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Wine Authentication

After the major wine fraud by Rudy Kurniawan and his arrest in 2012, the wine industry, and science for that matter, became more interested in the wine authentication. Fraudulent wines cause major economic damage and are difficult to identify. Current analysis methods for wine authentication are expensive and laborious, but what do they actually measure? In recent years, several new authentication methods have been developed, but are these already applicable on a large scale?

Many different forms of wine fraud are possible. For example, wine can be diluted with water, illegal additions of alcohol, glycerol, colorants or flavorings can be made, the wine can be replaced or blended with wine of a lower quality, or the wine can be incorrectly labeled. In the latter case, the indication of the grape used, the winemaker, the geographical origin or the wine’s harvest year may be incorrect.

Expensive bottles

The label, bottle, capsule, cork or wax that seals the bottle can be analyzed. Is the label of a historic bottle glued on with synthetic glue? Is the weight and shape of the bottle correct? Is the label printed with a modern printer? Is the label’s paper actually as old as the bottle should be? and the cork? There are many ways in which a fraudulent bottle can be identified. However, these procedures are laborious and require a keen eye from a specialist. It is therefore only for the higher segment of bottles, often of many thousands of euro’s, for which it is worthwhile to perform these analyzes for an individual bottle. Opening of these (pretended) unique bottles for testing is in general not desirable.

Sour grapes Poster Netflix documentary

Figure 1. The poster of the documentary Sour Grapes on Netflix. The documentary tells the story of the wine fraud committed by Rudy Kurniawan at the beginning of this century.

Inferior quality

The authentication of bulk wines, new vintages or less rare bottles is also important. Not in the least because a Barolo really needs to be made from the Nebbiolo grape, or an Amarone della Valpolicella really has to come from the Valpolicella region in Veneto. In addition, unauthorized additions to the wine may affect food safety. But above all, producers do not want their good name to be used on fraudulent bottles, and consumers do not want to buy an expensive Burgundy, only to discover that the content is a bulk wine of inferior quality. In contrast to the unique bottles mentioned above, these bottles are available in large quantities and can easily (without large economic or historic loss) be opened and used for testing.

The methods and the difficulties

To authenticate a bottle of wine, the contents of the bottle can be checked on roughly four different aspects, namely:

  • the DNA of the grape variety,
  • the isotope ratios,
  • the trace elements, and
  • the organic compounds in the wine

Various, often laborious and costly genetic, chromatographic and spectroscopic methods are used to measure these components. With these different techniques it is possible to determine the geographical origin, harvest year (vintage), grape varieties used, or for example the addition of water, sugar or glycerol (see Table 1).

Table 1. Overview of elements in the wine and the most important type of fraud that can be detected with it.

Geographical originVintageGrape varietyAddition of water / sugar / glycerol
DNAX
Isotope ratiosXXX
Trace elementsX
Organic compoundsXX

Unfortunately, authentication is not straightforward, and the analysis of the wines is hampered by multiple factors, for example:

  1. Breakdown of grape DNA. DNA authentication methods seem a solid resolution, but are difficult to implement because the grape DNA largely breaks down during the wine making process.
  2. Vinification. Every vinification process occurs differently and therefore increases the variability. In addition, specific actions during the vinification can have huge effects on the composition of the wine. For example, the fining agent bentonite, has a major effect on the amount of trace elements in the wine because it exchanges trace elements for the proteins that it binds. Determining a geographical origin on the basis of the trace elements is therefore made impossible.
  3. Aging. Aging has an effect on the composition of the wine and adds to the variability in the signal.
  4. Legislation. It is often permitted to add up to 15% of another wine, or grape, without having to state this on the label. Further, in some wine regions (outside Europe), a limited amount of water may be added to the wine.
  5. Climate and year-specific conditions. Climate and the year-specific conditions have a major influence on the composition of the wine, which means that there is not an invariable fingerprint for a wine from a certain area.

As such, the isotope ratios, the amount of trace elements, and the composition of the organic compounds in the wine vary each year and are influenced by several factors in the vineyard and actions in the wine cellar1-3.

European Wine DataBank for isotope values

The analysis techniques do not simply give a simple answer to the question whether a wine is authentic or not. To determine whether the analyzed wine is authentic, the measurement data must be interpreted and this requires a reference data set. A Cabernet sauvignon from Bordeaux, for example, has a different characteristic “fingerprint” for its isotope ratio or trace elements than a Pinot noir from Burgundy. To determine the approximate appearance of this fingerprint per region, year and grape variety, standardization of the analysis techniques used is required and a database must be built up to serve as a reference set.

