Indoxyl derivatives in woad in relation to medieval indigo production

The two precursors of indigo in the woad plant, Isatis tinctoria, were quantified by a new spectrophotometric method involving the formation of a red adduct from indoxyl and rhodanine. In young leaves, approximately 24% of the dry weight was found to be the indoxyl derivatives, indoxyl-3-(5-ketogluconate) (isatan B) and indoxyl 3-O-beta-D-glucoside (indican), in the ratio of approximately 3 : 1. The older leaves contained a lower concentration of the indoxyl derivatives (up to 14% dry weight). Isatan B in the leaves was found to be relatively unstable towards heat and drying. The traditional method of indigo production used since at least medieval times was reproduced. Analysis revealed that conversion to indigo took place in the initial step, the formation of woad balls. The traditional method reduced side reactions leading to indirubin formation. The conversion efficiency of the indoxyl precursors to indigo was approximately 14% of the theoretical maximum yield. (C) 1998 Elsevier Science Ltd. All rights reserved.