Monitoring the spreading of commercial wine yeasts in the vineyard

[摘要] ENGLISH ABSTRACT: Traditionally, wine has been produced by the spontaneous fermentation of grapejuice by yeast that originate from the grapes and winery equipment. Research hasshown that the population composition and dynamics of these yeasts and othermicroorganisms are very complex. Kloeckera and its anamorph, Hanseniaspora,dominate the yeast population found on the surfaces of grapes, although prevailingSaccharomyces cerevisiae strains complete the fermentation process.The yeast S. cerevisiae is an important factor contributing to the quality of winesand, therefore, the improvement of wine yeasts receives considerable attentionworldwide. Apart from classical yeast breeding studies, genetic engineering andrecombinant DNA techniques are increasingly being used in strain developmentresearch programmes. These techniques might enable the wine yeasts to produceheterologous enzymes that degrade polysaccharides, convert malic acid to lacticacid, increase glycerol production, release roam and flavour compounds, secreteantimicrobial peptides, etc. The release of recombinant yeast strains (geneticallymodified organisms, GMOs) is subject to statutory approval. Therefore, it is importantto answer several questions prior to the use of such genetically improved yeast in thecommercial production of wine. For example, will recombinant yeast strains be ableto multiply and spread in nature, and will this GMO be able to out-compete thenatural microflora because of its newly acquired genetic traits. Since existingcommercial wine yeasts are used in the abovementioned strain developmentresearch, it is essential to determine already at this early stage to what extent thesewine yeast strains survive and spread in nature and to what extent they influence thefermentations of the following vintages.This study is divided into two sections. The aim of the first section is to sample arepresentative number of yeast strains from various vineyards in differentclimatological areas, mainly in the Western Cape, South Africa. These yeast strainswere identified mainly by electrophoretic karyotyping (contour-clamped homogenouselectric field electrophoresis; CHEF).The second part of the study summarises the results obtained when Fouriertransform infrared (FT-NIR) spectroscopy was used to differentiate commercial wineyeast strains. Sets of data, containing the spectra of the mostly used commercialwine yeast strains, were constructed and used as a reference library. The spectra ofthe isolated yeast strains were then compared to the reference dataset with specificFT-NIR computer software using mathematical calculations.In conclusion, the two methods used in conjunction with one another proved thatthe commercial wine yeast strains do not easily disperse from the cellar into thevineyard. The commercial wine yeast strains are also more likely to be found nearthe cellar and the places where the grape skins are dumped. Therefore, should arecombinant yeast strain be used in winemaking, it would not be dispersed into thevineyard. It therefore appears that the commercial use of genetically improved yeast does not pose a high risk in terms of dominance of the indigenous microbialpopulation in the environment

[发布机构] Stellenbosch University