The words “contains sulphites” appear on just about every wine label. As a result, most consumers are familiar with the phrase – but this doesn’t necessarily mean that we really understand the impact of sulphites on the wine industry.
The amount of sulphites that a wine can contain is highly regulated around the world. Any wine which contains a sulphite concentration of more than 10 parts per million (ppm) must include the phrase “contains sulphites” on the label.
Sulphites are classified as an allergen, which means that in high concentrations they can pose a risk to people who are sensitive to these kinds of substances. Wine labels include this information to alert a small percentage of people of the fact that the product contains the additive.
Sulphites are chemical compounds that contain sulphur. This non-metallic chemical element is very common in nature – in fact, it is essential to life itself.
We find it in combination with a great number of other chemical compounds in the earth. It has a distinctive yellow colour and has been used as a food additive since the 17th century.
Sulphur is used in the vineyard to prevent fungal diseases like powdery mildew. From a biological standpoint, the fungus needs the plant tissue in the vineyard to survive. On contact, sulphur penetrates the plant tissue, thus offering the plant protection from the disease. One of the main problems associated with the use of sulphur is its excessive absorption and accumulation in the soil.
Acidification can affect the soil, groundwater, surface water, living organisms, and ecosystems in numerous ways. In the specific case of vineyards, it prevents the plants from absorbing water and nutrients, leading to atrophied vines that display symptoms of nutrient deficiency.
The acidification of the environment provides a clear example of how the soil, water, and living organisms are interrelated. Soils with a low pH block the decomposition of organic matter and suffer a reduction in microbial life, which ultimately has a direct impact on the quality of the vineyard.
Given how the use of sulphur affects the vineyards, winegrowers have, over the past decade, begun paying greater attention to its application and dosage. They have also begun searching for alternative treatments such as the use of plant-based phytovaccines, biofungicides, and a combination of inorganic fertilizers in the soil.
How does sulphur find its way into wine?
When sulphur combines with oxygen, it produces what we refer to in oenology as sulphur dioxide (SO2). It is a chemical compound with anti-microbial and anti-oxidant properties that is widely used in winemaking.
Sulphur dioxide is a normal component of wine and a natural by-product of yeast metabolism. Most yeast strains produce sulphur dioxide levels ranging from 10–30 mg/litre during the process of alcoholic fermentation.
The judicious use of SO2 confers certain protective and beneficial properties to the wine. Using it excessively, however, can generate negative effects, including toxicity to humans, the inhibition of desired fermentations, unpleasant flavours, and the reduction of colour and varietal aromas in a wine. This is why maximum legal limits are necessary
Sulphur dioxide in wine comes in two forms, either “free” or “bound” with other compounds suspended in the wine or must such as aldehydes and sugar. The sum of both portions represents what is known as Total Sulphur Dioxide (TSO2). Free SO2 is what provides the wine with the aforementioned anti-oxidant and anti-microbial properties. When added to the wine, it interacts with water to produce sulphurous acid, which dissociates into different fractions depending on the pH of the wine.
The higher the pH, the lower and therefore less effective the available free SO2 will be.
A growing awareness among wine producers of the demand for products that are free from chemical compounds is leading to a decrease in the use of additives, including sulphur dioxide, in the winemaking process.
This philosophy aligns with the approach of organic wine producers.
That being said, it does require a much more controlled vinification process and can occasionally cause problems in terms of the consistency, ageing potential, and stability of the wines.
During my most recent trip to Priorat, I saw a clear example of a winery that is opting for a reduced use of sulphur dioxide in its production process.
Rocarella is a winery in the foothills of the Montsant mountains where llicorella slate is the predominant soil component. The winery’s founder, Roger Oferil, explains that the key to working with low amounts of SO2 requires several conditions, such as maintaining the winery as clean as possible, controlling temperature, and avoiding excessive oxygen exposure of the wine during the vinification process.
The natural wine trend, which has remained steady for several years now, means the number of wineries that are reducing the amount of sulphites in their wines is on the rise.
Communities like Raw Wine, which focus on producers, winemakers, and consumers of low-intervention organic and biodynamic wines, are a clear example of how these types of winemaking practices are growing rapidly in the vinicultural sector.
Environmental sustainability is becoming an ever greater concern in the wine industry, and this, coupled with greater health consciousness, puts chemical additives like sulphur compounds in a difficult position.
Although there is no infallible technique that would eliminate these chemicals from the vineyard and winery entirely, the industry is moving towards a reduction in their levels thanks to sustainable strategies and market trends. The road might be long, but the goal is clear: to find treatments that offer an alternative to the on-going use of sulphur.
Marta Juega, PhD.