NATURAL INDICO FROM ISATIS TINCTORIA L. FOR THE VALORISATION OF SICILIAN CROPS

Abstract

Isatis tinctoria L. or woad (Brassicaceae) is an upright herbaceous biennal species up to 120 cm in height. It is in Mediterranean counlries, a common plant cultivated through out centuries to produce the blue dye indigo. With increasing concem for sustainability and a demand from consumers for naturally sourced products, there is a revival of interest in naturai indigo as an agricultural crop produci. 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. With woad, the addition of alkali to the steep water releases free indoxyl, which forms indigo after a vigorous aeration. Indigo is hydrophobic and insoluble in water, so that it sedimcnts readily, and the solid indigo can be readily washed and dried. Indigo is synthesized from two precursor molecules of indoxyl, deriving from plant secondary metabolism. Recently clarified the nature of indigo precursors 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 the related isatan B (indoxyl-3-0-b-D-ribohexo-3-ulopyranoside). The seasonal variation of indoxyl glyeosides in woad leaves was investigated and the influence of various post-harvest treatments was studied and Isatans A and B disappeared completely when the leaf material was submitted to a conventional drying process. In contrast, the indican coment in leaves increased significantly, and the extent of this increase was temperature-dependent. These precursor can be identified and quantified 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 indigo precursors in steeping the leaves in hot water. The precursors are broken down to indoxyl and sugar moieties by endogenous enzymes in plant (glycosidases), but in the extraction method this is done by alkali with aeration. The purity of plant-derived indigo even with the modem extraction method is 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 authors indicate that indigo can be extracted from woad with a purity of 90% if three conditions are met: the leaves contain a sufficiently high yield of indigo precursors; the leaves are rinsed free of soil; and the indigo is sedimented in an acid medium.