A three-year PhD project at the University of Auckland, in collaboration with Bragato Research Institute, has unpicked the complex web of factors behind reductive off-aromas in New Zealand Pinot Noir. The findings have some practical implications for how winemakers think about vineyard inputs, cellar decisions, and even closure strategy.

Reductive off‑flavours are a persistent and complex challenge for New Zealand Pinot Noir producers. Volatile sulphur compounds (VSCs), compounds responsible for aromas such as rotten egg, onion and struck match, have a very low sensory threshold, which means even small changes in their concentration can create a wine that is considered faulty. The question of why some wines become reductive and others don’t has been difficult to answer, as the chemistry involved is complex and involves a shifting interplay of vineyard inputs, yeast metabolism, fermentation conditions, and what happens to the wine long after it’s bottled.

A recently completed University of Auckland PhD project by Sukhpreet Kaur Gill and supervised by Dr Rebecca Jelley, Dr Jennifer Muhl, Dr Rebecca Deed, and Professor Bruno Fedrizzi, set out to investigate those interactions. Over three vintages (2022-24), wines were made at BRI’s Research Winery under controlled conditions and then subjected to accelerated ageing. The compounds tracked included a broad suite of VSCs, dimethyl sulphide (DMS), ethanethiol (EtSH), carbon disulphide (CS2), methanethiol (MeSH), diethyl disulphide (DEDS), and benzothiazole (BZ), as well as varietal thiols, methoxypyrazines, esters, amino acids, and polysulfides.

 

Monitor elemental sulphur early

Elemental sulphur (S0) is used widely in New Zealand vineyards as a fungicide against powdery mildew, and it is well known that residues left on fruit at harvest can serve as a precursor to VSCs during fermentation.

Across three years of the study, elemental sulphur was the most consistent and consequential variable. High elemental sulphur additions drove significantly elevated concentrations of CS2 and DMS during ageing. In the lab, high sulphur additions led to significantly higher concentrations of glutathione polysulfide compounds. These molecules could be latent precursors of VSCs, capable of degrading to release sulphur-containing off-aromas long after the wine has been bottled. Wines made with high elemental sulphur also showed markedly lower residual copper concentrations in the finished wine, most likely because copper was being tied up in complex formation with H2S produced during fermentation.

For New Zealand Pinot Noir producers, it is important to treat elemental sulphur as a wine-quality risk as well as a vineyard disease-control tool.

The timing of sulphur sprays in the vineyard matters, as applications made close to harvest can leave more residual sulphur on the fruit, which feeds directly into fermentation chemistry. Where powdery mildew pressure demands late-season applications, it’s worth considering the risk to the wine’s reductive trajectory and implementing appropriate mitigation strategies, particularly in wines intended for bottle ageing.

 

The interaction between nitrogen and SO2

Nitrogen availability during fermentation is known to influence H2S production, as yeast under nitrogen stress tends to release H2S as a by-product of sulphur amino acid metabolism.

In the trial wines, neither nitrogen level nor the timing of SO2 addition (pre-crush versus post-crush) produced consistent and significant differences in VSC concentrations. However, the interaction between these variables and elemental sulphur level was significant for a range of compounds, including ethyl phenylacetate, phenylethyl alcohol, isoamyl acetate, benzyl alcohol, and BZ.

This suggests that managing nitrogen status in the juice or must isn’t just about ensuring fermentation health; it may also be about limiting the synergistic effect that high nitrogen can have in the presence of residual elemental sulphur. The implication for winemakers is that YAN monitoring and supplementation strategies should probably be considered alongside, rather than in isolation from, any assessment of vineyard sulphur inputs.

 

Optimise whole bunch inclusion

In 2023, whole bunch inclusion was added to the winemaking protocol, at 0%, 25%, and 75% of the fermentation weight. As expected, whole bunch inclusion significantly increased concentrations of stem-derived compounds. Both IBMP and IPMP, the methoxypyrazines associated with green, herbaceous, and earthy characters, increased with the proportion of whole bunches added, which is consistent with published literature. It’s a trade-off that whole bunch advocates generally accept, stem inclusion adds complexity, but green notes come along for the ride.

What’s perhaps less anticipated is the potential connection between whole bunch inclusion, elemental sulphur, and nitrogen on VSC formation. Wines made with 75% whole bunches, under conditions of both high elemental sulphur and high nitrogen, showed significantly elevated concentrations of EtSH (ethanethiol) and DEDS (diethyl disulphide) — both of which carry onion and cooked cabbage characters at elevated concentrations. Methionol (cooked potato/cauliflower) was also affected by whole bunch inclusion, though in a somewhat more complex pattern depending on elemental sulphur level.

For Pinot Noir producers who favour whole bunch fermentation for structural and aromatic reasons, this research shows that the complexity comes with an elevated reductive risk when elemental sulphur levels are also high.

 

Time copper additions carefully

The 2024 vintage experiment tackled copper fining, which remains one of the most commonly used tools for dealing with reductive aromas post-fermentation. Wines were made with either no copper, pre-fermentation copper additions (at two doses), or a standardised post-fermentation addition at bottling.

The research found that pre-fermentation copper additions, at high doses, significantly reduced concentrations of BZ, 3-methylthio-1-propanol (3MPO), and MeSH in the finished wines. This is consistent with the idea that copper added early is taken up by yeast and fine solids. Critically, pre-fermentation additions did not result in elevated residual copper in the finished wines — the yeast lees and pomace could adsorb it effectively, and it is removed during racking.

Post-fermentation copper fining told a different story. Not only did it show no significant reduction in the most problematic VSCs for the majority of treatments, but residual copper concentrations in the finished wine were measurably higher than in the pre-fermentation addition wines, and notably, higher than the amount of copper added. The research team noted that this residual copper remained stable throughout accelerated ageing, meaning it stays in the wine and continues to participate in ongoing chemistry during bottle storage.

This matters for a couple of reasons. First, the copper-sulfhydryl complexes that form when copper is added post-fermentation are not as readily removed as the chemistry might suggest; only a fraction of added copper can be filtered out in practice. Second, and perhaps more concerning, those copper complexes can slowly release H2S and MeSH during bottle storage, potentially contributing to the very reductive aromas the copper addition was intended to resolve.

 

The role of ageing

A consistent thread running through all three years of the research was the role of ageing itself. Regardless of fermentation treatment, accelerated ageing, carried out at 28–30°C for periods equivalent to roughly three months to over a year of real-time bottle ageing, drove significant changes in the concentration of almost every class of compound analysed.

DMS increased consistently during ageing across all experiments. This is well documented in literature and is attributed to the thermal degradation of its precursor S-methyl methionine, but it’s worth emphasising that this increase occurred regardless of elemental sulphur level, yeast strain, nitrogen status, or copper treatment. DMS is coming for every wine. At higher concentrations, DMS carries cooked vegetable and asparagus notes; at lower concentrations, it can contribute positively to complexity.

Glutathione trisulfide (GSSSG)  decreased during ageing, which is consistent with its potential role as VSC precursors. Their degradation suggests that wines high in polysulfides at bottling may be storing up reductive risk for later in their bottle life.

Fruity and floral esters — ethyl octanoate, ethyl hexanoate, ethyl decanoate, hexyl acetate — decreased during ageing, while certain branched-chain ethyl esters increased. The net effect on wine aroma is a gradual shift away from fresh, vibrant fruit character toward something more developed and savoury. This trajectory is well understood in the context of Pinot Noir maturation, but the research underscores how strongly the pace and direction of that evolution can be influenced by what happens in the vineyard and fermentation.