The Dual Role of Sulfites in Grape Preservation
Sulfites, primarily in the form of sulfur dioxide (SO₂), are a crucial tool in modern winemaking and the global distribution of fresh grapes. Their addition is not a recent innovation; historical evidence suggests that ancient Romans used sulfur to sanitize their wine vessels, foreshadowing the modern application. However, the precise, controlled use of sulfites we see today is a product of 20th-century food science and has two main functions: acting as an antioxidant and an antimicrobial agent.
Antioxidant Protection
Grapes are rich in organic compounds that are susceptible to oxidation when exposed to oxygen, a process that can lead to undesirable changes in color, aroma, and flavor. For winemakers, premature oxidation can destroy delicate fruit flavors, cause browning in white and rosé wines, and lead to 'stale' characteristics. For fresh grapes, oxidation can cause discoloration and decay during long periods of storage or transportation. Sulfites combat this by readily reacting with oxygen, essentially 'capturing' it before it can alter the wine or fruit's components. This reaction prevents unwanted chemical changes and preserves the intended character and freshness of the grape product.
Antimicrobial and Preservative Action
In addition to battling oxidation, sulfites are natural antimicrobials that inhibit the growth of unwanted bacteria and wild yeasts. In winemaking, controlling microbial activity is critical. Uncontrolled bacteria could convert alcohol into vinegar, and wild yeasts could introduce off-flavors. Sulfites allow the winemaker to control fermentation by inhibiting these undesirable microbes while allowing the desired yeast strains to thrive. For fresh grapes, sulfite addition, often through a gas release mechanism in the packaging, prevents the growth of mold and fungi that can cause decay, thereby extending their shelf life significantly.
Natural vs. Added Sulfites
It is a common misconception that only winemakers add sulfites to grapes. In reality, all fermented grape products contain some level of naturally occurring sulfites. During the fermentation process, yeast produces small amounts of SO₂ as a metabolic byproduct. Therefore, even wines marketed as 'no added sulfites' contain trace amounts. However, the majority of sulfites in wine are intentionally added by the winemaker to ensure stability and preservation. For fresh table grapes, sulfites are almost exclusively added to enable long-distance shipping and storage, as tested by the CFIA.
The Application of Sulfites in Winemaking
The use of sulfites in winemaking is a precise and regulated process, with additions made at different stages.
Common stages for sulfite addition include:
- At Crushing: A dose of sulfite is added to the crushed grapes or juice (must) to prevent wild yeast or bacteria from taking hold before the controlled fermentation begins.
- Post-Fermentation: After the desired fermentation is complete, sulfites are added to stabilize the wine and protect it during aging.
- Before Bottling: A final dose ensures the wine remains stable and protected from spoilage during its journey to the consumer.
Sulfites in Table Grapes vs. Wine
| Feature | Table Grapes | Wine |
|---|---|---|
| Primary Use | Preservation during storage and transport. | Antioxidant and antimicrobial during winemaking and aging. |
| Application Method | Typically a gas-releasing pad in packaging. | Added as a liquid or solid at various stages. |
| Sulfite Source | Almost entirely added. | Both naturally produced (fermentation) and added. |
| Detectability | Usually undetectable in final product, but regulated during application. | Requires label if exceeding 10 ppm, with significant variation. |
| Effect on Flavor | Designed to have minimal to no effect on taste. | Can affect flavor if added in excessive amounts. |
Addressing Health Concerns and Sulfite Sensitivities
While sulfites are safe for most people, approximately 1% of the population suffers from a sensitivity to them, which can manifest as allergy-like symptoms. These reactions are most often seen in individuals with asthma. For this reason, regulations in many countries mandate that any wine with a total sulfite level exceeding 10 parts per million (ppm) must include a 'contains sulfites' warning on its label.
For those with sensitivities or who simply prefer to avoid added preservatives, there are alternative options. Some winemakers, particularly those in the natural wine movement, use minimal or no added sulfites. For fresh grapes, consumers can seek out organically grown varieties, which are less likely to have added sulfites for preservation.
Conclusion
In summary, the addition of sulfites to grapes is a deliberate and controlled practice rooted in food preservation and quality control. Whether protecting fresh grapes during transit or ensuring the stability and character of a finished wine, sulfites play a crucial, dual role as antioxidants and antimicrobials. Their use allows for a global market of fresh, high-quality grapes and ensures that the wine you drink tastes as the winemaker intended, free from spoilage and oxidation. While not without concerns for a small segment of the population, the benefits of using sulfites for preservation and stability are widely accepted within the industry, as evidenced by their continued, regulated use.
International Organisation of Vine and Wine (OIV) - Review on SO2 and Wine
The Many Functions of Sulfites in Grapes
- Preservation: Extending the shelf-life of both fresh grapes and wine by preventing microbial spoilage.
- Antioxidation: Protecting the fruit and wine from browning and loss of flavor due to exposure to oxygen.
- Microbial Control: Inhibiting the growth of undesirable bacteria and wild yeasts that can spoil the final product.
- Fermentation Management: Allowing winemakers to control the fermentation process and prevent it from restarting.
- Freshness: Maintaining the color and freshness of table grapes during long-distance shipping and storage.
The Controversy Surrounding Added Sulfites
- Health Concerns: Sulfites can trigger allergy-like symptoms, particularly in asthmatics.
- Labeling Requirements: Due to sensitivity concerns, products containing over 10 ppm of sulfites must be clearly labeled.
- Winemaker Preference: Some winemakers, especially in the natural wine movement, choose to use minimal or no added sulfites to create a different style of wine.
- Dulling Flavors: Although rare and only at high concentrations, some argue that too many sulfites can dull a wine's natural flavors.
- Long History: Despite the debate, sulfites have been used as a preservative for centuries and remain a common tool.
