Gianfranco Rosini

Yeast flora of grape berries during ripening

The yeast flora associated with the surface of grapes during ripening was studied with regard to different sectors of the grape skin and the position in the bunch by means of traditional as well as more vigorous preisolation and precounting treatments. The yeast number per square centimeter of skin increases with ripening and is highest in the area immediately surrounding the stem. The cluster sector closer to the peduncle seems to constitute a favorable substrate for yeasts, hosting a resident flora about 10 and 100 times higher than the central and lower parts of the bunch, respectively.Kloeckera apiculata was the normal resident species of grapes regardless of the sector or the ripening period, and constitutes the fermenting flora of mature grapes. The ecological implications of the results of this survey are discussed.

Systematics of the species of the yeast genus Saccharomyces associated with the fermentation industry

SummaryThe so-called wine yeasts Saccharomyces cerevisiae, S. chevalieri, S. bayanus, S. italicus and S. uvarum are characterized by high ethanol tolerance and fermentation velocity. They are ecologically related, being predominantly associated with grape must and wine, and are taxonomically indistinguishable. The only significant physiological differences are between the ability to ferment certain sugars. A taxonomic revision of more than 1,000 strains isolated during the past 50 years and belonging to the above species showed extreme instability in the ability to ferment different sugars. The relationships between these yeasts were examined for DNA base composition and DNA-DNA reassociation. The G+C ranged from 37.6% to 39.0% while optical reassociation experiments defined a first group of species (Saccharomyces cerevisiae, S. chevalieri and S. italicus) exhibiting high base sequence complementarity (>90%). S. bayanus and S. uvarum also showed a high degree of relatedness. Low homology values (∼30%) indicate that the two groups of species are not closely related. While it is proposed to combine S. cerevisiae, S. chevalieri and S. italicus into one single species under the oldest epithet Saccharomyces cerevisiae, a study of a larger number of strains is recommended before considering the taxonomic position of S. bayanus and S. uvarum.

Killer sensitivity patterns as a tool for the fingerprinting of strains within the yeast speciesKluyveromyces lactis andK. marxianus

A simple procedure based on differential sensitivity to a given panel of “killer yeasts” allows the discrimination of strains within the speciesK. lactis andK. marxianus. The proposed method was found to be reproducible, readily discriminatory, and easy to perform without specialized equipment. All strains within the species are characterized by a specific, individual sensitivity pattern (killer formula).

Wine-making by cell-recycle-batch fermentation process

SummaryIn order to check the overall validity of more efficient fermentation systems to reduce wine-making costs, we carried out an off-skins fermentation of clarified Trebbiano toscano grape-juice, making use of a non-conventional “cell-recycle-batch fermentation” process. The results showed that the process causes a reduction of the fermentation length as well as an improvement in ethanol productivity and yield and can be conveniently applied to the production of ordinary table wines.

Sparkling-wine production by cell-recycle fermentation process (CRBF)

SummaryThe influence of the cell-recycle fermentation process (CRBF) on sparkling wine in-tank production was studied to improve the kinetics of “prise-de-mouse”. The results showed that the process causes a reduction in fermentation time as well as an improvement in ethanol productivity and yield without modifying the analytical characteristics of the sparkling wine obtained.

Killer Yeasts: Notes on Properties and Technical Use of the Character

In 19631 it was observed that some laboratory yeast strains belonging to the species Saccharomyces cerevisiae were able to kill other yeasts. Three different phenotypes of this property have been determined: ‘Killer’, ‘Sensitive’ and ‘Neutral’.

OXYGEN CONSUMPTION AS A MEASURE YEAST STARTER CULTURE VITALITY FOR THE WINE INDUSTRY.

A simple technique for rapid determination of fermentation starters vitality which eliminates the need for determination of viable cells counts is described. The mathematical relationship between cell number and oxygen consumption of eight strains of Saccharomyces cerevisiae was studied. Results confirmed the possibility of utilizing a pO2 probe as an indicator of cell viability for fermentation starter.