Italy, December, 2007

ECOLOGY OF THE Gfs1 STRAIN IN THE CASE BASSE ECOSYSTEM

“Il Chianti e le terre del vino” 

Professor Massimo Vincenzini*
*Professor of Agricultural Microbiology of Florence University

 

Since 1994 the Case Basse estate has collaborated with the microbiology section of the Agricultural Biotechnology department (DiBA) of Florence University. This research aims at monitoring and studying the microbiology of vinification (alcoholic fermentation and malolactic fermentation), that the winery traditionally carries out naturally, that is without the use of yeast cultures and commercial bacteria. In 2005 the Case Basse estate, together with the University of Piacenza (Professor Fregoni), started experimentation aimed at reducing water stress in vines. Since the experimental plan also foresaw the vinification of grapes coming from different experimental rows and the chemical, physical and sensory assessment of the corresponding wines, a yeast strain was needed to carry out the fermentation process. In full respect of the winery’s policy, the yeast strain could only be a native one and, if possible, characteristic of the Case Basse ecosystem. For this aim, a yeast strain of the Saccharomyces cerevisiae, called GfsI, was used, isolated at the Case Basse winery from must in fermentation and currently being part of the collection of microbic strains of DiBA. The reasons for this choice of strain came from results of research on the genotypic biodiversity of S. cerevisiae in natural vinification at the Case Basse winery, starting with the 1994 harvest. The research came about from the premise that, in natural vinification, unlike vinification carried out by inoculating the must with commercial selected yeast, the population of S. cerevisiae is made up of numerous strains that intervene simultaneously and/or in succession in the fermentation process and are able to carry out their own metabolic activity, releasing several metabolites into the must in quantities and ratios relative to the specific strain. This fact would be the basis of more complexity of taste and aroma, often recognised more in wines produced with natural fermentation with respect to those obtained with fermentation induced by selected yeasts, normally consisting of a single yeast strain preparation. The research on natural biodiversity of S. cerevisiae yeast at Case Basse has foreseen, for eleven years in a row (from 1994 to 2004), the monitoring of microbic populations in natural vinification and the isolation of colonies referable to S. cerevisiae, always taking them from the same fermentation tank where must obtained from grapes of the same vineyard has always been put. Furthermore, between the 2003 and 2004 wine seasons, further isolation was carried out, starting from soil samples taken from the Case Basse vineyard, from the grapes of this vineyard and lastly from the water used to wash out the wooden fermentation tank. All the isolates (531 in total) were differentiated by strain through restriction fragment length polymorphism analysis of the profile (RFLP) of the mitochondrial DNA (mtDNA), a method considered suitable for identifying different strains within the same species of yeast. The analysis consists of extracting total DNA from the yeast cells, followed by digestion with suitable restriction enzymes that cut the DNA in correspondence to specific nucleotidic-based sequences and from the subsequent agarose gel electrophoresis to highlight the DNA fragments originating from the digestive reaction. On the basis of the number and dimension of the fragments, the profile that is characteristic of a single strain is obtained. The results have enabled us to identify a total of  99 different native strains. For each fermentation a variable number of strains was obtained, between 8 and 18, demonstrating a high level of genotypic polymorphism of the S. cerevisiaeyeast. 75% of the strains found during the course of vinification were not found in consecutive years: this behaviour was noticed mainly with reference to those strains defined as minorities, generally present during the course of alcoholic fermentation in low percentages. On the other hand the dominant strains, that is those mainly responsible for carrying out the fermentation process, were recurrent through time. In particular, the strain called Gfs1 was found consecutively for 10 years, from the 1995 harvest to the 2004 one, and for five vinifications it turned out to be the dominant strain in alcoholic fermentation. Furthermore, this strain was found at high frequencies both in the soil samples of the Case Basse vineyard and in the samples of water used to wash out the fermentation tank. Such results suggest that the Gfs1 strain may be established in the wine growing and producing ecosystem of Case Basse and that its continuous presence may be the result of the adaptation of the strain to the particular style of vinification adopted by the winery. The use of the Gfs1 strain in experimentation on water stress, therefore, appears to be totally justified. Under the purely enological profile, the strain used is endowed with good fermentation vigour, high alcoholigenous power and good fermentation purity, though producing high quantities of glycerol.