Rhubarb
Despite its crunchy stringiness, rhubarb provides only small amounts of fiber, including the insoluble cellulose and lignin in the stiff cells of its stalk and “strings” and the soluble pectins in the flesh. Rhubarb has some sugar, no starch, and only a trace of protein and fat.
Rhubarb is a relatively good source of dietary fiber and vitamin C. One-half cup cooked rhubarb has 2.4 g dietary fiber including insoluble cellulose and lignin in the “strings” and soluble pectins in the flesh. One-half cup cooked rhubarb has 2.4 g dietary fiber and 4 mg vitamin C (5 percent of the RDA for a woman, 4 percent of the RDA for a man).
Rhubarb also has some calcium (174 mg per serving), but oxalic acid (one of the naturally occurring chemicals that give rhubarb its astringent flavor) binds the calcium into calcium oxalate, an insoluble compound the body cannot absorb. The other astringent chemicals in rhubarb are tannins (also found in tea, red wines, and some unripe fruits) and phenols. Tannins and phenols coagulate proteins on the surface of the mucous membrane lining of the mouth, making the mouth “pucker” when eating rhubarb.
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Cooked. Only the stalks of the rhubarb are used as food;
the leaves are poisonous, whether raw or cooked.
Low-oxalate diet (for people who form calcium oxalate kidney stones)
Look for: Crisp, bright, fresh stalks of rhubarb. Although color is not necessarily a guide to quality, the deeper the red, the more flavorful the stalks are likely to be. The mediumsize stalks are generally more tender than large ones, which, like large stalks of celery, may be stringy.
Wrap rhubarb in plastic and store it in the refrigerator to keep cool and humid. Rhubarb is fairly perishable; use it within a few days after you buy it.
Remove and discard all leaves on the rhubarb stalk.
rhubarb leaves are not edible; they are poisonous, raw or cooked.
Wash the rhubarb under cool running water. Trim the end and cut off any discolored parts. If the stalks are tough, peel them to get rid of hard “strings.” (Most of the rhubarb we buy is grown in hothouses and bred to have a thin skin that doesn’t have to be peeled.)
Rhubarb is colored with red anthocyanin pigments that turn redder in acid and turn bluish in bases (alkalis) and brownish if you cook them with sugar at very high heat. If you cook rhubarb in an aluminum or iron pot, metal ions flaking off the pot will interact with acids in the fruit to form brown compounds that darken both the pot and the rhubarb.
Kidney stones. More than 50 percent of all kidney stones are composed of calcium oxalate or calcium oxalate plus phosphate. People with a metabolic disorder that leads them to excrete large amounts of oxalates in their urine or who have had ileal disease or who eat large amounts of foods high in oxalic acid are the ones most likely to form these stones. Rhubarb, like beets, cocoa, nuts, parsley, spinach, and tea, is high in oxalic acid.
