the OAV the more the compound will usually also impact the palate or taste profile, bearing in mind that most of the taste of a wine is derived from retronasal sensations. Of course, the strength of an odour is not simply a matter of is OAV, and strong odours can mask weaker odours.
1.9 Consumer Rejection Thresholds (CRTs)
There is often a marked difference between the concentration of a compound at which a consumer may be able to perceive a fault – the consumer detection threshold (CDT), and the level that would lead them to reject the wine – the consumer rejection threshold (CRT). With all consumer goods, the buyer is often prepared to accept minor blemishes and performance idiosyncrasies as long as the goods remain fit for purpose – in the case of wine, the consumer may accept it as fit for purpose as long as it is drinkable and has an element of enjoyment. Of course, consumers frequently reject wines that are simply not to their taste, or are of general low quality, and are often confused as to whether an individual aroma or taste characteristic should, or should not, be present. Visible faults such as hazes and the aforementioned tartrate crystals are very likely to lead to rejection, and on occasions sound wines may be rejected simply due to careless or inappropriate handling. Mature red wines very often throw sediment in the bottle which, in the absence of decanting, may become suspended in the wine or fall to the bottom of the glass when the wine is poured.
1.10 Basic Categories of Wine Faults
1.10.1 The Origin of Wine Faults
There are three basic categories of wine faults, according to their nature:
Microbiological
Chemical
Physical
However, the defining of some faults as being of microbiological or chemical origin or nature is not necessarily straightforward: chemical reactions consequential to microbiological synthesis may produce fault compounds, and some faults, e.g. excess acetaldehyde and oxidation, may be formed by microbiological activity and/or chemical reactions.
I will briefly examine each category, and consider factors that may lead to, or help prevent related faults.
1.10.2 Microbiological Faults
1.10.2.1 Types of Microorganisms Involved
This group comprises faults that result from the actions of microorganisms; these may be grouped as (i) yeasts, (ii) bacteria, and (iii) moulds. It is perhaps worth looking briefly and very generally at each of these.
Yeasts
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.
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