I'm not much of a cook nor a particularly mindful eater, but the funny feeling I get from spinach has given me some food for thought. Do you know what I'm talking about? Not the grit left behind if you don't wash the leaves properly. Some have described the effect as chalky or fuzzy.
Turning Over a New Leaf
The most common answer seems to be oxalic acid, also known as ethanedioic acid, which spinach has lots of. Other fruits and vegetables have to a lesser degree. Rhubarb leaves, however, have so much oxalic acid they are considered poisonous. It may have evolved as a way for plants to discourage animals from eating it.
A dentist said the residual feeling was from the oxalic acid combining with the calcium in your saliva, to form crystals of calcium oxalate. But spinach is already has lots of calcium oxalate in the leaves and saliva doesn't seem to have much. I haven't come across any direct experimental evidence to explain the feeling, so I looked for some more corroborating information.
Your Mileage May Vary
The funny teeth syndrome seems to be more associated with cooked spinach than raw, supported by my own anecdotal Facebook survey. Cooking breaks down cell walls so perhaps more oxalic acid or calcium oxalate gets out into your mouth as compared to eating fresh. Also the volume shrinks with cooking, so you would get more spinach mass in a given mouthful.
Get Into the Kitchen
But cooking may break down oxalic acid. I wonder if calcium oxalate is more stable or even gets formed by cooking. Cooking spinach makes it more nutritious in some ways than eating it raw. I am no dietician, and I'm not going to get into the real and imagined nutritional benefits of eating spinach advocated by Popeye and others. Cooking with calcium-rich foods may also reduce the spinach effect. If anyone has any recipes that tend to reduce the spinach effect on a consistent basis, it might be interesting to see if has chemical components that alters the spinach tooth effect.
When I tried to test the spinach effect on myself, the difference between cooked and uncooked was minor. But as I said, I'm not much of a cook nor an especially subtle taster. As well, the amount of oxalic acid can differ by variety of spinach, the age of the spinach, season, and growing conditions. The tip of my tongue is now hurting, though I don't know if eating spinach was necessarily the cause of it. I'll let someone else test that idea on themselves.
So I'm still chewing on this one. I can't decide which explanation to swallow. But at least the next time I discover spinach in my teeth at a job interview, I'll have an excuse — I'm studying the dental effects of oxalic acid and calcium oxalate.