Magnetic resonance sniffs out bad winePhysics in Action: November 1998, Claude Guillou and Fabiano Reniero
Nuclear magnetic resonance (NMR) has come a long way since it was discovered in the 1940s. This physical phenomenon, which arises from the intrinsic spin possessed by many nuclei, has provided a better understanding of the physics of nuclei and molecules. It has also been exploited to analyse the properties of many different materials in chemistry, physics, polymer science and biomedicine.
Recent developments in the use of high magnetic fields and pulsed NMR techniques have made it possible to probe the structure of organic compounds as complex as proteins. Imaging machines based on the NMR principle have also been developed, and now provide a powerful and non-invasive tool for diagnosing a variety of medical conditions. However, less well known are the applications of NMR for analysing food and drink. At the Joint Research Centre at Ispra, we are using a technique to detect whether a wine has been adulterated with foreign substances. This method is based on an NMR measurement of the deuterium content of wine.
Nuclear magnetic resonance is observed for nuclei with non-zero nuclear spin, which includes both the hydrogen nucleus (a proton) and the deuterium nucleus (a proton and a neutron). However, the physical properties of these two isotopes dictate that the NMR signal produced by deuterium nuclei is over 100 times weaker than that produced by the same number of hydrogen nuclei. The natural abundance of deuterium is also extremely low, with typical samples of hydrogen containing only about 0.015% of deuterium. This means that the NMR signal due to deuterium in a natural sample containing hydrogen is about a million times weaker than the signal due to hydrogen.
Despite this drawback, deuterium has very interesting properties for quantitative NMR. Deuterium has a quadrupole magnetic moment rather than a dipole moment, which means that it is unaffected by the nuclear Overhauser effect. This effect – in which radiofrequency radiation applied to the nucleus changes the strength of the resonance – is often exploited to enhance the NMR signal, but it also degrades the precision of quantitative techniques. Indeed, deuterium spectra generally show distinct peaks that are suitable for quantitative purposes.
An important advantage of NMR is that the deuterium content can be determined for each of the sites in a hydrogen-containing molecule that are not magnetically equivalent. For ethanol, for example, it is possible to determine separately the deuterium content of the methyl group (CH2D) and the methylene alcohol group (CHD) in the deuterium NMR spectrum. The low natural abundance of deuterium means that it is only necessary to consider molecules containing a single deuterium atom.
How does the deuterium content of wine indicate whether it has been adulterated? This is possible because the deuterium content of water in the hydrosphere and biosphere is not a constant. As an extreme example, ice at the South Pole has very low deuterium content, with a deuterium-to-hydrogen ratio of about 90 parts per million (ppm), while ocean water has a value of about 156 ppm. This natural variation is due to thermodynamic and kinetic effects that take place during the water cycle, when water evaporates from the ocean and precipitates over land. The transpiration of water from plants also favours lighter isotopes, leading to a greater abundance of deuterium in the water contained in plants.
The deuterium content of the water in any plant, including the vines used in wine production, therefore depends on several factors that can be related to the geoclimatic conditions during plant growth. Moreover, the water in the plant is used in the photosynthesis of different chemicals, in particular the production of glucose. This transfers the isotopic content of the water to the glucose and other sugars present in the plant, which means that both the metabolism and physiology of the plant influence the final deuterium content of the sugars. The deuterium content of the sugars therefore provides a good indication of their botanical origin.
Although sugars are particularly difficult to study with deuterium NMR, it is possible to detect the deuterium content of the methyl group in ethanol, which is produced during the yeast fermentation of wine. Ethanol is responsible for most of the alcoholic content of wine, and it retains a deuterium-to-hydrogen ratio representative of the sugars from which it is produced. Quantitative NMR can therefore be used to determine whether the ethanol present in wine originates from the sugars naturally present in the grapes or whether other sugars have been added to boost the alcoholic content. This practice – known in the trade as “chaptalization” – is allowed in the European Union, but only within specified limits.
A simple way of using NMR to detect sugar in wine is to compare the deuterium content of the wine being tested with a genuine wine from the same geographical origin. This requires an accurate determination of the NMR signal from the genuine wine, which is being done for all European wine-producing countries by our laboratory and other official laboratories of the European Union. The NMR data of more than 10 000 samples, together with an exhaustive description of the wines, have been collated since 1991 and now provide a powerful tool against fraudulent practice.
Other isotopic indicators, such as the content of oxygen-18 in wine or carbon-13 in ethanol, can be used to help detect other types of fraud, such as watering down the wine or false declarations of geographic origin. These parameters are usually measured by mass spectrometry, but this does not provide the site-specific information given by deuterium NMR.
Isotopic techniques, particularly the NMR analysis of deuterium, can also be used to control the authenticity of fruit juices by first converting the sugars into ethanol using controlled fermentation. Deuterium NMR can also be used to characterize the origin of natural flavours such as vanillin or raspberry. In future the combination of nuclear magnetic resonance and mass spectrometry will almost certainly lead to many other applications in detecting frauds in food.
About the authorClaude Guillou and Fabiano Reniero are at the Joint Research Centre of the European Commission, Ispra, Italy.
