(to include any taint or other contamination) that results in a significant off‐odour or flavour, a marked reduction of quality, or is adverse to human health. For the sake of both clarity and simplicity, it is generally used as such in this book.
1.7 Distinguishing Between Faults and Flaws
In a tasting assessment, the condition of a wine may be described as
Fault‐free (sound);
Flawed (showing minor defects);
Faulty (showing one or more serious defects).
As stated, the use of the word ‘fault’ should be reserved for major defects, including those off‐odours and off‐flavours that have a significant organoleptic impact upon a wine, or for compounds that may cause accelerated deterioration, or are harmful to human health. Where the impact is minor, including defects that result in a reduction of typicality (the word tipicité is often used amongst wine lovers and critics), or a modest reduction in quality or ageing potential the term ‘flaw’ is generally more appropriate, although some authors do regard a lack of typicality of style as a fault [14]. The level, or concentration, of causal compounds is obviously key here, but whilst the physical level is generally relatively easy to quantify by laboratory analysis, the sensory impact upon wines of differing styles and aroma and flavour matrices may be less easy to qualify: much will depend upon the style and matrix of the wine. So the boundaries between faulty, flawed, and ‘in good condition’ may, on occasions, be somewhat blurred. However, there are some compounds which, even if present at a low level, cause a such a reduction in organoleptic attributes and a loss in quality, that affected wines must always be regarded a faulty. TCA and other haloanisoles (see Chapter 3) are classic examples.
Whilst there is little doubt that a wine contaminated from an external source should always be regarded as faulty, the internally produced ‘off’‐aromas and flavours may be subject to dissent as to the concentration at which they become unacceptable and are considered to be flaws or faults. When tasting a wine, the judgments made are, to a large degree, subjective. Individuals have varying sensitivities, responses and reactions to aromas, odours and flavour compounds based on their culture and education [15], experience, and age [14]. Members of a panel of professional tasters and critics may be unanimous in their judgements when assessing a wine or there may, on occasions, be out and out dissent. There can also be disagreement between professionals and consumers as to what constitutes a fault or flaw, for example, the acceptability or otherwise of sediments. Precipitated crystals of potassium bi‐tartrate or calcium tartrate are sometimes found in the bottom of wine bottles and, of course, these will often appear in glasses of wines when poured. They are most likely in high acidity wines from cool climates, but can appear in reds too. The crystals are harmless and have no negative organoleptic impact, but to the consumer they may be cause for concern and even rejection of the bottles in question. Accordingly, although many industry professionals are unconcerned by tartrates, some do consider them to be an ‘appearance’ fault (see Chapter 15).
1.8 Sensory Detection (Perception) Thresholds and Sensory Recognition Thresholds
1.8.1 Sensory Detection Thresholds
There are many compounds that may give rise to faults but, with the notable exception of TCA and other haloanisoles, their presence usually only becomes important when their concentration is at or above, or has the potential to reach, their individual sensory detection thresholds. To complicate matters, there are also some compounds which, even if below their individual sensory detection thresholds, may produce off‐odours or‐off‐tastes if present with other compounds, which may also be below their individual sensory detection thresholds.
Sensory perception thresholds may be divided into odour detection thresholds (ODTs) and taste detection thresholds (TDTs), but many authors 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