Rhubarb

Botanical Profile

Note on drug differentiation: The pharmacopoeial rhubarb context refers primarily to Rheum palmatum, Rheum officinale and defined hybrids; culinary rhubarb must be distinguished from these.

Note on species differentiation: Depending on the botanical and raw-material context, the term rhubarb may refer to different Rheum species.

Rhubarb comprises several species and cultivated forms of the genus Rheum. In the raw-material context, culinary rhubarb with fleshy petioles must be clearly distinguished from traditionally used rhubarb roots of Asian Rheum species.

Rheum species are robust perennial herbaceous plants with large basal leaves, fleshy petioles, strong rhizomes and upright paniculate inflorescences. The genus belongs to the Polygonaceae and is distributed in temperate to montane regions of Eurasia. In food culture, the sour petioles of cultivated rhubarb forms are central. In medicinal-plant and trade history, by contrast, the underground organs of certain Asian Rheum species were primarily described. The leaves themselves are not suitable as a usual food raw material because of their oxalic-acid content. For assessment, it is essential to establish whether petiole, root, rhizome or extract is being considered.

Phytochemistry

Rhubarb petioles contain water, organic acids including malic and oxalic acids, sugars, pectins, minerals, anthocyanins in red-coloured cultivars and accompanying phenolic compounds. Underground organs of certain Rheum species contain anthranoids, including derivatives of rhein, emodin, aloe-emodin and chrysophanol, as well as tannins, stilbenes, flavonoids and other phenolic components. This distinction between petiole and root raw materials is central because composition, activity spectrum, safety and regulatory assessment differ substantially. Origin, species assignment, plant part, plant age, drying and extraction solvent strongly influence the profile.

Traditional Uses

Rhubarb has two distinct traditions. The sour petioles are used in European cuisine for compotes, baked goods, syrups and fruit-like preparations. Dried rhubarb roots of Asian origin, by contrast, were important traded commodities for centuries and appear in Chinese, Central Asian and European herbal traditions. Historical root uses must not be equated with the food use of the petioles. Culinary significance is based on acidity, freshness and suitability for processing, whereas root raw materials are highly characteristic plant materials requiring their own assessment.

Current Research

Current research on Rheum species addresses culinary-rhubarb quality, oxalic acid, anthocyanins, polyphenols, anthranoid chemistry and quality control of root raw materials. Rhein and emodin are frequently investigated in experimental models, while culinary rhubarb is studied more strongly with regard to cultivars, acid profile, processing and sensory quality. Anthranoid-containing root preparations are subject to separate regulatory and safety attention. Results from isolated anthranoid fractions or root preparations cannot be transferred to rhubarb petioles as food.

Sources

• Plants of the World Online: Rheum L.; Royal Botanic Gardens, Kew.
• EMA/HMPC documents on Rhei radix and anthranoid-containing herbal preparations.
• Standard pharmacognosy references on Rheum species.