We've been having pretty good luck in our garden with swiss chard. But our first batch of chard (sauteed with lardons of bacon) had a chalky, gritty finish in the mouth; the same thing that sometimes happens with spinach. No, it wasn't sand or dirt -- we'd thoroughly washed the leaves, and sand would have been much coarser -- this was more like a "hairy teeth" feeling.
Did some research -- Harold McGee's On Food and Cooking
- The culprit is oxalic acid, naturally present in chard
- Oxalic acid reacts with calcium -- also present in chard -- causing the calcium to form tiny crystals
- The calcium crystals are what cause the chalky/gritty sensation
- In high does, oxalic acid is toxic -- it's what makes rhubarb leaves poisonous (BTW, a few bites of rhubarb leaf isn't fatal; you'd need to eat a lot of it for it to kill you)
- Oxalates are present at safe/edible levels in the following foods (listed in descending order of highest concentration): buckwheat, star fruit, black pepper, parsley, poppy seed, rhubarb stalk, amaranth, spinach, chard, beets, cocoa, chocolate, most nuts, most berries, and beans.
I'm not really sure what's up with our swiss chard, because we've cooked tons of it before without the chalky/gritty effect. Perhaps it's the soil chemistry, or maybe even some quirk in our seaside climate -- who knows (thoughts, anyone?).
At any rate: how to reduce oxalic acid in swiss chard (or spinach, etc):
- Use the stalks/stems only -- oxalic acid is in higher concentrations in the leaves (traditional Italian cooking uses the stalks, not the leaves)
- Blanch the leaves in boiling water (30 sec to 1 min) -- this leaches out the oxalic acid reasonably well
- Add something alkaline -- such as baking soda -- to neutralize the acid
- Our approach was to blanch them with a teaspoon of baking soda added to the boiling water. We tried this with the same swiss chard from our garden, and it eliminated all traces of grittiness. Success!!! (said with Borat accent)
For further reading: link.
