Two cooking methods that lower the oxalate content of food are blanching it for 10 minutes and cooking it in a pressure cooker for 15 minutes. These methods work best for lowering the oxalate content of green vegetables.
Blanching vegetables for 10 minutes lowers their oxalate content by at least 40 percent, and sometimes up to 120 percent, depending on the vegetable. Green leafy vegetables such as spinach, Swiss chard and kale are all high-oxalate foods. To blanch these foods, place them in water heated to at least 200 degrees Fahrenheit and let them cook for the recommended amount of time. It is very important not to consume or reuse the water the vegetables were boiled in, as it will have a very high oxalate content left over from the boiling process.
Steaming works just as well as blanching or boiling to lower the oxalate content of vegetables. To steam vegetables, place them in a steamer basket and position it on top of a pot of boiling water. Different vegetables require different cooking times. Spinach and arugula generally require 3 minutes of steaming, while tougher greens like kale and collards need 10 minutes. Root vegetables such as carrots, potatoes and turnips take anywhere from 8 to 20 minutes.
Once the vegetables are boiled or steamed, it is acceptable to brown them in oil for extra flavor. Doing this won't add to the oxalate content of the food nor negate the process of lowering it.