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.
Moulds
Moulds are filamentous fungi that grow as hyphae, which are multicellular filaments. A mould is a member of one of two distinct groups of fungi:
Ascomycota (sac fungi) – there are some 64 000 species;
Zygomycota (bread moulds) – there are some 1000 species.
In order for moulds to grow, they require food in the form of organic matter, moisture, warmth, and oxygen. Certain moulds are implicated in spoilage and product deterioration, and filamentous fungi have a major role in the formation of haloanisoles, which give rise to so‐called ‘corkiness’ that can render affected wines undrinkable.
Bacteria
There are more species of bacteria than any other form of life. Bacteria can be single or multi‐celled microbes. Unlike yeasts, they do not have a nucleus and are classified as prokaryotes. A simple way of describing bacteria is by their morphology, i.e. their shapes. There are five groups of shapes; the two most important groups that may be found in wine are cocci that are round or oval shaped and bacilli that are rod shaped. These descriptors often appear as part of genus names. Some species of bacteria exist as individual cells; others group themselves together in pairs, rods, chains, or clusters.
Many species of bacteria grow only in aerobic conditions – these are termed obligate aerobes, and include the dreaded Acetobacter, which can turn wine into vinegar. There are also species, termed obligate anaerobes, which grow in anaerobic conditions. The presence of oxygen poisons some of the key enzymes of obligate anaerobes, so they will not grow in aerobic conditions. There are some obligate anaerobes which require a high level of carbon dioxide for growth, as will be the case during fermentation. Facultative anaerobes are organisms that manufacture adenosine triphosphate (ATP) by aerobic respiration in the presence of oxygen, but are able to change to fermentation or anaerobic respiration in the absence of oxygen. Bacteria that grow in an acid environment are termed acidophiles, but these are very rarely present in grape musts and wines as the pH is invariably higher than they can tolerate.
The pH of musts and wines varies considerably between extremes of pH 2.7 and 4.3 depending, inter alia, upon growing conditions including soil type, aspect, climate, and weather in the growing season. As grapes ripen, pH increases. A high pH will not only have a negative influence upon the taste profile, but increase the risk of bacterial growth. Although there are exceptions, the ideal pH of must for white wines lies in the range of pH 2.9–3.4, and for red wines pH 3.3–3.7, although in recent years this latter figure is often exceeded. Generally speaking the lower the pH the less the risk of the growth of unwanted microorganisms, but there are some bacteria, including acetic acid bacteria, that may grow in wines with a low pH.
Whilst the different species of bacteria vary considerably in the other conditions in which they thrive, they all require food, water, and suitable environmental conditions. During grape growing and winemaking bacteria have numerous sources of nutrients, including sulfur, carbon (particularly six carbon sugars), nitrogen, and phosphorus. Although, with some notable exceptions, sugars are largely consumed by yeasts during fermentation, wines that have completed all production processes will contain other nutrients, including typically between 80 and 230 mg/l of phosphorous. Whilst generally bacterial growth is slower at low temperatures, most species flourish in the wide range of 5 °C (41 °F) and 60 °C (140 °F). Pre‐fermentation heat treatments of red musts, including pre‐fermentation hot maceration, flash détente, and thermo détente which, depending upon the individual process, involve heating the must to between 75 and 85 °C (158–185 °F) will kill any bacteria present at that stage, as will pasteurisation, which may be also undertaken when the finished wine is bottled.
Although many species of bacteria are implicated in microbiological wine faults some, particularly certain lactic acid bacteria of the genus Oenococcus, perform a most useful role in the winemaking process, being responsible for the MLF, which leads to textural changes that are almost always desirable for red wines. The MLF is often undertaken for certain styles of white and sparkling wines too, but whether or not it is desired it is crucial that it never spontaneously takes place subsequent to bottling – see Chapter 9.
1.10.2.2 Examples of Microbiological Faults
Faults of microbiological nature that may be found in wine include
Contamination with haloanisoles – see Chapter 3;
Brettanomyces related faults – see Chapter 4;
