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64 p r a c t i c a l w i n e r y & v i n e ya r d d e c e M B e r 2 0 1 4 W I N E M A K I N G Neil Scrimgeour and Eric Wilkes, Australian Wine Research Institute, Glen Osmond, Adelaide, Australia BY Closure trials demonstrate volatile sulfur compound formation KEY DRIVERS IN THE SHELF LIFE OF WINE S ince its initial groundbreaking clo- sure trial in 1999, the Australian Wine Research Institute has con- ducted many major commercially funded closure trials, which have provided further understanding of the impact clo- sure selection has on wine development in-bottle and the importance of factors such as total package oxygen (TPO) and storage temperature on shelf life. From the beginning, these closure trials have sought to understand the complex relationship between the wine matrix, oxy- gen introduced at bottling, oxygen trans- mission rate (OTR) through the closure, free SO 2 concentration and development of volatile sulfur compounds. To date, the following concepts have been established: • High levels of oxygen introduced at bot- tling (TPO) can result in a dramatic reduc- tion in shelf life (sometimes reducing it to less than six months). • In commercially bottled wines with well controlled TPO, evolution of wine style is typically driven by the closure OTR, es- pecially with longer term storage periods (more than one year). • White wines are more susceptible to ox- ygen introduced at bottling and through closure permeation than red wines, re- sulting in a more pronounced impact on wine style. Numerous closure trials have shown that oxygen ingress (OTR) is one of the key drivers in determining how a wine will develop after bottling and, ultimately, its expected shelf life. However, understand- ing of how volatile sulfur compounds form and evolve under different wine closures is still developing. A significant number of volatile sulfur compounds are found in wine, and the three listed in Table 1 have been shown to be significant in closure studies. While at low levels they can add complexity to a wine, generally they are considered undesirable. Reversible reactions The simplified reaction scheme in Fig- ure 1 outlines how sulfur-containing compounds are inter-related in wine. An important point to note is that the bal- ance between oxidized sulfur compounds (OSCs) (which have lower sensory impact) and thiols (which have greater sensory impact) is reversible, dependent on the amount of available oxygen and presence of metals. Even when compounds are present in their oxidized form (with minimal sen- sory impact), a change in conditions can drive re-formation of the more potent thiols, negatively impacting wine aroma. In numerous closure trials, dimethyl sulfide (DMS) has been the dominant sulfur-containing compound, with levels typically approaching the aroma threshold (20–25 µg/L) after one year in bottle. Figure 2, which presents data from the current AWRI red wine closure trial, shows that during the first six months of the trial, the DMS levels decreased as the oxygen introduced at bottling was uti- lized in oxida- tion reactions. After this point, the DMS levels increased, rep- resenting a dif- ferent phase in the wine chem- istry that was related, at least in part, to the relatively slow introduction of oxygen through the closure. Wines under closures with a lower OTR, such as screwcaps, have consistently ex- hibited higher levels of DMS, suggesting that the higher levels of oxygen intro- duced through higher OTR closures are involved in the conversion of DMS into other components. Variable performance Free SO 2 levels in closure trials invariably show a rapid downward trend during the first six months of storage, as the o x y g e n i n t ro - duced at bottling (headspace oxy- gen and oxygen entrained in the closure) drives oxidative reac- tions in the wine. After that point, the reduction in SO 2 is primarily At a glance: • Closure trials continue to provide important information about factors influencing wine oxidation, shelf life and formation of volatile sulfur com- pounds. • Concentrations of undesirable vola- tile sulfur compounds in wine experi- ence peaks and valleys over time in bottle. • Closure oxygen transfer rate (OTR) can have a significant impact on wine development over time. Vola le sulfur compound Odor descriptor Threshold Literature AWRI (µg/L) review Hydrogen sulfide H 2 S ro en egg, sewage-like 1 nd-370 nd-56 Methanethiol MeSH ro en cabbage, burnt 1.5 nd-16 nd-11 rubber, putrefica on Dimethlyl sulfide DMS Black currant (at low levels), 25 nd-474 nd-980 cooked cabbage, asparagus, canned corn, molasses Table I: Three important volatile sulfur compounds in wine and their aroma threshold levels. Common concentration ranges as reported in published literature and seen at the AWRI are provided. (Not detected is represented by nd.) R-S H thiols (mercaptans) ~1.5 µg/L O � CH 3 C – S-R Thioacetates >40 μg/L OSC Oxidized Sulfur compounds >10 µg/L H 2 S hydrogen sulfide 1 µg/L Wine components reduc on oxida on acetyla on hydrolysis Figure 1. Sulfur reaction scheme.