The overview of the above-mentioned methods shows that the measurement of the isotope ratios is the most versatile method for wine authentication and uncovering various types of fraud. To enable the isotope ratios for the authentication of wines, the European Union started the European Wine DataBank in 1991. This databank includes the isotope values ​​of authentic and representative wines from EU countries with wine regions from the harvest of 1992 onward. Every year, more than 1400 grape samples (of 15 kg each) are taken by official inspectors in the different European vineyards. These grapes all undergo a standardized microvinification in the European Joint Research Center and their isotope values ​​are subsequently included in the database. To promote the standardization of these values, the International Organization of Vine and Wine (OIV), in collaboration with the EU, has validated and approved a number of spectroscopy techniques for the measurement of these isotope values. In order to be able to check wines from outside the EU, a database for wines outside the EU was started in 1994 in Germany that nowadays contains the data of commercial wines from 26 different countries worldwide2.

What are isotopes?

Now the chemical part, because what exactly are those isotopes? Isotopes are atoms of the same chemical element (such as hydrogen, carbon and oxygen) but with a different number of neutrons in their nucleus. For example, carbon usually has 12 neutrons in its nucleus, but a small proportion of carbon atoms have 13 neutrons. These carbon atoms are referred to as 12C or 13C respectively. The same applies to hydrogen (H) that can have 1 or 2 neutrons, or oxygen (O) that has 16 or 18 neutrons. The number of neutrons of an atom, and therefore the ratio between the different isotopes, can be measured by spectroscopy techniques. The ratio in which these isotopes occur in molecules – such as water, glucose, ethanol, glycerol, carbon dioxide or tartaric acid – is characteristic of a harvest year (vintage) or geographical location, but also depends on how these molecules were created. Are the carbon atoms of the ethanol molecules in the wine derived from grape sugars or from cane sugar? Is the added glycerol synthetic, or animal? Has there been much or little evaporation of water in the grapes? Table 2 shows an overview of which frauds can be detected with the measurement of the different isotope ratios.

Table 2. An overview of the different types of wine authentication and the stable isotope ratio that is measured.

Type of authentication
(method of fraud)
Measurement of stable isotope ratio
Ethanol
(chaptalization)
(D/H)I, (D/H)II R-value δ13C sugars / ethanol
Sugars
(addition of beet sugar, cane sugar or a mixture)
(D/H)I, (D/H)II R-value δ13C sugars / ethanol
Water
(addition of water)
δ18O water (D/H)II ethanol
Glycerol
(addition of synthetic or animal glycerol)
δ13C glycerol
Carbon dioxide
(addition of synthetic or fossil CO2)
δ13C carbon dioxide
Tartaric acid
(addition of synthetic tartaric acid)
δ13C tartaric acid
Geographical origin
(incorrect labeling)
(D/H)I, (D/H)II, R-value δ13C ethanol, δ18O water
Harvest year (vintage)
(incorrect labeling)
(D/H)I, (D/H)II, R-value δ13C ethanol, δ18O water

(D / H) is the ratio of hydrogen (H, hydrogen with a neutron) with deuterium (D, hydrogen with two neutrons) in the molecule of ethanol. The addition I or II to (D / H) indicates whether it is the methyl or the methylene group of ethanol. δ: means “difference”, so “δ13C glycerol” means “difference in carbon 13 (ratio)  of the glycerol molecule”. Adapted from Christoph, 2015 via CC by 4.0.

Statistics

Yet the “fingerprints” of the isotope values ​​of two different Pinot noir wines from the same geographical location (e.g. Burgundy) are never the same. Differences in microclimate in the vineyard, vinification, or the soil type can influence the isotope values ​​in the wine. So there is not one correct value per wine region, but it is a spectrum of values. But how is it possible to distinguish all these different values ​​from Burgundy Pinot noir wines from, for example, the different values for Pinot noir wines from the German Ahr? The answer to this is statistics. Complex multivariate statistical models such as a “principal component analysis” or a “cluster analysis” can be used to determine common characteristics of wines and how they differ from wines from another region, from another grape variety or to which sugar or water has been added. Figure 2 shows practical examples of these statistical analyses. The tested wines fall outside the spectrum of the reference data and are therefore labeled as not authentic. Because so many factors influence the isotope values it is, even with statistical models, not always possible to say with 100% certainty if a wine is authentic or not. However, the extreme cases are recognized and can be withdrawn from the market.

authentication isotope ratios graphs

Figure 2: Practical examples of isotope measurements for the authentication of wines. Left: The isotope values ​​of the ethanol and the water in the wine do not correspond to the reference wines. Middle: Wines for which not all information was available compared to wines from the European reference database. For a number of wines, water or sugar has been added (the circled wines) and for a number of other wines there are doubts (circled with a dotted line). Right: The isotope values ​​of a wine do not match those of the reference wines from the same area. The geographical origin of the wine is therefore incorrect. Adapted from Christoph, 2015 via CC by 4.0.

