Peptidase activity and the ability of wine yeasts to utilise grape must proteins as sole nitrogen source

Abstract

Peptidase activity of 14 Saccharomyces cerevisiae wine strains was tested with API ZYM kits LRA ZYM AP‐III and LRA ZYM AP‐IV, each containing 10 different enzymatic substrates. The ability of the yeast strains to utilise grape must proteins directly as sole nitrogen source was also examined by growth and fermentation tests in synthetic nutrient medium without additional nitrogen compounds, using a washed alcoholic precipitate of grape must. These observations strongly support the existence of an extracellular grape must protein hydrolytic enzymatic system in wine yeasts. Free amino acid content evolution in clarified and unclarified Chardonnay and Cortese pressed and free‐run juices, with or without growing yeast cells, was investigated by high‐performance liquid chromatography. Clarification does not seem to affect the grape amino acid contents. These remained almost constant from 0 to 8 h of contact with suspended solids when no growing yeast cells were detected. No grape proteolytic activity on grape proteins was seen after the first hours from crushing in spite of the reported existence of very active grape proteases. Hypotheses on possible causal factors for these results are discussed.