Oxalic acid blocks absorption of iron. I disagree with including spinach in the list as it has very little if any oxalic acid...thus it’s desirability as a great food source for iron!(Remember Pop-eye anyone?)
That “bite” you talk about is the oxalic acid itself!
Actually spinach is a food with one of the highest level of oxalic acid. It is a problem because it binds with calcium and interferes with absorption. It's also implicated in kidney stones. Cooking may reduce the level somewhat.
Also, the iron in spinach is a nonheme iron so spinach is not a good source of *usable* iron. Other foods, e.g., oysters, chicken livers, will give you better iron that your body can use. Not spinach.