In the effort to create high quality wines, it is critical to maintain proper free sulfur dioxide (SO2) levels during all steps of the winemaking process. Keeping SO2 levels at optimum values is key to preventing the discoloration, off-flavors and unpleasant aromas resulting from oxidation and microbial contamination. Suffice to say that all those award winning wines going for $200 a bottle or more have the correct levels of sulfites added to them!
The problem with the task of maintaining SO2 levels is the tendency for this component to disappear over time. The SO2, typically added as a sulfite like potassium metabisulfite, undergoes an equilibrium between free and bound forms which is pH- and temperature-dependent. In addition, oxidation by atmospheric oxygen continually reduces SO2 levels especially when the wine is briefly exposed to air as occurs during racking and bottling. Therefore it is important to have a reliable method handy for measuring free SO2.
There are Several Approaches one can take to determine free SO2. However, as a large number of winemakers rely on the simple Ripper titration, we thought it a good idea to elaborate on the virtues and cautions of using this method.
In the Ripper titration, standard iodine is titrated into an acidified wine sample. The iodine reacts with free SO2, and when the SO2 has all been titrated, free iodine appears at the endpoint. Originally described in 18981, the original Ripper method is fast but its accuracy and precision are notably poor, primarily because it relies on the appearance of a dark blue starch-iodine complex to detect the end point, which is difficult to see (especially in red wines) and subject to errors of interpretation2.
However, the method’s speed and simplicity are attractive; many winemakers have claimed that they can train themselves to read the endpoint reproducibly.
Some modifications to the Ripper method improve the reliability of detecting the endpoint. For example, the use of electrochemical sensors that respond to the appearance of free iodine removes the burden of trying to visualize the starch endpoint against a dark background in red wines.
The Hanna Instruments autotitrators like the HI 84100 (also sold by Milwaukee Instruments) use the ORP electrode in an automated device. This electrode relies on a change in the oxidation status of the solution that occurs as free iodine appears. Vinmetrica’s SC-100 uses an amperometric probe to detect appearance of free iodine. This provides robust, sensitive results that compare favorably with other methods.
There have been numerous reports describing the various sources of potential error and interferences in the Ripper method. The acidification step (also required in some other methods) induces slow dissociation of bound SO2 to form more free SO2, leading to a systematic overestimation of free SO2. This effect can be minimized by performing the Ripper titration quickly (within 2 minutes), such that it probably contributes less than a few percent error.
Another source of error that can lead to overestimation of free SO2 is the presence of other oxidizable compounds in the wine. These can react with the iodine titrant, again leading to overestimation of the SO2 level. In some white wines, there are appreciable levels of ascorbic acid which react readily with the titrant. In red wines, the presence of phenolic compounds can also lead to overestimation.
In this case the error may be estimated by a modified Ripper method3 in which duplicate samples are titrated, one having been pretreated with a drop of 3% hydrogen peroxide, the other untreated. The difference between the two values provides an estimate of the “true” SO2 ppm value (since peroxide primarily removes only the free SO2).
In our hands, using the Vinmetrica SC-100 (or the SC-300 in SO2 mode), we have not seen significant differences between the “true” and uncorrected value of free SO2 in the wines we have worked with to date. This implies that phenolic compounds are not significant sources of interference with Vinmetrica instruments.
Indeed, the aeration oxidation (AO) method, which is theoretically free of these interferences, has been shown in one independent study to give identical results to those of the SC-1004 . Although it seems to be generally accepted that Ripper methods produce higher values and are less accurate5 than other methods, it is possible that the Vinmetrica’s improved endpoint detection may change this perception
In summary, the Ripper method is quick and can be made reliable and accurate enough to guide the winemaker in maintaining proper levels of sulfur dioxide. The SC-100A and SC-300 are simple and inexpensive SO2 meters for accurate SO2 analysis.
1 M. Ripper, Die Schwelflige Saure im Wein und deren Bestimmung, J. Praia. Chem. 46, 428-73 (1898)
2 James E Vahl and Jean E. Converse, Ripper procedure for determining sulfur dioxide in Wine: Collaborative study. J Assoc Off Anal Chem 63, 194-9 (1980)
5 J.W. Buechsenstein and C. S. Ough, SO2 Determination by Aeration-Oxidation: A Comparison with Ripper (1978) Am. J. Enol. Vitic. 29:3:161-164