Discussion of the Rankine method - probably too radical for the conservative wine industry - by Tom Smith, research director of Multitrator
For winemakers, getting the correct sulphur dioxide level in wine is a serious thing.
Sulphur dioxide is used as a microbial control agent to control spoilage of wine, both during the vintage and wine making and later during storage.
The addition of sulphur dioxide, or salts which release sulphur dioxide on hydrolysis starts when the grapes enter the crusher to eliminate unwanted micro-organisms which may interfere with the controlled fermentation processes which modern winemaking practices demand. During the fermentation and storage stages, the sulphur dioxide content is carefully monitored and controlled.
This is particularly important in the production of white wines, as red wines have a higher content of the polyphenolic compounds which exhibit a natural anti-oxidant effect.
The Rankine method (named after its inventor, Dr Bryce Rankine) is widely used to analyse for sulphur dioxide.
In this method, sulphur dioxide is scavenged by bubbling nitrogen through the wine, and reacting the entrained sulphur dioxide with hydrogen peroxide.
The resultant sulfuric acid is then titrated with standard base.
While this method permits the determination of free and 'bound' sulphur dioxide by varying the conditions of analysis, it is a long-winded method which requires a complex set-up of flasks, condensers, dropping funnels and absorbers.
It is not well suited to coping with large numbers of assays required during the frantic days of the vintage.
Further, according to an industry source, the results produced by this technique do not correlate directly with other methods, and conversion tables are often used.
Regardless of its limitations, it is regarded as the industry standard, and is widely accepted.
However, its slowness and complexity frustrates many in the industry.
Recently, Multitrator was approached by one of its wine industry contacts to look at this problem once again.
The method it came up with is an automated thermometric back-titration.
The analyst simply pipettes an aliquot of wine into a titration beaker, puts it on the stirrer, and clicks the start button on the titration software.
One minute later, the result is computed on the dedicated spreadsheet.
The titration sequence starts with a pre-determined volume of dilute hydrogen peroxide solution being dispensed into the stirred wine sample.
Some of the peroxide reacts with the sulphur dioxide in the wine, and the excess is back-titrated with a dilute solution of potassium metabisulfite. The reaction is very strongly exothermic and very sharp, highly reproducible endpoints are obtained with quite dilute solutions. Typically, analytical precisions of 0.2mg/l (0.2ppm) sulphur dioxide are obtained.
Sulphur dioxide levels down to at least 5mg/l (5ppm) can be routinely measured.
Because the peroxide and metabisulphite solutions are relatively unstable, they should be made up fresh daily.
The potassium metabisulphite is calibrated against standard potassium iodate solution by an automated thermometric iodometric titration.
Make-up and calibration of solutions can be accomplished in approximately 20 minutes.
Due to the innate conservatism of the wine industry and legislative considerations, this new method would not be considered suitable at this stage for the certification of the sulphur dioxide content of wines.
However, its very speed and precision lends itself to routine process and quality control in wineries who are concerned with increasing productivity and running to tighter process control parameters.
Copies of application notes are available by e-mail from Multitrator.
