Isatis tinctoria L. or woad (Brassicaceae) is an upright herbaceous biennal species up to 120 cm inheight. It is in Mediterranean counlries, a common plant cultivated through out centuries to producethe blue dye indigo. With increasing concem for sustainability and a demand from consumers fornaturally sourced products, there is a revival of interest in naturai indigo as an agricultural cropproduci. Indigo is formed after the extraction of indigo precursors in the leaves of these plants:mainly isatans in woad These compounds are extracted by steeping leaves in warm water. Withwoad, the addition of alkali to the steep water releases free indoxyl, which forms indigo after avigorous aeration. Indigo is hydrophobic and insoluble in water, so that it sedimcnts readily, and thesolid indigo can be readily washed and dried. Indigo is synthesized from two precursor molecules ofindoxyl, deriving from plant secondary metabolism. Recently clarified the nature of indigoprecursors in woad (Isatis tinctoria L.), by identifying the major indoxyl glycoside as isatan A(indoxyl-3-O-(60-O-malonyl-b-D-ribohexo-3-ulopyranoside)), and by correcting the structure of therelated isatan B (indoxyl-3-0-b-D-ribohexo-3-ulopyranoside). The seasonal variation of indoxylglyeosides in woad leaves was investigated and the influence of various post-harvest treatments wasstudied and Isatans A and B disappeared completely when the leaf material was submitted to aconventional drying process. In contrast, the indican coment in leaves increased significantly, andthe extent of this increase was temperature-dependent. These precursor can be identified andquantified by HPLC method since their quantity in dependent on the species and the harvest period.The modem extraction method of indigo from woad uses the water solubility of the indigoprecursors in steeping the leaves in hot water. The precursors are broken down to indoxyl and sugarmoieties by endogenous enzymes in plant (glycosidases), but in the extraction method this is doneby alkali with aeration. The purity of plant-derived indigo even with the modem extraction methodis somewhat low when compared to the synthelic indigo. Naturai indigo contains besides indigo,impurities such as indirubin, indigo-brown, indigo gluten and minerai matter. Several authorsindicate that indigo can be extracted from woad with a purity of 90% if three conditions are met: theleaves contain a sufficiently high yield of indigo precursors; the leaves are rinsed free of soil; andthe indigo is sedimented in an acid medium.
|Number of pages||1|
|Publication status||Published - 2012|