Impact of copper residues in grape must on alcoholic fermentation: effects on yeast performance, acetaldehyde and SO₂ production

Abstract

A relevant trend in winemaking is to reduce the use of chemical compounds in both the vineyard and winery. In organic productions, synthetic chemical fertilizers and pesticides must be avoided. The use of copper and sulphur-based molecules as an alternative to chemical pesticides, is a common practice in organic vineyards, but is in turn gradually decreasing, for several reasons. Beyond regulatory changes that impose lower limits, indeed, the use of high doses of copper seems to affect the composition of grape must, and therefore have an impact on the winemaking process. Previous studies have established that high concentrations of Cu2+ can be toxic to yeasts, inhibiting growth and activity, causing sluggish fermentation and reducing alcohol production. Nevertheless, with the objective to ascertain toxicity limits for yeasts, most of these works employed Cu2+ concentrations much higher than those normally found in grape juices (>20 mg/L), whereas very few information is available about lower copper amounts.

In this context, the aim of our work is to verify the potential effect of real copper contaminations from the vineyard on alcoholic fermentation. In fact, the goal of the trial was to compare different commercial strains of Saccharomyces cerevisiae in order to observe the effect of copper on their metabolism (in terms of: fermentation kinetics, production of free and total SO2 and production of acetaldehyde), rather than to verify at what copper concentration the fermentation goes sluggish or stuck. A second objective is also to identify the yeast strains whose behavior is less affected by the presence of copper.

For this reason, a first test was conducted on different natural white musts, contaminated or not with 10 mg/l of Cu2+, trying to mimic as much as possible what happens in the cellar. 15 active dry yeasts have been compared, outlining a common effect of acetaldehyde increase due to copper presence, but displaying a high strain and must-dependent individual behavior in terms of amounts of SO2 and acetaldehyde produced.

To investigate the impact of Cu2+ on yeast metabolism under more controlled conditions, a subset of relevant strains was compared in a synthetic must [5], added or not with Cu2+. The influence of fermentation temperature was also evaluated, carrying out the experiments at 16, 18 and 20°C. Fermentation kinetics, free and total SO2 production, and acetaldehyde production were monitored. The results confirm the common observation of increased acetaldehyde production in the presence of copper, while also confirming strain-specific responses which resulted more stable than in natural must. Regarding temperature effects, beyond the expected impact on fermentation duration, our initial findings suggest that temperature influences the magnitude of the difference between copper and control conditions.