Color spectrum and electronic tongue

The techniques for measuring isotopes in the wine are often laborious and expensive. A lot of research has therefore been done in recent years into cheaper and faster alternative techniques. For example, it is possible to authenticate wines based on their color. By analyzing the RGB color spectrum of the wine it is possible to determine whether a Rioja Gran Reserva that has to mature for at least 5 years is mixed with the younger and therefore cheaper Crianza or Joven Rioja wines4. With the help of this technique, it was also possible to fairly accurately distinguishing wines from the São Francisco Valley in Brazil on the basis of their origin, grape variety and even winemaker5.

Another new method is the so-called ‘electronic tongue’. This is a small device that measures with seven different sensors the chemicals that are responsible for taste. These same chemical substances such as sodium chloride, sugars, hydrogen ions and magnesium chloride are also detected by the receptors on the human tongue that send this “taste signal” to the brain. The amount of these chemicals is a measure of the intensity of the flavors. The electronic tongue produces a characteristic taste pattern for the different wines, which can be used to compare the wines. In this way, the electronic tongue has succeeded in distinguishing between sweet Hungarian Tokaji wine made from by noble rot affected grapes and wine sweetened with sugar6.

The RGB color spectrum and the electronic tongue measurements were both used to distinguish a specific wine type, grape variety or region within a known and limited group of wines. It is unclear whether these techniques can still demonstrate reliable differences (and similarities) between the wines in a larger set of wines with more variability in vintage, grape variety, vinification methods, geographical location, etc.. Further, a wide (large-scale) application of these techniques can also only take place when there are reference data sets for all these different wines. The further development of these techniques and the sampling of the necessary reference data will take years, but their employability and low costs offer many possibilities for a more accessible way for wine authentication.

Future wine authentication

The methods of wine fraud have become increasingly sophisticated in recent decades and the developed isotope techniques are a powerful tool to combat this. However, wine fraud will always persist. It is therefore important that the techniques for authenticating wine continue to be developed, and that easier and cheaper alternative techniques are introduced with which fraudulent wines can be recognized quickly and locally. The current database of isotope values and the newer authentication methods to come will definitely make future fraudsters work for their money.

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References:
1. Boccacci P, Chitarra W, Schneider A, Rolle L, Gambino G. Single-nucleotide polymorphism (SNP) genotyping assays for the varietal authentication of ‘Nebbiolo’ musts and wines. Food Chem. 2020 May 15;312:126100. https://doi.org/10.1016/j.foodchem.2019.126100
2. Christoph N, Hermann A, Wachter H. 25 Years authentication of wine with stable isotope analysis in the European Union – Review and outlook (2015) BIO Web of Conferences 5, 02020
https://doi.org/10.1051/bioconf/20150502020

3. Kamiloglu S. Authenticity and traceability in beverages. Food Chem. 2019 Mar 30;277:12-24. https://doi.org/10.1016/j.foodchem.2018.10.091
4. Herrero-Latorre C, Barciela-García J, García-Martín S, Peña-Crecente RM. Detection and quantification of adulterations in aged wine using RGB digital images combined with multivariate chemometric techniques. Food Chem X. 2019 Jul 5;3:100046. https://doi.org/10.1016/j.fochx.2019.100046
5. Lima CM, Fernandes DDS, Pereira GE, Gomes AA, Araújo MCU, Diniz PHGD. Digital image-based tracing of geographic origin, winemaker, and grape type for red wine authentication. Food Chem. 2020 May 15;312:126060. https://doi.org/10.1016/j.foodchem.2019.126060
6. Zaukuu JZ, Soós J, Bodor Z, Felföldi J, Magyar I, Kovacs Z. Authentication of Tokaj Wine (Hungaricum) with the Electronic Tongue and Near Infrared Spectroscopy. J Food Sci. 2019 Dec;84(12):3437-3444. https://doi.org/10.1111/1750-3841.14956

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