Читать книгу Wine Faults and Flaws - Keith Grainger - Страница 26

Yeasts are single celled microbes, belonging to the fungi kingdom. The genetic content of a yeast cell is contained within a nucleus which is enclosed within a nuclear membrane – this classifies them as eukaryotic organisms, unlike their single‐celled counterparts, bacteria, which do not have a nucleus and are considered prokaryotes. There are approximately 1500 species of yeast. Yeasts mostly reproduce by budding, usually multilateral budding where buds appear from different points in the shoulder of the cell, but polar budding, where the buds repeatedly grow from the same site are common for some ‘wild’ yeasts, including the genus Kloeckera [19].

The main species responsible for fermentation of grape must into wine is Saccharomyces cerevisiae. Most non‐Saccharomyces yeasts do not ferment to a high alcoholic degree, perhaps 4–5% abv. Although present in relatively low quantities on grape skins, together with many other yeasts and microflora, S. cerevisiae becomes dominant in the vast majority of fermentations. It is the main ‘non‐spoilage’ yeast species that is able to produce high levels of alcohol and survive in such a hostile environment. It has the ability to consume all the sugars generally present in grape must (other than must from late harvested or botrytised grapes destined for sweet wines) and its predominance minimises the risk of sluggish or stuck fermentations. It is also particularly associated with enzymatic activities involved in the transformation of aromatic precursors contained in grapes [20]. Many winemakers use laboratory cultured strains of S. cerevisiae for the alcoholic fermentation: there are approximately 700 strains, each with different behavioural and flavour characteristics. However, there are a great many (indeed a growing number of) winemakers particularly at smaller properties who prefer to leave their fermentations to the various natural yeasts present on the grapes and in the winery. Of course, this as was always the case until the last 30 or 40 years. Many artisan producers and wine lovers regard natural yeasts as being an extension of ‘terroir’. Whichever is their choice, winemakers usually try to ensure that S. cerevisiae is the species that dominates in the alcoholic fermentation process, and other species of yeasts and other microorganisms (with the exception of certain lactic acid bacteria) may be suppressed. However, for some wines, there are other Saccharomyces species that may be encouraged or even inoculated, including Saccharomyces bayanus (which is also often used for the ‘prise de mousse’ or second [bottle] fermentation of Champagne and other quality sparkling wines) and Saccharomyces beticus, being one of the key yeasts that produces the ‘flor’ essential for the production of Fino Sherries. Very occasionally, and particularly with producers who are thinking and working ‘outside of the box’, there are other genera and species that may be encouraged or co‐inoculated, including Lachancea kluyveri, Lachancea thermotolerans, Metschnikowia pulcherrima, and Torulaspora delbrueckii [21]. However, non‐Saccharomyces species can rise to off‐odours and flavours and produce compounds that may lead to faults or spoilage, so extreme care must be taken when using these, whether they are inoculated or naturally present on grapes or in the winery. In particular, Brettanomyces bruxellensis and Brettanomyces anomala are regarded by most oenologists as major spoilage organisms in wine, although the aromas and flavours metabolised by these yeasts can, at low levels and in some wine matrices, add interest and complexity. Several ‘film‐forming’ yeasts may grow on the surface of wine in the presence of oxygen, such as will be the case in ullaged vats or barrels. These too can result in off‐odours and flavours and even spoilage. Figure 1.1 shows film‐forming yeasts on the surface of wine in a small vat, when the ‘floating’ lid had been carelessly fitted a few centimetres above the surface.

Figure 1.1 Film forming yeasts on wine surface.