K. Raes

Fruit and vegetables contain molecules that have particular colors, which can potentially be an environmentally attractive substitute for their synthetic counterparts in (non )food applications. The most sustainable source for such natural colorants would be by the valorization of by-products from the fruit and vegetable industries, but qualitative and quantitative characteristics of food by-products for this purpose remain scarce. Natural dyes also show mediocre stability and affinity toward textile fibers, which questions their potential feasibility for application and level of sustainability to overcome these issues. This review describes three dye classes (i.e., anthocyanins, quinones, and carotenoids) along with their occurrence, mass, and concentration in by-products that are generated from agricultural losses as well as the fruit and vegetable processing industries. To tackle the shortcomings of natural dyes on fibers, several application techniques were collected from the literature. A discussion on techno-economic potential and environmental sustainability is included. The latter is done by including a life cycle assessment (LCA) to investigate the environmental impact of extracting anthocyanins, quinones, and carotenoids from fruit and vegetable processing by-products and their subsequent application to the dyeing process. The mapping of by-products for each natural dye class illustrates the vast availability of agro-food residues (>0.1 Mt annually in the EU-28) with a natural dye content of up to 56 kg/t DW for anthocyanins, 18 kg/t DW for quinones, and 593 kg/t DW for carotenoids. Metallic mordants are mostly favored for improving the fixation of natural dyes but entail potential environmental issues. Greener approaches, such as biomordants and enzymes, still show room for improvement, chemical modification methods might also guarantee dye fixation, though questionable in environmental sustainability. The different valorization scenarios of anthocyanins, quinones, and carotenoids from food waste, analyzed with LCA, showed the environmental competitiveness of these natural dyes, applied as a crude extract, compared to synthetic dyes. The valorization routes design shows that agricultural losses and food processing waste streams are adequate sources of natural dyes, especially to be applied in niche scale applications.

Natural dyes might be more environmentally sustainable compared to their synthetic counterparts, however in general their performance is worse. Therefore, typically metallic mordants are applied to improve the natural dye's affinity towards substrates, but this is not a suitable technique in a ‘green story’. In this paper, we test the potential of using anthocyanins from blueberry waste for dyeing cotton with biomordants, which are selected to tailor the intermolecular interactions such as hydrogen bonds, ionic bonds and π-π interactions with the dye molecule. In the experimental part, parameters during extraction and dyeing were optimized (e.g. temperature, pH, dyeing time and concentration). The effect of the (bio)mordants was monitored by Fourier transform infrared spectroscopy, spectrophotometric measurements and standard ISO wash and light tests. It was shown that stannous chloride stands out as metallic mordant, while no biomordants show sufficient intermolecular interactions to replace this metal salt. The experimental study has been corroborated with a series of molecular modeling calculations to obtain more insight into the intermolecular interactions between dye and (bio)mordants. To this end, both static Density Functional Theory based calculations as semi-empirical and force field based molecular dynamics calculations have been performed. The results indeed confirm that, in general, too small interaction energies for the biomordants of interest with the dye molecules are found, in correspondence with experimental findings. Overall, by performing systematic experiments in combination with the interpretation of the molecular models, this study yields valuable insights into the development of green routes towards use of anthocyanins as a natural dye for cellulose-based materials.