2.1.2 Rhubarb
Synonyms: Rheum; Radix rhei; Rhubarb rhizome.
Biological Source: Rhubarb is the rhizome and roots of Rheum officinale Bail., R. palmatum L., Rheum emodi Wall ; R. webbianum Royle, belonging to the family Polygonaceae. The rhizome and roots are mostly collected from 6-7 year old plants just prior to the following season. They are commercially available either with intact cortex or partially decorticated.
Geographical Source It is obtained largely from cultivated as well as wild species of Rhubarb grown in regions extending from Tibet to South East China. It is also found in Germany and several European countries. In India it is grown extensively in Kashmir, Kullu, Sikkim, Uttar Pradesh, Panjab. It is also found in Nepal. It is cultivated in Southern Siberia and North America.
Preparation The rhizomes are collected either in spring or in autumn from 6 to 10 year old plants., grown at an altitude of more tha 3, 000 meters. These are duly cleaned, decordicated and dried. The relatively larger rhizomes are cut into small pieces either longitudinally or transversely. The cut fragments are threaded and dried in the shade. They are also dried artifically in an atmosphere of hot wooden boxes and exported for commercial consumption.
Description Rhubarb is usually found to be compact, rigid, cylindrical conical or barrel shaped with 8-10 cm length and 3-4 cm thickness. They appear to be mostly longitudinally wrinkled, ridged or furrowed; whereas a few of them do exhibit transverse annulations or wrinkles. Interestingly, the flat pieces are prepared from large rhizomes that are normally cut longitudinally and, therefore, they appear to be largely as plano-convex with tapering at both ends. These two varieties of pieces possess a sharp characteristic odour and a bitter astringent taste. The surface is often smeared with a bitter yellowish powdery substance, which on being removed gives rise to a rather smooth surface that appears to be pale brown to red in colour.
Chemical Constituents Rhubarb essentially contains mainly the anthraquinone glycosides and the astringent components. The former range between 2 to 4.5% and are broadly classified into four categories as stated below:
(a) Anthraquinones with —COOH moiety—e.g., Rhein; Glucorhein;
(b) Anthraquinones without —COOH moiety—e.g., Emodin; Aloe-Emodin; Chrysophanol; Physcion;
(c) Anthrones and Dianthrones of Emodin—as shown below:
(d) Heterodianthrones—e.g. Palmidin A, B, and C, which are produced from two different anthrone molecules, as stated under:
Palmidin A : Aloe-emodin anthrone + Emodin anthone
Palmidin B : Aloe-emodin anthrone + Chrysophanol anthrone
Palmidin C : Emodin anthrone + Chrysophanol anthrone
However, the astringet portion of rhubarb chiefly comprises with the following components, namely: gallic acid as α- and β-glucogallin; tannin as d-catechin and epicatechin.
Rhubarb in addition to the above constituents, consists of rheinolic acid, pectin, starch, fat and calcium oxalate. The calcium oxalate content ranges between 3-40% in various species of rhubarb which reflects directly on the corresponding ash values (i.e., total inorganic contents).
Chemical Tests
1. The Rhubarb powder on being treated with ammonia gives rise to a pink colouration.
2. Rhubarb gives a blood-red colouration with 5% potassium hydroxide.
3. It gives a positive indication with modified Borntrager’s test (see under Aloes).
Uses
1. It is used mainly in the form of an ointment in the treatment and cure of chronic eczema, psoriasis and trichophytosis—as a potent keratolytic agent.
2. It is employed as a bitter stomachic in the treatment of diarrhoea.
3. It is also used as a purgative.
