30
Appearance of Reduced Aromas during Bottle Storage

30.1 Introduction

Each year, over 10 000 wines are entered into the International Wine Challenge (IWC) in London, one of the largest and most prestigious such competitions in the world. Receiving a Gold Medal at the IWC can be a winery’s ticket to exploding sales, but faulty wines are still routinely observed – typically, about 6% of entries [1]. The major faults deemed to be present in these wines are associated with cork taint (Chapter 18), Brettanomyces spoilage (Chapters 12 and 23.3), oxidation (Chapter 24), and reduced sulfur aromas (Chapters 10 and 22.4) (see Figure 30.1). Of these major faults, the factors responsible for the appearance of reductive aromas in bottle are arguably the most mysterious.

Figure 30.1 Distribution of faults in wines from 2008 International Wine Challenge in London, UK. Of the 10 000+ wines entered, approximately 6% were described as faulty [1]

30.2 Potential latent sources of compounds responsible for reduced aromas

Reduced aromas are correlated with wines possessing suprathreshold concentrations of sulfurous compounds – namely, H₂S, CH₃SH, and dimethyl sulfide (DMS) (Chapter 10). Reductive aromas are more commonly observed in wines bottled in packaging with low oxygen ingress (i.e., under a tin‐lined screw cap) [2], presumably because of the absence of oxidative reactions.¹ DMS formation is well established to arise from non‐oxidative hydrolysis of S‐methylmethionine (Chapter 23.3). However, the identity of the precursors capable of releasing H₂S and CH₃SH during storage is not as clear. Two early hypotheses are listed below:

• Classic hypothesis 1. Mercaptans can be oxidized to their corresponding disulfides during wine production; for example, CH₃SH will form dimethyl disulfide. Because disulfides have higher sensory thresholds than their corresponding mercaptans, they can enter the bottle unnoticed, but will reform mercaptans during anaerobic storage [3].
• Classic hypothesis 2. Mercaptans arise during bottle storage from acid hydrolysis of thioacetates, for example, methyl thioacetate [4].

There is evidence that both disulfides and thioacetates can be formed during fermentation, and that they can release mercaptans early in a wine’s life. However, as noted in a review by Ugliano, there is scant evidence that these hypotheses are valid for actual bottled wines [5]. For example, the appearance of CH₃SH is uncorrelated with either dimethyl disulfide or methyl thioacetate disappearance. On the other hand, oxidation of wines containing thiols results in their loss – but through coupling with quinones (Chapter 24), not disulfide formation. Finally, the classic hypotheses cannot explain the formation of H₂S during storage.

Searches for alternate precursors of these latent compounds are an active area of research. Metal‐catalyzed hydrolysis of sulfur containing amino acids (cysteine, methionine) has been proposed because addition of transition metals will enhance the rate of H₂S and CH₃SH formation [5]. Other work has demonstrated that copper–thiol complexes (e.g. CuS) can stay dispersed in wine following copper additions, and do not necessarily fully precipitate – leading to the hypothesis that these complexes serve as latent thiol precursors of H₂S and other thiols during storage [6].

Still other latent sources of H₂S and CH₃SH may exist. For example, the potential contribution of non‐volatile asymmetric disulfides or trisulfides (e.g., adducts with other thiol compounds) has not been well explored. Also, quinone–thiol adducts formed following wine oxidation (Chapter 24) are assumed to be stable – but this is not beyond doubt. Perhaps these adducts can subsequently release H₂S and CH₃SH, or other thiols, during prolonged storage. Finally, other potent (and common) contributors to reduced aromas may have been overlooked. If the standing hypotheses are not valid, then these questions are founded on unknown chemical processes; that is, what “reducing” agent/s or reactions are participating or occurring? Wine storage is a rare situation of very long lifetimes of reactive solutions where kinetically slow processes, perhaps those not commonly observed otherwise, have time to occur.

Determining the likely latent sources of H₂S, CH₃SH, or other causes of reduced aromas is more than interesting chemistry – it is also a crucial step in eliminating an increasingly vexing problem. Determining their identity should lead to better prevention, remediation, and detection strategies for reduced aroma formation in bottle. Currently, some winemakers will add ascorbic acid to predict the appearance of reduced aromas, but the rationale for this test is based on disulfides as latent precursors, and these tests have not been validated. In recent years, winemakers have benefited from accelerated aging tests, for example, contact tests to predict the likelihood of potassium bitartrate instability (Chapter 26.1), and it is expected that a validated accelerated reduction assay would be equally beneficial.

References

1. 1. Goode, J. and Harrop, S. (2008) Wine faults and their prevalence: data from the world’s largest blind tasting, 16èmes Entretiens Scientifiques Lallemand , Horsens.
2. 2. Godden, P., Francis, L., Field, J., et al. (2001) Wine bottle closures: physical characteristics and effect on composition and sensory properties of a Semillon wine – 1. Performance up to 20 months post‐bottling. Australian Journal of Grape and Wine Research , 7 (2), 62–105.
3. 3. Limmer, A. (2005) Do corks breathe? Or the origin of SLO. Australian and New Zealand Grapegrower and Winemaker , 497, 89–98.
4. 4. Rauhut, D., Kurbel, H., MacNamara, K., Grossmann, M. (1998) Headspace GC‐SCD monitoring of low volatile sulfur compounds during fermentation and in wine. Analusis , 26 (3), 142–145.
5. 5. Ugliano, M. (2013) Oxygen contribution to wine aroma evolution during bottle aging. Journal of Agricultural and Food Chemistry , 61 (26), 6125–6136.
6. 6. Franco‐Luesma, E. and Ferreira, V. (2014) Quantitative analysis of free and bonded forms of volatile sulfur compouds in wine. Basic methodologies and evidences showing the existence of reversible cation‐complexed forms. Journal of Chromatography A , 1359, 8–15.

Note

1 Although many issues may be encountered in the winery, the closure or packaging type eventually plays a relatively large role in the presence of faults. Cork taint and oxidation can be associated with cork closures, and the use of screw cap and some technical closures to overcome this brings a new conundrum, that of reductive characters.