Many of my cookbooks recommend fresh citrus

zest as a flavoring agent. But they always caution against getting any of the white pith beneath it into the food. They say its bitterness will ruin the dish.

But my new Vita-Mix blender cookbook recommends grinding the pith -- along with the seeds and pulp -- into the smoothie or soup being prepared.Both points of view can't be right. Is it possible that the bitterness of the pith is an urban myth? I'm told, for example, that the French eat rhubarb leaves, which American authorities say are poisonous. Has anyone ever tested that one, either? I wish all urban myths were as easy as those to prove or disprove. I'll answer your question in three parts: the pith, the rhubarb and the Vita-Mix.

The PithAn incorrigible experimenter, I personally carried out the appropriate scientific research to test the hypothesis of bitterness in the pith of citrus fruits. My experimental protocol was relatively straightforward: I tasted the stuff. With a very sharp knife, I removed wide strips of colored zest (the fruit's exocarp) from four varieties of orange, two varieties of lemon and two varieties of grapefruit. (Limes, I found, have extremely thin layers of zest and pith -- too thin to separate.) I then pared a slice of pure white pith (the endocarp) from each fruit and submitted it to a panel of discriminating tasters (my wife and me) for sensory evaluation.

Results: The grapefruit piths were quite bitter, even leaving a lasting sting on the tongue; the lemon piths were only mildly bitter; and the orange piths were almost tasteless. All had a rather unpleasant, spongy chewiness.

Except for grapefruit, then, the "bitter pith" legend appears to have little basis in fact, especially for oranges. Is it possible that all citrus fruits have been tarred with the brush of bitterness because of an overexuberant generalization from grapefruit? Many food myths have flourished from roots less firmly anchored in fact.

My work was far from a definitive study, of course. As scientists often say when they want more funding, "further research is necessary."

But even if all citrus pith were indeed bitter-tasting, so what? Bits of pith might be visually unwelcome as white fragments in a dish that calls for the zest alone, but they certainly wouldn't ruin the dish's flavor. In fact, a touch of bitterness is a highly desirable component of what many regard as the three basic food groups: coffee, beer and chocolate.

The RhubarbI'd never heard that the French eat rhubarb leaves. But it is well known that they eat many things that would gross out most Americans, such as snails, frogs' legs, tripe, brains and -- also common in many other cultures -- horse.

A case in point: Years ago when I was traveling alone in France, a young American latched onto me because he was lonely and wanted to speak English to someone. He was blissfully unaware of the fact that all he had to say -- in any language -- was an endless stream of annoying inanities and non sequiturs. I tried to shake him with various excuses, but I had no success until we entered a bistro for lunch. Scanning the menu, I had an idea. I ordered the tête de veau -- a boiled calf's head. It arrived on a large platter complete with brains, tongue, thymus gland and other anatomical apparatus, some of it unidentifiable. My "friend" excused himself and wandered off, presumably to resume his search for a hamburger, and I never saw him again. (The tête de veau, incidentally, wasn't bad at all. For graphic, slash-by-slash instructions on preparing one yourself, see Viewer discretion is advised.)

But back to the rhubarb. All parts of the rhubarb plant contain oxalates, which are poisonous in sufficient quantity. Rhubarb also contains anthraquinone glycosides, which probably contribute to its toxicity. The highest levels of oxalic acid -- about 0.6 or 0.7 percent -- are found in the leaves, while the stalks contain much less. The lethal dose of oxalic acid for a 145-pound person is about 25 grams, which would require the consumption of about 4 kilograms, or 9 pounds, of rhubarb leaves. There are easier ways to commit suicide.

Have the lethal consequences of eating rhubarb leaves ever been tested, you ask? Undoubtedly. I think you can imagine the inadvertent experiments that must have been performed in early human history involving plants we now know are poisonous. Once such an experiment yields its unmistakable result, word tends to spread.

In the case of rhubarb, it is said that in Britain during World War I, fresh vegetables were so scarce that authorities recommended eating rhubarb's large leaves as a supplement. The recommendation was quickly withdrawn after illnesses and, reportedly, several deaths occurred. So don't eat 'em.

Even if you're French.

The Vita-MixEver since 1922, when Steven J. Poplawski invented a tall container with a spinning, vortex-producing blade in its narrowed bottom, there have been many brands of blenders, including Hamilton Beach, Waring and Oster, among others.

In 1937, William G. Barnard introduced the Vita-Mix. Today, the Vita-Mix Corp. is a gung-ho promoter of "health and wellness" (there's a difference?). The company urges us to throw whole fruits and vegetables -- often with rinds, seeds and all -- into (of course) their machines, to make smoothies that will help you "improve your health, get more nutrition, and feel better."

As you can tell, Vita-Mix is an advocate of "no nutrient left behind." It should be no surprise, then, that some of its recipes include the zest, the pith, the seeds or the whole darn plant.

But hold the rhubarb leaves.

Robert L. Wolke ( is professor emeritus of chemistry at the University of Pittsburgh. His latest book is "What Einstein Told His Cook 2, The Sequel: Further Adventures in Kitchen Science" (W.W. Norton, 2005). He can be reached at