Different yeasts were isolated from spontaneous fermentation of Assyrtiko grape must in Santorini Island, Greece. Molecular typing revealed the presence of three Saccharomyces cerevisiae strains (S9, S13, S24) and one strain of the yeast species Nakazawaea ishiwadae (N.i). The four isolated strains were further tested in laboratory scale fermentations of Assyrtiko must in pure inoculation cultures and in sequential inoculation (72 hours) of each S. cerevisiae strain with the strain of N. ishiwadae. All fermentation trials were realised in duplicate.
Fermentation kinetics were followed by HPLC, while the volatile composition of the final products was determined by GC-MS (qualitative analysis) and GC-FID (quantitative analysis). Sensory evaluation of the samples took place by a panel of 10 trained panellists. In general, the fermentation rate in trials with S.13 and N.i. was lower than the rest, while trials with S9 and S24 resulted in higher ethanol contents in the final product but without statistically important differences. The wines fermented with the S24 and N.i. strains were characterised by the highest concentrations of acetic acid (0.9 and 0.7 g/L respectively) and with S13 by the highest concentration of glycerol (15g/L). In terms of aromatic profile, the trials contacted with S9 were up to 3.5-folds richer in volatile compounds responsible for the fruity character in wines. In addition, the fermentations with S13 and N.i. were about 3-folds richer in compounds characterized by floral character (e.g. phenethyl alcohol, tyrosol etc.), while the most abundant group of compounds in fermentations contacted with S24 strain were the oxidation esters (e.g. ethyl hydrogen succinate). In the sequential inoculations apart from a delay in the completion of alcoholic fermentations, a comparable with single strains fermentations trend in ethanol production and reducing sugar consumption was observed. Intensification of the production of acetic acid, oxidation esters, several ethyl esters and higher alcohols (C5, C6) was also observed. Significantly lower (5-fold) contents of higher alcohols and their corresponding esters, responsible for floral aromas for ferments with N13 compared to ferments with S13 was also noted. Regarding the production of esters responsible for tropical and citrus aromas (e.g. isoamyl acetate, ethyl hexanoate), the highest content was observed in ferments with N13 (1.32 ppm) and N24 (1.97 ppm) while the lowest in ferments with N9 (0.99 ppm). The concentration of most esters was increased for all trials after sequential inoculation compared to the corresponding trials contacted with pure cultures. The results from the organoleptic analysis are in line with the chemical analysis. Even though, all four newly isolated strains have the ability to ferment and produce dry wines, the most preferred wines by the panel were those produced by S9 and S13 strains.
Authors: Kallithraka Stamatina1, Christofi Stefania1, Dimopoulou Maria1, Tsapou Evangelia Anastasia1 and Papanikolaou Seraphim1
1Department of Food Science and Human Nutrition, Laboratory of enology and alcoholic drinks, Agricultural University of Athens
*corresponding author: email@example.com
Keywords: Saccharomyces cerevisiae, Nakazawaea ishiwadae, wine volatile content, sensory analysis, fermentation kinetics