Understanding Oxalates and Their Impact
Oxalates, or oxalic acid, are naturally occurring compounds found in many plants, especially greens, nuts, seeds, and certain vegetables. They are often considered "antinutrients" because they can bind with minerals, primarily calcium, forming insoluble calcium oxalate crystals. For most healthy people, consuming moderate amounts of oxalates poses no issue. However, for individuals predisposed to kidney stones, or those with sensitive digestive systems, a high oxalate intake can be problematic. Calcium oxalate stones are the most common type of kidney stone, and dietary oxalate intake is a significant contributor.
How Lacto Fermentation Breaks Down Oxalates
Lacto fermentation is a process where lactic acid bacteria (LAB) convert carbohydrates into lactic acid, creating an acidic, probiotic-rich environment. This microbial activity is key to reducing oxalates through two primary mechanisms:
Microbial Degradation
Certain strains of LAB possess the unique ability to break down oxalates directly. These bacteria produce enzymes, such as oxalyl-CoA decarboxylase and formyl-CoA transferase, which are part of a metabolic pathway that breaks down oxalic acid into simpler, non-harmful compounds like formate and carbon dioxide. Studies have isolated powerful oxalate-degrading LAB strains like Lactobacillus fermentum, Lactobacillus acidophilus, and Lactiplantibacillus plantarum from fermented foods and the human gut.
Leaching into Brine
Another important mechanism is the leaching of water-soluble oxalates into the fermentation brine. As vegetables are submerged in a salty solution, water-soluble oxalates are drawn out of the plant cells and into the liquid. Research on kimchi made from silver beet, for example, found that while soaking in brine helped, fermentation itself further significantly reduced total and insoluble oxalates, indicating a dual-action process of leaching and microbial degradation.
Scientific Evidence of Oxalate Reduction
Numerous studies confirm the efficacy of lacto fermentation in lowering oxalate levels in various foods:
- Spinach: Research has repeatedly shown that lactic acid fermentation can cause a significant reduction in oxalate content in spinach, a notorious high-oxalate food. One study noted that combining blanching with fermentation was even more effective.
- Kimchi: In a study on kimchi made from silver beet, researchers found a significant reduction in total oxalate content after only five days of fermentation. The process resulted in a 70.4% reduction in insoluble oxalate, largely attributed to the activity of LAB like Lactiplantibacillus plantarum.
- Koreeb Seeds: A study on fermented kisra bread from Koreeb seeds revealed that 72 hours of fermentation led to an impressive 80.9% reduction in oxalic acid content, significantly enhancing the bioavailability of minerals like calcium and zinc.
- Edible Grass: In a study published in Foods, lacto fermentation of edible grass reduced oxalic acid to about 24% of the control sample, enhancing its functional properties.
Factors Influencing Oxalate Reduction
The degree of oxalate reduction via lacto fermentation is not uniform and depends on several factors:
- Bacterial Strains: The specific species and strains of lactic acid bacteria present are critical. Some strains are more efficient at degrading oxalates than others. Using a starter culture with known oxalate-degrading properties, such as specific Lactobacillus strains, can yield more reliable results.
- Fermentation Time: As shown in studies on Koreeb seed fermentation, a longer fermentation time typically correlates with a greater reduction in oxalate content. The microbes need sufficient time to proliferate and metabolize the oxalates.
- Type of Food: The inherent composition of the food affects the outcome. Factors like fiber content, initial pH, and the type of oxalate (soluble vs. insoluble) can influence the rate and extent of reduction.
- Temperature: Temperature is a crucial factor that determines the balance and activity of microbial populations. Maintaining an optimal temperature range is important for a successful fermentation process and effective oxalate breakdown.
Comparison: Lacto Fermentation vs. Other Processing Methods
Processing high-oxalate foods is possible through several methods, but they differ in their effectiveness and impact on overall nutrient content. Here is a comparison:
| Feature | Lacto Fermentation | Boiling | Steaming | Soaking |
|---|---|---|---|---|
| Mechanism | Microbial degradation and leaching | Leaching into water | Limited leaching; some structural changes | Leaching into water |
| Effective on Soluble Oxalates? | Yes, via both degradation and leaching | Yes, very effective (discarding water) | Moderately effective | Moderately effective (discarding water) |
| Effective on Insoluble Oxalates? | Yes, changes in pH can release bound oxalates for microbes to degrade | No, less effective on insoluble forms | No, less effective | No, less effective |
| Impact on Nutrient Profile | Often enhances bioavailability of minerals and vitamins, adds probiotics | Can cause loss of heat-sensitive vitamins and minerals | Preserves more nutrients than boiling | Can cause some nutrient loss |
| Additional Benefits | Introduces beneficial bacteria, creates new flavors | Simple and quick | Simple and quick | Low-tech, easy |
Boiling and discarding the water is a highly effective way to reduce soluble oxalates. However, unlike fermentation, boiling does not produce beneficial bacteria and can reduce some heat-sensitive nutrients. Fermentation, on the other hand, provides the dual benefit of reducing oxalates while also producing probiotics and improving the bioavailability of other minerals. While combining methods like blanching and fermenting can be effective for some vegetables like spinach, fermentation alone offers unique nutritional advantages.
Conclusion
Evidence from numerous studies confirms that lacto fermentation is a highly effective method for reducing the oxalate content of many plant-based foods, including high-oxalate sources like spinach and silver beet. This is primarily achieved through the metabolic activity of specific oxalate-degrading lactic acid bacteria and the leaching of soluble oxalates into the brine. The efficiency of this process depends on factors such as fermentation time, temperature, and the specific strains of bacteria present. By incorporating lacto-fermented foods into a balanced diet, individuals can enjoy the nutritional benefits of many vegetables while mitigating the antinutrient effects of oxalates. This makes fermentation a valuable and ancient practice for improving nutritional quality, particularly for those managing a low-oxalate diet.
For more detailed scientific insight, a key review on this topic can be found here.
Disclaimer: Always consult with a healthcare professional before making significant dietary changes, especially if you have a medical condition like a history of kidney stones.
Final Thoughts and Practical Applications
While lacto fermentation is a powerful tool, it’s not a magic bullet. Combining it with other practices can maximize benefits. For example, ensuring adequate calcium intake alongside fermented foods will further bind any remaining oxalates in the gut, minimizing absorption. Additionally, variety in your diet prevents overconsumption of any single high-oxalate food. The scientific community continues to explore the complex relationship between gut microbiota, oxalates, and human health, but the existing evidence strongly supports the benefits of lacto fermentation in enhancing the nutritional profile of plant foods.
