removal after flowering and an untreated control. No effects on tannin and anthocyanin concentrations in grapes were observed. Both treatments reduced levels of 3-isobutyl-2-methoxypyrazine (IBMP) in the grapes and the derived wines, although the after-flowering treatment did so to a greater degree in the fruit specifically. Leaf removal before flowering can be used to reduce cluster compactness, Botrytis rot, and grape and wine IBMP concentration and to improve wine color intensity but at the expense of cluster weight and vine yield. Leaf removal after flowering accomplishes essentially the same results without loss of yield.

Sauvignon blanc wines often have complex aromatic profiles with flavours ranging from herbaceous to tropical. The formation of volatiles during fermentation is critical for the aromatic bouquet of a wine. Oxygen and sulfur dioxide additions to the must can influence the formation of these compounds. However, the effect of different, controlled oxygen and sulfur dioxide additions to Sauvignon blanc must and the changes it induces in the corresponding wines has not been investigated in detail. The present study evaluated how controlled oxygen and sulfur dioxide additions to Sauvignon blanc must affects the levels of volatile and non‐volatile compounds, including the methoxypyrazines, volatile thiols, esters, alcohols, fatty acids, monoterpenes, glutathione and phenolics. It was evident that the presence of sulfur dioxide resulted in wines with higher concentrations of certain compounds. Copyright © 2013 John Wiley & Sons, Ltd.

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Grape berries were classified according to diameter and total soluble solids (TSS) to study the effect of light and temperature on methoxypyrazines (MPs), glutathione (GSH), and hydroxycinnamates (HCAs) during the ripening of Sauvignon blanc. The light exposure of the fruiting zone was modified within leaf and lateral removal at the phenological stage berry of peppercorn size and no removal (control). In comparison to the control, the concentration of 3-isobutyl-2-methoxypyrazine (IBMP) was below the limit of detection in leaf removal 2 weeks before harvest. Leaf removal had no significant influence on GSH and HCAs in the grape juice at harvest. Berry diameter significantly influenced the concentration of IBMP in the grape juice and did not influence the concentration of GSH and HCAs. At harvest, the concentrations of IBMP in grape juices of similar TSS in the control were 12.6 and 5.2 ng/L in 15.5 and 13.5 mm berry diameter classes, respectively. Furthermore, the study showed that berries of the same diameter were not at the same physiological ripening level (not the same TSS).

Early leaf removal around the cluster zone is a common technique applied in cool climate viticulture, to regulate yield components and improve fruit quality. Despite the increasing amount of information on early leaf removal and its impact on total soluble solids, anthocyanins, and polyphenols, less is known regarding aroma compounds. In order to verify the hypothesis that defoliation, applied before or after flowering, could impact the biosynthesis of thiol precursors, we performed a two year (2013 and 2014) experiment on Sauvignon blanc. We provided evidence that differential accumulation of thiol precursors in berries is affected by the timing of defoliation, and this impact was related to modifications in the biosynthetic pathway. Furthermore, the possible interaction between leaf removal treatment and seasonal weather conditions, and its effect on the biosynthesis of volatile precursors are discussed. Our results suggested that in Sauvignon blanc the relative proportion of 4-S-glutathionyl-4-methylpentan-2-one (G-4MSP) and 3-S-glutathionylhexan-1-ol (G-3SH) precursors can be affected by defoliation, and this could be related to the induction of two specific genes encoding glutathione-S-transferases (VvGST3 and VvGST5), while no significant effects on basic fruit chemical parameters, polyphenols, and methoxypyrazines were ascertained under our experimental conditions.

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