“Low preservative” wines – just how low is low?

As part of a recent project to develop a new Test Kit for the determination of Sulfur Dioxide for use with Spectrophotometer and Discrete Analysers, we tested a batch of commercially purchased wines labelled “low preservative”, so we could be sure that the Test Kits worked across the full range of sulfur dioxide levels. To our surprise, a number of these wines were not low at all! This is of concern to our industry as some consumers are very interested in and aware of the preservative levels of the foods and drinks they purchase. Is it time for the wine industry to decide what the level of sulfur dioxide should be to qualify for a “low preservative” statement?

Sulfur dioxide

Sulfur dioxide (synonymous with sulfites) is widely used in the preservation of wine quality and so is an important ingredient in the production of wine. The maximum level of sulfur dioxide in wine is regulated in all wine-consuming countries due to its health impact at higher doses. In Australia the maximum level allowed in dry wines is 250 mg/L (ppm). For wines containing more than 35 g/L of sugars, the total sulfur dioxide content can be up to 300 mg/L.

Although the above levels are deemed safe, some individuals are allergic to the sulfites in various foods at much lower levels and so warning labels are required in most countries for food containing minimal levels of sulfites. In Australia the level in wine that requires an allergen warning under the Foods Standards code is for “added sulphites (such as SO₂or PMS) in concentrations of 10 mg/kg or more”. That is, any wines that have added sulfites (in whatever form it is added, e.g. PMS) of 10 ppm or more are required to have an allergens warning.

To complicate matters, it is well known that sulfur dioxide is produced during the fermentation process and can be well above the 10 mg/L minimum mentioned above. So not only can the sulfur dioxide come from additions, it can be present in the wine simply from the fermentation process. These amounts are cumulative so finished wines should definitely be tested prior to making a claim of ‘low preservative’.

Measuring sulfur dioxide

The measurement of sulfur dioxide has been done for decades by the quite cumbersome and elaborate aspiration/oxidation technique, commonly referred to as the Rankine method in Australia and New Zealand. This is widely performed in wineries and wine laboratories (ours included) throughout Australia and New Zealand, even though it is an old and laborious way of getting this data. Many other tests have been updated to more modern and efficient techniques; the use of Near Infra-Red (NIR) spectrometers to measure alcohol content in place of the time-consuming distillation/hydrometry or pycnometry is one good example.

There are other ways of measuring sulfur dioxide and as a productivity improvement for our labs we started to investigate other techniques some time ago. As we also make and sell Test Kits for use with spectrophotometers and discrete analysers, we were very interested to find out if there was a reliable kit on the market.

We didn’t find any kits that we considered safe to use (many contain the carcinogen formaldehyde) or that gave accurate results for all levels and wine styles. Red wine in particular is problematic for the colorimetric type of analysis, with some manufacturers basically ignoring this issue by advising to simply decolourise the wine – this does not provide accurate results.

Test Kit development

After much work we have developed and now produce very well-performing Test Kits for Free Sulfur Dioxide for use in Spectrophotometer and Discrete Analysers and we are about to launch a similar Test Kit for Total Sulfur Dioxide.

As part of our development work we wanted to be sure our kits covered the full range of sulfur dioxide levels found in wine, not just the levels in standard solutions. So we tested hundreds of wines that came through our labs to validate the new technique by comparing the results from the Rankine technique to our colorimetric procedure. One problem we had was testing wines with low levels of sulfur dioxide as out of the hundreds of wines, very few had very low levels – that is, below approximately 30 mg/L.

Low preservative wines

So we headed to the nearest Dan Murphy’s store and went to the ‘Low Preservative’ section and bought some commercial wines that were labelled as being low in preservatives. To our surprise some of the so-called ‘Low/Minimal Preservative’ wines were nothing of the sort. We got results close to 100 ppm which is actually typical or even high for the average of what we had tested elsewhere!

Our results for a small batch of low preservative Australian wines from Dan Murphy’s aisle were: