Rhubarb

Description

A vegetable, the stems of which are used for pies, jams, etc. The leaf is toxic, contain_x0002_ing a high level of soluble oxalates.

Chemical properties

R. palmatum, or Chinese rhubarb, is a large, herbaceous, perennial plant with thick rhizomes that branch out after the first 2 to 3 years of growth. Its radical leaves split when the plant is 2 years old. R. palmatum grows in Tibet and western China, especially in high-altitude areas with dry climate. R. officinale differs from R. palmatum in that the leaves are divided in large, indented lobes; the bark is not red-dotted as that of R. palmatum, and the inflorescence is enlarged at the top. The plant flowers after 3 years and forms a flower-bearing stalk up to 2 to 3 m (7 to 10 ft) in height, with a long ear consisting of small, whiteyellowish flowers (May to June). It grows in the high-altitude areas of Tibet and China. R. rhaponticum is a perennial, herbaceous plant growing up to 1 to 2 m (3 to 7 ft) in height. It has a thick rhizome, erect stalk, numerous leaves and white-yellowish flowers in a terminal cluster. It is native to central Asia and extensively cultivated in Europe. The parts used are the skinned rhizomes and roots of R. rhaponticum. The dried product has various appearances, depending on the origin of the plant. Chinese rhubarb is classified by shape—flat, derived from large, cylindrical rhizomes split in the middle, or round. The rhizome is heavy, hard, orange-yellow or reddish gray, exhibiting irregular, granular fractures. The characteristic star-shaped nodules, not present in the roots, are apparent. The Chinese rhubarb rhizome exhibits a radius larger than that of R. rhaponticum. The two varieties may also be differentiated by the harsher odor of R. rhaponticurn as compared to Chinese rhubarb. Under Wood’s fluorescent light, the reddish-brown color of Chinese rhubarb remains unaltered, whereas that of R. rhaponticum changes from violet to blue on addition of ammonia. Rhubarb has a bitter-tonic flavor.

Composition

Chinese rhubarb contains from 2.5 to 4.5% anthracene compounds both in the reduced and oxidized state (free or bound as glucosides). It contains chrysophanic acid, rhabarberone, alizarin, cinnamic and gallic acids and tannins. A main constituent is also calcium oxalate. The content in anthracene compounds of R. rhaponticurn is 1.9 to 2.5%. This variety also contains rhaponticin. Tyrosinase inhibitory activity-guided fractionation and chemical analysis of acetone extract of the rhizomes led to the isolation of two potent compounds, 3,4?,5-trihydroxystilbene-4?-O-β-D-(2?-O-galloyl)glucopyranoside and 3,4?,5-trihydroxystilbene4?-O-β-D-(6?-O-galloyl)glucopyranoside. These compounds showed a competitive inhibition against tyrosinase and also inhibit melanin biosynthesis.* Hydroxyanthraquinones from R. officinale Baill (Dahuang) have also been isolated.