Study of the microbial biodiversity of Irpinian grapevines for optimizing the production of typical wines (Aglianico and Fiano).
[Tesi di dottorato]
At the industrial level, the study of microbial biodiversity of different vine cultivars of the wine industry could provide an effective way to meet the demands for greater health, quality and typicality of the wine by a consumer, without colliding with the statutory prohibition or prejudice associated with the use of genetically modified vine and yeast. This research involves the study of microbial biodiversity of two cultivars of vine which originate from Irpinia, a region possessing an established vine industry: Aglianico and Fiano. As a first step, we created a collection of yeasts isolated from grapes and grape vines and these were later identified. Analysis of their morphology and biochemical characteristics identified them as belonging to four main genera: Saccharomyces spp., Kloeckera spp., Candida spp. and Metschinikowia spp. With regards to the Aglianico, Saccharomyces yeasts were found only in the musts, whereas in Fiano they were also present on grapes. In both cases the non-Saccharomyces genera were the predominant ones on grapes. Most of the yeasts selected were resistant to high concentrations of SO2 (100 ppm and 250 ppm), including non-Saccharomyces generally considered very sensitive to this compound. Also, only a small fraction of them were able to secrete β-glucosidase, an enzyme that, in enology, catalyzes the release in the must of odorous molecules in the form of volatile terpenes. The analysis of data obtained from the selections highlighted some yeasts, and Saccharomyces spp. and non-Saccharomyces spp., which have been used in various combinations for wine production on an industrial scale: these belong to the species S. cerevisiae, Rhodotorula mucilaginosa, Metschnokowia pulcherrima and Hanseniaspora guilliermondii. In the case of R. mucilaginosa, it was used alongside commercial yeast in the fermentation of both types of must for the 2007 vintage, while the others have been used in various combinations with each other, and the commercial yeast in the harvest of 2008, only with musts from the vineyards of origin. S. cerevisiae strains AGYP37 and M. pulcherrima AGYP28 were used in the fermentation of Aglianico while S. cerevisiae strains FYP69 and H. guilliermondii FWL66 in that of Fiano. As for the fermentation of 2007, the strain R. mucilaginosa AGWLR12 had better results in Aglianico than in Fiano, reaching an alcohol content and total acidity levels higher than the control, without producing too much acetic acid and completing fermentation one week in advance. Aromatic analysis of wines made with this yeast shows that it has influence on the content of methanol, and yields more higher alcohols and especially terpenes. Compared to the control, the latter increased by three times in Aglianico and by almost nine times in Fiano. This increase is due to the action of a β-glucosidase of AGWLR12 which is quite active under the conditions of the must and also responsible for increasing the content of resveratrol in the Aglianico wine, through its capacity for hydrolysis of the precursor trans-piceid. Sensory analysis confirmed the positive contribution of Rhodotorula AGWLR12 to both Fiano and Aglianico, resulting in a greater intensity of almost all parameters. Furthermore, on 2008 wines, it has shown some positive differences among samples as well, with regard the colour, the aroma and taste-tactile sensation. The analysis of wine and biochemical parameters of the wines of 2008 vintage shows that selected strains of native yeasts are efficient in completing fermentation and can affect wine quality. The best results were obtained in the fermentation of Aglianico grape where, in comparison to the control, native yeasts increased the total acidity without producing excessive amounts of acetic acid, reached more or less constant values of pH and alcohol content, increased colour, increased the content of polyphenols, flavonoids and coloured anthocyanins, and increased the reducing power but not the anti-radical activity of wine, except in one case. HPLC analysis showed a significant increase of gallic acid, catechin and once of resveratrol in experimental fermentation of Aglianico. The magnitude of these changes seems to be influenced by the interaction of yeasts. During the fermentation of Fiano musts, native yeasts achieved the same alcohol content, pH, polyphenol and flavonoid content, antiradical activity and reducing power of the control, not producing excessive amounts of acetic acid. A positive feature was an increase of catechin levels. These results indicate that indigenous yeasts, at least those isolated from Aglianico, can help to confer quality and wholesomeness to wine.
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