Rapid Sulphur dioxide testing: what difference does it make to wine quality?

31. Oct, 2018
By Richard Mills
Winemakers share their experience with the WineScan SO2 as a much faster and more convenient alternative to manual SO2 analysis methods.

“The WineScan SO2 is a technical innovation in the world of wine analysis. Our return of experience is very positive.” 

This comment reflects a shared view that having a Sulphur dioxide test that is very quick, yet still based on the reference method brings new possibilities to winemaking and wine quality. 
For more background, let’s hear what WineScan SO2 users in the field are saying and look at how the innovative WineScan approach can deliver reliable measurements in just three minutes. 

Aurélie Sabatier, Laboratory Manager, VIGNOBLES & COMPAGNIE:
“We control SO2 at all steps of the wine preparation from bulk wine reception, during wine processing (clarification, stabilization, wine assembly…) and until bottling.

The simplicity of routine use allows any operator to make controls by themselves.

Active SO2 is a very important parameter that allows us to adjust precisely the content of SO2 precisely in relation to wine composition. This allows us to add only the required quantity and avoid adding too much SO2. Our policy is to reduce as much as possible SO2 content in our wines.

I fully agree with the idea that WineScan SO2 is a real technical innovation. Compared to other analytical methods used in the past, WineScan SO2 is the only instrument that give very similar results to the reference method (Franz Paul), with a very good accuracy and repeatability.

The WineScan SO2 provides rapid and accurate results that allows strong reactivity in our decision making. We can bring the same care to high quality wines and standard wines and have a constant quality of our products.”


Pauline Brun, Laboratory manager Cave de TAIN L’HERMITAGE has similar views:
“We control SO2 on Must at reception and also during the fermentation. Checking SO2 on must allows us to control the fermentation from the beginning and explain any downturn during fermentation.

Having all the parameters at the same time is an important time-saving that allows immediate reaction on the process just after analysis. 

The WSC SO2 is a technical innovation in the world of wine analysis. Our return of experience is very positive (repeatability, coherence of results…). From grape harvest to wine bottling and at all intermediate stage of the vinification, we can manage the SO2 content more precisely.”

Central to the comments on the WineScan approach is an innovative take on a classical reference method that delivers free and total Sulphur dioxide results in three minutes instead of at least 30 minutes. 


Rapid Sulphur dioxide analysis: how it works 
In short, the WineScan approach can be summed up as following the reference method, but with a much faster capture of Sulphur dioxide with mid infrared analysis.  
The measurement principle is based on the aspiration-oxidation reference method which is the official analytical method for free sulphur dioxide and total sulphur dioxide as described by OIV (Organisation Internationale de la Vigne et du Vin). 

With this traditional method, the sample (the wine) is acidified and thereby the bisulphite and sulphite is ‘pushed’ to molecular form Sulphur Dioxide. This is then separated from the wine by the aeration. The liberated Sulphur Dioxide is then captured in a H2O2 solution and determinated by simple titration.


The WineScan SO2 configuration can be regarded as an automation of the aeration/oxidation method where the capture of the sulphur dioxide in the H2O2 solution is substituted by continuous infrared SO2 detection, as shown in fig.1. 


SO2 Animation

Measurement principle: The sample and the Phosphoric acid are sequentially pumped into and mixed in a reaction chamber. This reaction chamber is constantly heated to 85 °C. Through mixture in the reaction chamber, a fixed volume of gas (air) is continuously circulated and presented in a gas cuvette from which the mid-infrared gas sprectra are collected.

Three minutes instead of 30
Due to mathematical modeling, WineScan SO2 only requires 60 seconds of evaporation to predict what the sulphite content would have been if completely hydrolyzed – a process that normally takes 15 minutes according to the traditional reference method, or thirty minutes if both free and total Sulphur dioxide is required. Moreover, the accuracy and repeatability of the WineScan method is close to the performance of the reference method. 


Why not analyse directly in the liquid sample?
Ever since the first WineScan was launched in 1999, users have commented: ´It is great having all these quality parameters, but can’t you do SO2 as well?´ Developers at FOSS had to answer no due to the low concentrations of Sulphur dioxide in the liquid sample, but at the same time, they set about designing the solution which eventually took the form of the WineScan SO2. 

Sulphur dioxide has very distinct features in the mid-infrared spectrum, but the sulphur dioxide IR-spectrum overlaps to a certain degree with water, alcohol and acetic acid vapor and the actual concentration in wine is very low compared to alcohol and the others. Thus, it has not been possible to create a calibration for the liquid system alone. 

The separation of the molecular Sulphur dioxide into a gas volume increases the sensitivity and specificity multifold in the mid-infrared spectrum. This, coupled with the mathematical modeling already mentioned, allows a reliable and rapid test. 

Free and total Sulphur dioxide and over 20 other wine quality parameters
Experienced WineScan users will note however that the three minutes involved is longer than the 30 seconds or so it takes to test a raft of parameters in the liquid sample. Because of this, the WineScan SO2 measurement can be switched on and off depending on test requirements: three minutes for free and total Sulphur dioxide together with over 20 other parameters or about 30 seconds without SO2. 


More about WineScan SO2

More about FTIR analysis 


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