Sulfates and sulfites are both sulfur-based compounds, but they have distinct chemical structures and functions in food. While the body can process both, the vast majority of dietary concerns, particularly sensitivities, relate to sulfites, not sulfates. Most sulfites are converted into harmless sulfates in the body by the enzyme sulfite oxidase, but for a small portion of the population, especially asthmatics, this process can be compromised. Sulfates ($SO_4^{2-}$), which are fully oxidized, are generally unreactive, whereas sulfites ($SO_3^{2-}$), with one less oxygen atom, are used for their powerful preservative properties.
Natural Sources of Sulfates (from Sulfur Amino Acids)
Sulfur is a critical mineral for human health, and much of our dietary intake comes from the sulfur-containing amino acids: methionine and cysteine. These are found naturally in a wide variety of protein-rich foods.
Protein-Rich Animal Products
Many common animal-based foods are excellent sources of the sulfur-containing amino acids, methionine and cysteine.
- Meat and poultry: Especially high levels are found in beef, ham, chicken, duck, and organ meats like liver.
- Fish and shellfish: Most types of fish, as well as shrimp, scallops, and other crustaceans, are rich in these amino acids.
- Eggs and dairy: All types of eggs, and dairy products like cheese and cow's milk, contain significant amounts of sulfur.
Vegetables and Fruits with Natural Sulfur
Certain vegetables and fruits contain sulfur compounds that give them their characteristic flavor and aroma. These compounds are a natural source of sulfur in the diet.
- Cruciferous vegetables: Broccoli, cauliflower, cabbage, Brussels sprouts, kale, and radishes all contain glucosinolates, which are sulfur-rich compounds.
- Allium vegetables: Onions, garlic, leeks, shallots, and chives are known for their pungent organosulfur compounds.
- Other produce: Asparagus, watercress, and dried fruits like peaches and figs naturally contain sulfur compounds.
Added Sulfites as Preservatives
While sulfates and sulfur are naturally occurring, a substantial portion of the population's exposure comes from added sulfites used for preservation. These must be declared on food labels if their concentration exceeds 10 parts per million (ppm).
Wine and Beer
Sulfites are a natural byproduct of fermentation, but winemakers and brewers often add more to prevent spoilage and oxidation.
- Wine: Added sulfites act as antioxidants to prevent browning and spoilage. White wines and sweet wines generally have higher sulfite levels than red wines.
- Beer and cider: Some beers and ciders contain added sulfites, though their use is less common today.
Dried and Processed Foods
Sulfites are used as a preservative to maintain color, freshness, and shelf life in many processed foods.
- Dried fruits: Dried apricots, raisins, and other dried fruits are often treated with sulfites to prevent browning and preserve color.
- Processed potatoes: Dehydrated, pre-cut, and frozen potato products, like french fries, can contain sulfites.
- Condiments: Items like vinegar, salad dressings, and some mustards may contain added sulfites.
- Canned and bottled goods: Canned vegetables, fruit juices, and bottled lemon or lime juice are potential sources.
Comparison of Sulfate and Sulfite Sources in Food
To avoid confusion, here is a comparison highlighting the primary differences in sources for these sulfur-based compounds.
| Feature | Sulfates (from Amino Acids) | Sulfites (as Additives) |
|---|---|---|
| Source | Naturally occurring in proteins | Added as preservatives (E220-E228) |
| Function | Building blocks for proteins, enzymes, and other essential molecules | Antimicrobial and antioxidant agent |
| Primary Foods | Meat, poultry, eggs, dairy, legumes, cruciferous vegetables, allium vegetables | Dried fruits, wine, beer, processed potatoes, condiments, fruit juices |
| Labeling | Not listed as 'sulfates'; part of the protein content | Required if over 10 ppm; listed as "contains sulfites" or by E-number |
| Health Concerns | Generally safe for consumption | Potential for asthma reactions, hives, or other symptoms in sensitive individuals |
How to Identify Sulfites on Food Labels
For those with sulfite sensitivity, reading food labels is essential. Regulations require manufacturers to list sulfites if they are present above 10 ppm. Common names and E-numbers to look for include:
- Sulfur dioxide (E220)
- Sodium sulfite (E221)
- Sodium bisulfite (E222)
- Sodium metabisulfite (E223)
- Potassium metabisulfite (E224)
Additionally, be aware that items served in restaurants or fresh produce buffets might have added sulfites without individual item labeling. Choosing raw, whole foods and preparing them at home is the most reliable way to avoid added sulfites.
Conclusion
Sulfates are a critical and naturally occurring component of many nutritious whole foods through sulfur-containing amino acids, primarily methionine and cysteine. In contrast, sulfites are largely added to processed and dried foods as preservatives to maintain shelf life and appearance. Understanding the difference and knowing how to read food labels is key for managing dietary intake, especially for sensitive individuals. While natural sulfates are a necessary part of a healthy diet, vigilance is required to identify and avoid added sulfites in processed goods. Food Allergy Canada provides a detailed fact sheet on sulphites.
