Environmental degradation of the natural colorant curcumin processes through chemical autoxidation and microbial degradation of the respective products

While the environmental fate of non-degradable and fossil-based plastics is of high societal interest, the fate of plastic additives, such as colourants, is often overlooked. Some plastic additives are toxic and often poorly biodegradable, and could, thus, be considered as chemicals of emerging concern. The CurCol (curcumin-based sustainable colourants) project aimed to develop biodegradable colourants from natural and chemically modified curcumin. It was investigated whether these bio-based colourants are biodegradable and what happens to them when they are used as additives in bio-based plastics and exposed to the environment.

Various approaches were used to enrich and isolate bacteria from different environmental samples and the rhizosphere of curcuma plants using natural curcumin and structurally similar aromatic metabolites (ferulic acid, cinnamic acid, eugenol or vanillin). Neither the enrichment cultures nor the bacteria isolated with aromatic compounds were able to utilise curcumin as the sole carbon source. There was also no indication of co-metabolic conversion of curcumin. Studies on the chemical stability curcumin and various of its yellow and red colour derivatives indicated pH-dependent autoxidation of curcumin to a bicyclopentadione (BCP). At elevated temperatures curcumin was chemically degraded to vanillin. Also, purified BCP was decomposed to vanillin and feruloylmethane at temperatures from 60 °C on. Curcumin, BCP and vanillin were also released from coloured polymers in a temperature-dependent way. No toxic effects of the colourant and its degradation products were detected, and vanillin was readily degraded by most of the isolated bacteria as well as by model organisms such as Pseudomonas putida KT2440.

These experiments showed that curcumin and its derivatives can be chemically degraded under environmental conditions occurring during composting and that the respective degradation products can serve as carbon and energy sources for environmental bacteria. Thus, curcumin-based colourants could constitute a bio-based and environmentally friendly additive for polymers.