A dye–fibre system from food waste: Dyeing casein fibres with anthocyanins

Regenerated protein fibres manufactured from food side-streams offer significant potential as circular and sustainable fibres, but greater knowledge of their dyeing properties is required. In this research, coloration of casein fibres with dyes also extracted from blackcurrant skins left over from juice pressing is explored. Casein fibre was dyed with blackcurrant extract, rich in anthocyanins, from pH 2 to pH 6 and from 40 to 80°C, with and without alum. Casein fibres could be dyed with blackcurrant extract across all conditions tested, and under optimal conditions, dyeing is achieved with medium depths of colour with good wash fastness. Highest sorption of anthocyanins onto casein is observed at pH 4, where anthocyanins are a mixture of 60% neutral purple quinonoidal base form and 40% flavylium cation form; under these conditions dye–fibre interaction is optimal. At pH 2, casein fibre has a highly positively charged surface and anthocyanin is in the flavylium cation form, leading to some dye–fibre repulsion. At pH 6, the slightly negatively charged casein fibre demonstrates lower sorption of the mixture of 40% purple quinonoidal base form and 60% the anionic quinonoidal base form, again leading to some dye–fibre repulsion. Presence of alum in the dyebath enhances sorption of anthocyanins onto fibre at pH 4 due to formation of Al–anthocyanin complexes. Wash fastness of the dyeings is better as pH increases and as temperature increases.