What is Oxalic Acid and Why is it in Oxalis?
Oxalic acid is an organic compound found naturally in many plants, known for its chelating properties. It binds with minerals, most notably calcium, which can interfere with mineral absorption in the body and form insoluble calcium oxalate crystals. This is the compound that gives the leaves of wood sorrel, or Oxalis, their characteristic sour, lemony flavor. The name Oxalis itself is derived from the Greek word oxys, meaning sour or sharp, a direct nod to its high oxalic acid content. The presence of oxalic acid is a natural defense mechanism for the plant against herbivores.
Health Implications of Consuming Oxalic Acid
In small quantities, oxalic acid is generally harmless to healthy individuals, especially when consumed with calcium-rich foods that bind the oxalates in the digestive tract and aid excretion. However, consuming large quantities can be problematic, particularly for individuals predisposed to certain health conditions.
Potential risks associated with excessive oxalate consumption include:
- Nutrient Deficiency: Oxalic acid's binding of calcium can lead to a deficiency over time if high-oxalate foods are consumed in excess.
- Kidney Stones: The precipitation of calcium oxalate crystals is a primary cause of kidney stones.
- Aggravated Conditions: For those with rheumatism, arthritis, or gout, high oxalate intake can aggravate symptoms.
It is important to note that the danger lies in consuming large amounts over a prolonged period. A casual trailside nibble of a few leaves is unlikely to cause issues for most people.
Oxalic Acid Levels in Specific Oxalis Species
While the genus Oxalis is known for its high oxalic acid content, the concentration varies significantly among different species. Research and foraging guides provide insights into the varying potencies of these plants.
- Oxalis corniculata (Yellow Sorrel): The leaves of this common garden weed have a notable oxalate content, reported to be between 7% and 12% oxalate. Though a source of vitamin C, these leaves should be consumed in moderation.
- Oxalis pes-caprae: This species has been found to have higher oxalic acid concentrations than many other wild edible plants, which aligns with its genus name. The leaves, stems, and flowers all contain the compound.
- Oxalis tuberosa (Oca): The tubers of Oca contain oxalic acid levels ranging from 80 to 200 mg/100g. Interestingly, exposure to sunlight can decrease the organic acid content in the tubers, making them sweeter.
- Oxalis acetosella (Common Wood Sorrel): While some guides list it as potentially toxic due to oxalic acid, it is widely consumed by foragers in moderation. The tiny size of the leaves makes it difficult to ingest a dangerous quantity.
Comparison: Oxalis vs. Other High-Oxalate Plants
To put the oxalate content of Oxalis into perspective, it helps to compare it to other common foods known to contain the compound.
| Plant (Part) | Oxalate Concentration | Notes |
|---|---|---|
| Oxalis corniculata (Leaves) | 7,000-12,000 mg/100g (dry weight) | Higher than many common vegetables |
| Spinach (Raw) | >1,000 mg/100g (dry matter) | One of the highest oxalate sources |
| Chard (Raw) | >1,000 mg/100g (dry matter) | High in soluble oxalates |
| Rhubarb | High | Comparable to spinach |
| Oxalis tuberosa (Tubers) | 80-200 mg/100g | Lower than spinach leaves, comparable to carrots |
| Carrots | Comparable to Oca | Considered a moderate oxalate source |
Reducing Oxalate Content for Consumption
For those who wish to reduce their oxalate intake while still enjoying Oxalis or other high-oxalate plants, several cooking methods are effective.
- Boiling: Boiling vegetables in water is the most effective method, as oxalates are water-soluble and will leach into the cooking water. After boiling, discard the water and use the drained greens. One study showed boiling reduced soluble oxalate in spinach by over 60%.
- Pairing with Calcium: Consuming high-oxalate foods with calcium-rich foods like cheese, milk, or yogurt helps bind the oxalates in the digestive system, preventing absorption.
- Soaking: Soaking tubers or other parts of the plant can help reduce oxalate content, especially if followed by cooking. For example, studies on Oca tubers show that post-harvest exposure to sunlight can reduce their acidic taste.
- Steaming: While less effective than boiling, steaming can also reduce soluble oxalate levels.
Safe Foraging and Culinary Use
Foragers frequently enjoy the bright, sour taste of wood sorrel in small amounts. Safe culinary practices include:
- Grow your own: To ensure no chemicals were used, grow Oxalis from seed yourself.
- Use as a garnish: Add a few leaves or flowers to salads and dishes for a lemony accent rather than eating large quantities as a main vegetable.
- Identify correctly: Ensure you can correctly identify edible Oxalis species, though their distinct leaves and flavor make it relatively easy.
Remember that while some Oxalis is considered a common garden weed, it can be a delightful and nutritious wild edible in moderation. If you have underlying health conditions like kidney stones, gout, or rheumatism, it is best to avoid consuming Oxalis entirely due to its oxalic acid content.
Conclusion
In conclusion, the amount of oxalic acid in Oxalis varies considerably by species, with some like O. corniculata having quite high levels. The characteristic sour taste is a direct result of this compound, which can pose health risks if consumed in excess, particularly for individuals with certain medical conditions. However, for most people, consuming Oxalis in small, sensible amounts is safe. Culinary methods such as boiling and pairing with calcium-rich foods can further mitigate the effects of oxalates. Understanding the potency of wood sorrel is key to enjoying its unique flavor responsibly, making it a safe and tasty addition to your kitchen in moderation. For more detailed information on specific plant properties, consult reputable botanical resources like the Plant For A Future database.
