Use of glutathione under different grape processing and winemaking conditions and its impact on the formation of sulfide off-flavors, colour, and sensory characteristics of Riesling, Sauvignon blanc, and Chardonnay

Abstract

The use of glutathione (GSH) in winemaking has been legitimated recently, according to OIV resolutions OENO 445-2015 and OENO 446-2015 a maximum dose of 20 mg/L is now allowed to use in must and wine. Several studies have proven the benefits of GSH, predominantly in Sauvignon blanc. Thus, oxidative coloration of must and wine is limited, aroma compounds such as volatile thiols are preserved, and the development of ageing flavors such as sotolon and 2-aminoacetophenone is impeded. The protective effect may be explained by the high affinity of GSH to bind o-quinones which are formed during phenolic oxidation and which are known to initiate browning and other oxidative changes. Some researchers have proposed the hydroxycinnamic acid to GSH ratio (HGR) as an indicator of oxidation susceptibility of must and could show that lower ratios yielded lighter musts. In contrast to the advantages of GSH, other researchers found that GSH can foster the formation of H2S and other sulfide off-flavors during fermentation. Even during bottle aging, reductive odors may occur as a late consequence of high GSH levels during winemaking. In order to examine the impact of GSH on the formation of sulfide off-flavors, colour, and sensory characteristics, Riesling, Sauvignon blanc, and Chardonnay grapes were processed under different conditions to obtain musts with high and low phenolic content. Based on the original GSH concentration the HGR was adjusted using GSH or GSH-enriched IDY. The resulting wines were either racked off the lees or submitted to sur lie aging for 4 months. As already observed by others, GSH additions increased the GRP concentration in must and preserved their green color. At the same time, these musts tended to form higher concentrations of H2S, methyl and ethyl mercaptan during fermentation suggesting that excessive GSH is responsible for the production of volatile mercaptan metabolites. Normally, these compounds were degraded at the end of fermentation and dropped below sensory threshold as soon as the wines were racked off the gross lees. However, the decrease in mercaptan content, partly explained by the oxidative formation of disulfides, was strongly impaired when o-diphenols were low in concentration (e.g. in free run juice) or when musts were treated with ascorbic acid and SO2. This observation suggests that an effective mercaptan deodorization in young wines depends on the oxidizability of o-diphenols. Bottled wines were generally lower in GSH than musts. However, elevated levels of GSH could be determined after sur lie aging, possibly explaining the protection against oxidation in this aging regime. Sensory analysis after bottling revealed that the fruity odor of Riesling and Sauvignon blanc wines was enhanced when GSH was added to must in moderate concentrations. Excessive GSH, especially in musts with a low phenolic content (e.g. from whole-cluster pressing), could lead to sensorially noticeable sulfide off-flavor in the later wines.