Relationship between pH, copper treatment, and ‘reductive’ aroma formation in post- bottling

Marlize Z. Bekker, Paul A. Smith, and Eric N. Wilkes The Research Institute, PO Box 197, Glen Osmond (Adelaide) SA 5064, Corresponding author’s email: [email protected] Introduction and aims Methods

Volatile sulfur compounds (VSCs) have a significant impact on the overall and wines (2014 , South Australia) were subdivided aroma of wines due to their low sensory thresholds. The main compounds into two portions, where one portion was pH adjusted to 3.0 and the other was

associated with ‘reductive aromas’ are hydrogen sulfide (H2S), methanethiol kept at the original pH (Chardonnay: pH 3.46; Shiraz: pH 3.72). One half of (MeSH), and dimethyl sulfide (DMS). These compounds are associated with each wine portion was treated with CuSO4.5H2O to give a final concentration unpleasant aromas described as ‘rotten egg’, ‘sewage’, ‘rubber’, ‘cooked of 0.5 mg/L Cu (Figure 1). vegetables’ and ‘canned corn’. The VSC profiles of the wines were analysed using GC-SCD. Factors that contribute to increased VSC formation post-bottling include low- oxygen conditions as well as increased residual Cu concentrations. Wine pH (1) + Cu also has the potential to influence chemical reactions in wine, by altering the Unadjusted pH Chardonnay pH 3.46 structural characteristics of precursor compounds, and affecting the way in Shiraz pH 3.72 which catalytic compounds such as Cu interact with precursor compounds (2) No Cu during VSC formation. However, the effect of pH on VSC formation has not yet Chardonnay wine been demonstrated. Shiraz wine (3) + Cu This study aimed to determine how wine pH, and the interaction between pH Low pH and Cu, affects the formation of VSCs in wines post-bottling. (pH 3.0) (4) No Cu Results Figure 1. Experimental design Four VSCs were significantly affected by pH, with lower wine pH associated with decreased H2S, MeSH, DMS, and CS2. Hydrogen sulfide Methanethiol Chardonnay Chardonnay

• Cu increased H2S • Cu decreased MeSH formation formation (Month 3, • But at lower pH less Month 6)

H2S was produced • But at lower pH less • Lower pH decreased MeSH was produced Cu effect (Month 3, Month 6)

Shiraz Shiraz

• Cu increased MeSH

• Cu increased H2S formation (Month 1, formation Month 3) • Lower pH had no effect • But at lower pH less

on H2S formation in MeSH was produced Shiraz • Lower pH decreased Cu effect

Dimethyl disulfide Carbon disulfide Chardonnay Chardonnay

• Cu had no effect on • Cu increased CS2 DMS formation formation • At lower pH less DMS • But at lower pH slightly was produced less CS2 was formed • Lower pH decreased • Lower pH decreased DMS formation CS2 formation

Shiraz Shiraz

• Cu had no effect on DMS formation • Cu had no effect on • At lower pH less DMS CS formation was produced 2 • Lower pH had no effect • Lower pH decreased on CS formation DMS formation 2

Conclusions

The post-bottling formation of H2S, MeSH, DMS, and CS2 in the wines studied was significantly affected by wine pH. For some VSCs, the interaction between pH and Cu treatment also had significant effects on their final concentrations post-bottling (Bekker et al. 2016). Winemakers have some flexibility in managing wine pH and Cu additions and therefore could take these results into account when making pH and Cu management decisions.

Reference: Bekker, M.Z., Mierczynska-Vasilev, A., Smith, P.A., Wilkes, E.N. 2016. The effects of pH and copper on the formation of volatile sulfur compounds in Chardonnay and Shiraz wines post-bottling. Food Chem. 207: 148-156.

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