Biological acidification of “Vino Santo di Gambellara” by mixed fermentation of L. thermotolerans and S. cerevisiae. Role of nitrogen in the evolution of fermentation and aroma profile

In this work, Lachancea thermotolerans was exploited as a biological acidifier of Vino Santo di Gambellara, a traditional Italian wine made from long-dried grapes. The drying of grapes before winemaking is a traditional technique widely applied in Italy in the production of sweet wines, but in recent years, global warming reduces dramatically the acidity of grapes, causing microbial instability during drying and winemaking. The ability of L. thermotolerans to convert sugars in lactic acid was already applied in the acidification of red wines, but the peculiar features of dried grape (e.g., osmotic stress, lack of nutrients, presence of mould-derived toxic compounds during drying) impose specific tests. L. thermotolerans was employed in sequential fermentation in combination with a strain of Saccharomyces cerevisiae. Considering the lack of information about the nutritional requirement of the non-Saccharomyces yeast, three protocols of nitrogen supplementation (mineral, organic and organic at high dosage) were tested. Alcoholic fermentation experiments were followed by plate counts onto differential media to discriminate between the two yeast species and by chemical analysis. Moreover, a GC-MS-MS approach carried out a complete characterisation of the volatile profile of wines. Results evidenced a long permanence of L. thermotolerans during alcoholic fermentation, which remained over the 7 log units until the 14th day of fermentation. The nitrogen supplementation protocol influenced cell growth and fermentative activity. Inorganic nitrogen supplementations allowed the accomplishment of alcoholic fermentation and the maintenance of pH below 3.35, with respect to the control wine (made only by S. cerevisiae), which was over pH 3.50. L. thermotolerans also influenced the wine's volatile aroma profile. Statistical differences were found in the main families of the yeast-derived aroma: acetate, esters, lactates, fatty acids and C6 compounds.