Study of five strains of <em>Saccharomyces cerevisiae</em> with regard to their metabolism towards geraniol

Geraniol is the main monoterpenic alcohol of some red grapes occurring both in the free form (aromatic) and in the glycosilated (neutral) form. Free geraniol is transformed into other terpenic compounds (linalool and α-terpineol) through acid hydrolysis. During fermentation, yeasts also change must free geraniol into citronellol. The final aroma of wines is mainly due to both the unchanged geraniol and the formed citronellol, thus the key role played by yeasts in the vinification of those aromatic wines is proved. In the present work the metabolism towards geraniol of 5 different strains of S. cerevisiae were studied. The laboratory tests were carried out using a must from neutral grapes as the fermentation medium. Two different amounts of geraniol were added to the medium to evaluate the influence, if any, of the initial concentration of monoterpenic alcohol on the transformation rate of this compound. At different stages of fermentation geraniol and citronellol, as well as other terpenic compounds, were evaluated by means of GC on samples extracted by SPE on a C18 cartridge. The results proved that the yeast strain dramatically affects the amount of both geraniol and citronellol occurring in the medium at the end of fermentation. Therefore in the fermentation of aromatic musts in which geraniol is the main component, the choice of the yeast proves decisive for the quality of wines. The influence of tween 80, added to the medium as a source of oleic acid, on the biosynthesis of citronellol was evaluated as well. The results showed that tween 80 increases the production of citronellol, though no remarkable changes in the consumption of geraniol were observed. It can therefore be concluded that the production of citronellol by yeasts is a minor way of utilizing geraniol when no longer needed for the synthesis of cell fats

Study of five strains of <em>Saccharomyces cerevisiae</em> with regard to their metabolism towards geraniol

Geraniol is the main monoterpenic alcohol of some red grapes occurring both in the free form (aromatic) and in the glycosilated (neutral) form. Free geraniol is transformed into other terpenic compounds (linalool and α-terpineol) through acid hydrolysis. During fermentation, yeasts also change must free geraniol into citronellol. The final aroma of wines is mainly due to both the unchanged geraniol and the formed citronellol, thus the key role played by yeasts in the vinification of those aromatic wines is proved. In the present work the metabolism towards geraniol of 5 different strains of S. cerevisiae were studied. The laboratory tests were carried out using a must from neutral grapes as the fermentation medium. Two different amounts of geraniol were added to the medium to evaluate the influence, if any, of the initial concentration of monoterpenic alcohol on the transformation rate of this compound. At different stages of fermentation geraniol and citronellol, as well as other terpenic compounds, were evaluated by means of GC on samples extracted by SPE on a C18 cartridge. The results proved that the yeast strain dramatically affects the amount of both geraniol and citronellol occurring in the medium at the end of fermentation. Therefore in the fermentation of aromatic musts in which geraniol is the main component, the choice of the yeast proves decisive for the quality of wines. The influence of tween 80, added to the medium as a source of oleic acid, on the biosynthesis of citronellol was evaluated as well. The results showed that tween 80 increases the production of citronellol, though no remarkable changes in the consumption of geraniol were observed. It can therefore be concluded that the production of citronellol by yeasts is a minor way of utilizing geraniol when no longer needed for the synthesis of cell fats

Karakterizacija mlečnokislinskih bakterij v avtohtonem kmečkem italijanskem siru

We studied the natural lactic bacteria population of an artisanal Italian cheese, Toma piemontese POD, from Piedmont (Northwest Italy), in order to select new strains to be used as starters in large-scale production. Isolates collected from curd and ripened artisanal cheeses were identified by the combined use of PCR 16S-23S rDNA spacer analysis, species-specific probes and 16S rDNA sequencing. Lactococci constitued 67% of the coccal isolates. Enterococci were also isolated together with strains of Streptococcus macedonicus and S. thermophilus. Lactobacilli were only detected in three samples of curds. Acidification and proteolytic activity and aroma production were also determined for each isolate. On the basis of the results a few isolates were selected and used as starters in cheesemaking trials with both raw and pasteurised milk. The produced cheeses were sensory evaluated and two of them showed typical Toma piemontese taste and flavour. The results suggest the possibility to use these new starters in both dairy industry and artisanal cheesemaking to improve product quality.Preučevali smo naravne populacije mlečnokislinskih bakterij v avtohtonem kmečkem italijanskem siru "Toma piemontese POD" iz Piemonta v severozahodni Italiji, da bi pridobili nove seve, ki bi jih lahko uporabljali v industrijski proizvodnji. Izolate, ki smo jih osamili iz sirnine in zrelega avtohtonega sira smo identificirali s pomočjo kombinirane uporabe različnih metod in sicer analizo vmesne regije 16S-23S rDNA z verižno reakcijo s polimerazo, uporabo vrstno specifičnih začetnikov in sekvenciranjem 16S rDNA. Lactococci so predstavljali 67 % vseh izoliranih kokov. Enterokoke smo izolirali skupaj s sevi vrst Streptococcus macedonicus in S. thermophilus. Laktobacile smo odkrili samo v treh vzorcih sirnine. Za vsak izolat smo ugotavljali tudi proteolitično aktivnost, acidifikacijsko sposobnost in proizvodnjo arome. Na osnovi rezultatov smo izbrali nekatere isolate in jih kot starterske kulture uporabili v poskusih izdelave sira in sicer iz surovega in pasteriziranega mleka. Sire smo senzorično ocenili in dva med njimi sta imela za "Toma piemontese" značilen okus in vonj. Naši rezultati nakazujejo možnost izboljšanja kakovosti proizvodov z uporabo teh novih starterskih kultur v sirarski industriji in v kmečki avtohtoni proizvodnji sirov