Protective effect of vegetal extracts against acidic and alcoholic stress in wine yeasts

Protective agents may minimize osmotic and thermal stress in wine yeasts (Caridi 2002). In the present work, the tolerance of 10 strains of Saccharomyces cerevisiae to three stressful agents - acetic acid, ethanol, and pH - was studied by spot dilution assay, with or without the addition of six protective agents. Six extracts were prepared using olive pulps (cv. Carolea), black grapes (cv. Gaglioppo), black grape skins and black grape seeds (cv. Magliocco canino), pomegranate peels and pomegranate pulps. For each strain, a suspension 0.1 OD600 was prepared, ten-fold diluted and spotted (5µl) on YPD agar modified adding different concentrations of the stressful and/or protective agents. The antioxidant activity of the six extracts was determined by DPPH and ABTS assays, both expressed as percentage of inhibition. The highest antioxidant activity resulted in the extract of pomegranate peels (94.60% and 60.07%) and black grape seeds (91.26% and 88.41%), followed by pomegranate pulps (37.81% and 21.33%), black grapes (16.75% and 5.00%), black grape skins (9.34% and 10.55%), and olive pulps (3.19% and 5.52%). The total phenolic content, determined by Folin-Ciocalteu assay and expressed as gallic acid equivalent mg/100 g of dry weight, was 6,670 - 8,589 - 1,518 - 868 - 756 - 664, respectively. The six extracts exhibited significantly different effects on the 10 strains and, in general, they allowed a better growth of the yeasts in the presence of the stressful agents. These results and their implication on the possibility to use protective agents in winemaking are discussed.