Colour, fragrance, aroma and the subtlest of flavor
notes on the palate – wines really allow us to experience the complexity
of nature. The ability to deduce the grape, growing region or vintage
simply from experiencing the taste and smell is an extremely difficult
discipline which remains the preserve of a few experts around the world.
The discerning consumer, however, doesn’t have to rely solely on their own judgment of taste – and indeed prefers not to. Where food is concerned they place their trust in high quality products and manufacturer information.
‘Labor&more’ spoke recently with Dr. Manfred Spraul, Director of NMR Applications at Bruker Biospin, Rheinstetten, about the new possibilities for quality analysis offered by NMR spectroscopy. (Labor&more is Europe’s leading magazine for decision makers in the scientific areas of chemistry, the pharmaceutical industry, biotechnology and the food processing industry.)
labor&more: Dr. Spraul, Bruker’s second generation WineScreenerTM was awarded the 2013 innovation prize at “Intervitis Interfructa” in Stuttgart. What can this analytical procedure deliver?
Dr. Manfred Spraul: Wine profiling, as it is known, provides a variety of quality and safety relevant parameters using a single measurement in just a few minutes. Conventional analysis is targeted at specific known compounds or properties of a wine. The NMR method provides the same targeted parameters, but also, within the same measurement, statistical analyses, such as classification and discrimination of wines using statistical models from authentic samples. This enables the verification of grape variety, geographical origin and vintage.
You have conducted studies using the German Riesling. What were your findings?
First of all, for product quality, it is essential to verify the information on the bottle label and to check that the concentrations of particular compounds comply with OIV (Organisation ¬International Du Vin) standards or those of other national authorities. For Riesling there is an international model which detects the grape variety independently of origin. For Germany and Austria there are models prepared on the basis of authentic samples. The origin can also be broken down into very tightly defined regional areas. In addition, the vintage can also be distinguished. Models currently exist for the 2001 and 2012 vintages; the systematic recording of authentic samples was first started in 2011. It is also important to have a statistical process that reliably records unknown compounds which were not the subject of the targeted search. It also detects whether other grape varieties have been added in large amounts.
What is the significance of the ‘fingerprint’ for the new NMR screening process and what is the role of the database which you have set up together with Winespin-Analytics?
The ‘fingerprint’ records compounds visible to NMR, and their proportions e.g. in wine or fruit juice. A distinctive feature of NMR profiling is its unique reproducibility. The technique reliably detects changes in concentration for many compounds simultaneously, making it possible, for example, to detect geographic origin. By means of its multi-marker approach, NMR can reliably detect both simple as well as more complex counterfeits. This is not possible using conventional methods which record only a few, or even just a single, parameter. Deviations from the norm require that this is well described. In a statistical sense this means that many authentic samples are needed in order to describe the variability which occurs normally. This is where the significance of the joint database lies. More international partners are now also collaborating on the database.When and why is an accurate pH value for NMR screening necessary?
NMR spectra are dependent on the pH value. If this can be controlled precisely, then the dependency can be eliminated and any changes resulting from origin, vintage and grape variety can thus be seen more clearly. The statistical models are much more accurate in the detection of deviations. For wines, the influence of pH values on NMR data is eliminated almost entirely.
How has it been possible, up to now, to deal with the issue of accurately adjusting the pH value of a sample in the order of only 1 – 2 ml?
A high degree of concentration and experience from the individual carrying out the preparation is demanded to achieve the reproducibility required. The sample is presented in the appropriate quantity (for wine = 900 µl) and 100 µl of buffer added and mixed by shaking. The target pH value is adjusted manually to a pH reference solution by means of titration with micro pH electrodes. The use of 1N reagent means, however, that in order to achieve a pH accuracy of <0.03 pH units, ½ or even just ¼ of a drop from a 100µl pipette must be dispensed. Also, between dispensing the reagent and checking the pH value arrived at, the samples must be shaken on each occasion.
That sounds very time consuming. You have presented the new ‘one stop titration’ that automatically adjusts the pre-defined pH value. How does this differ from the existing methods?
Preparation is simplified significantly with the recently launched BTpH Unit. Now the only preparatory work required is to put the correct sample aliquot into a 1.8 ml cryo vial and place the vessel into a shaker. The unique aspect of the patented pH electrodes are three capillaries, which we have integrated for the dispensing of buffer, acid and base. The small exterior diameter of the electrode shaft, less than 5 mm, allows us to conduct our titration in small vessels such as 1.8 ml cryo vials and in 2 ml deep well plates.
Which measurement process is used to record the target value of the pH value adjustment?
Electrodes with ISFET (Ion-Sensitive Field-Effect Transistor) technology are used with the BTpH unit. These electrodes are very robust and can even tolerate dry storage for short periods. ISFET electrodes also exhibit what is called a wall effect, which, with glass electrodes can sometimes lead to a slight distortion of the pH value.
Will the process be able to offer even more in the future?
The range of ISFET electrodes and their robust characteristics suggest that full automation would seem logical.
You originally developed the unit for wine analysis. What other possible areas of application are you pursuing for the pH unit?
In the analysis of honey it has already been shown that the accurate adjustment of the pH value significantly improves the analytical outcome. The same effect is also seen with spirits.
Manfred Spraul, born in 1953, studied technical chemistry at the University of Karlsruhe where he completed his doctorate in 1982. Following this, he began his professional career with Bruker Analytische Messtechnik [Analytical Measurement Technology] (today know has Bruker BioSpin GmbH) as an application specialist for NMR and LC. In 1987 he became manager of the NMR Applications Group in Rheinstetten and since 2002 he has been director for NMR applications with Bruker Biospin and worldwide coordinator of coupling techniques. In 2012 he was appointed Head of the Business Development Applied NMR, Applied NMR Method Development and Hyphenation. Manfred Spraul has 105 publications to his name and currently holds 13 patents. Developments in NMR, originating from a working group under his leadership, have received a number of awards, most recently, for example, the Intervitis / Interfructa 2013 innovation award for the WineScreener.