and researchers do not distinguish between these. A simple definition of a sensory perception threshold is ‘the level at which the aroma, flavour, taint, or fault will be detected by the 50% of the general public’. An alternative definition is ‘the lowest concentration at which individuals can reliably perceive a difference between a sample and its corresponding control, with 50% performance above chance’ [13]. Of course, individuals vary markedly in their perception thresholds to aromas and tastes. Threshold values may be stated for detection in air, water, wines generally, red, white, or sparkling wines, and even wines made from individual grape varieties or in different styles, such as light‐ or full‐bodied. However, the determination of such thresholds is perhaps far from a precise science, and individual research papers will often state markedly differing figures. The ‘Triangle Test’ is one widely used method of determining the sensory detection thresholds of compounds in wine and foodstuffs generally and is included in the methodology of the Society of Sensory Professionals. In this method members of a panel are presented with three samples: one different from the two identical. All three samples should be presented to each panel member at the same time, and the samples are tasted from left to right. There are six possible order combinations, and these should be randomised across panel members [16]. There is an ISO standard for measuring sensory detection thresholds by a three‐alternative forced‐choice (3‐AFC) procedure: ISO 13301 (2018) [17]. ASTM International, formerly known as the American Society for Testing and Materials (ASTM) has produced document E679‐04 (2011): Standard Practice for Determination of Odor and Taste Thresholds by a Forced‐Choice Ascending Concentration Series Method of Limits [18].
1.8.2 Sensory Identification (Recognition) Thresholds
Whilst an ODT is the lowest concentration at which a particular odour is perceivable upon nosing, the identification of the odorant responsible may not be achieved at that level. The ‘odour identification (or recognition) threshold’ is the concentration at which a particular odorant is not only detected, but is also recognised and can be named. ISO 5492 defines and odour identification threshold as the ‘minimum physical intensity of a stimulus for which an assessor will assign the same descriptor each time it is presented’ [11]. However, different compounds can give arise to aromas which may have a similar or possibly the same descriptor, or be difficult to distinguish, even by trained, expert tasters. As with detection thresholds, recognition thresholds may vary according to the matrix of the wine and the interplay between numerous compounds. Most research papers only refer to detection thresholds when discussing individual odorants, and accordingly the thresholds detailed in this book are generally those of detection, not identification.
1.8.3 Odour Activity Values
The levels at which a fault compound in wine is detected and identified will very much depend upon the wine matrix. Additionally, there may be multiple compounds responsible for an off‐aroma or flavour in a wine, and the threshold for determining the multiplicity of these may differ from that of the individual components. This is particularly so in the case of the compounds metabolised by yeasts of the genus Brettanomyces, as discussed in Chapter 4. Depending upon the fault in question, sensory detection thresholds for fault compounds may be measured in concentrations of milligrammes per litre (mg/l), which equates to parts per million (ppm), microgrammes per litre (μg/l), which equates to parts per billion (ppb), or even nanogrammes per litre (ng/l), which equates to parts per trillion (ppt). The human nose can actually detect some odours at levels of picogrammes per litre (pg/l) equivalent to parts per 1000 trillion, and possibly and incredibly at concentrations measured in femtogrammes per litre (fg/l). A femtogramme is 0.000 000 000 000 001 of a gramme! Only volatiles smell – we cannot smell most wine acids, with the exception of the volatile acids (mainly acetic acid).
Usually the greater the concentration of a compound, particularly if above detection threshold, the more it will impact upon odour. Accordingly some researchers consider ‘odour activity values’ (OAVs), which may be defined as the concentration of a compound present in a matrix divided by the ODT for that compound in that specific matrix [13]. Of course, the higher 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
