Stabilization of luteolins from weld (Reseda luteola L.) and sawwort (Serratula tinctoria L.) by microencapsulation for natural dyeing

Weld and sawwort, are herbaceous plants that have been used, since ancient times, for natural dyeing, concerning to the presence of luteolins and derivatives. However, natural dyes have been limited due to lower stability and higher production prices when compared with synthetic dyes. The weld plant was acquired in France and sawwort was produced in June 2020 in Portugal, with a low success rate. The leaves having been harvested at flowering stage (July 2021) for extraction of the coloring matter. The aim of this work was to study the effect of luteolins microencapsulation by freeze-drying and spray-drying on color stability, using maltodextrin and Arabic gum as wall material. Weld extracts and microencapsulates showed higher total phenolic compounds, and encapsulation efficiency (EE) for freeze-drying process, than extracts and microencapsulates of sawwort, but lower (EE) for spray-drying process. The analysis of the results shows that the solubility of the dye increases with microencapsulation and the stability of encapsulated pigments and the color of the plant extracts, in buffer solutions, was higher at pH 3 for weld, and at pH 4 for sawwort. LC-HRMS/MS analysis have shown that the main yellow chromophore are luteolin-O-glucoside. Weld extract contains luteolin-3’,7-O-glucoside isomers, luteolin-7-O-glucoside and luteolin-4´-O-glucoside, apigenin-O-glucoside, chrysoeriol-O-glucoside, luteolin, apigenin and chrysoeriol whereas glucoside and glucuronide isomers of luteolin and quercetin-3-O-methyl ether are the color compounds in the sawwort extract. Microencapsulation by spray-drying and by freeze-drying produces microcapsules with different morphologies and sizes, allowing to maintain their original color, contrary to what happens with the extracts. Microencapsulation is a promising process to improve the stability of natural dyes because it allows them to be obtained in powder form, enabling better preservation and easy transport. Weld and sawwtort plants have the potential to be applied as natural dyes, in different technological processes: handmade, semi-industrial and industrial, by companies with good environmental practices.