The influence of the copper content in grape must on alcoholic fermentation kinetics and wine quality: a survey on the performance of 50 commercial active dry yeasts

The effects of copper on the viability and fermentative activity of 50 active dry yeasts purchased on the northern Italian market were studied, and revealed that Copper excess may cause massive death of yeast cells, leading to a significant delay in the start and progress of alcoholic fermentation. A two-log units reduction in cell viability was observed when copper content of musts was around 20 mg/L. Despite this, the difference noted in the kinetics after 20 days’ fermentation was lower than that observed 48 hours after in the grape must. An excess of copper in must affected also the composition of the produced wines. The increase in acetic acid and in the sulphur dioxide concentration, observed in wines made using grape must with a high copper concentration, raises serious doubts both as regards the possibility of obtaining good wines from these raw materials and in relation to the progress of subsequent steps of the winemaking, such as malolactic fermentation. While it is an important tool in preventing vine disease, copper must be used very carefully to avoid serious troubles during wine fermentation, even if some yeasts seem more suited to ferment musts containing up to 20-30 mg/L copper.