Single-step conversion of starch-based bioplastics to bioethanol with recombinant yeast strains

Starch-based bioplastics account for the second-highest global production capacity among all bioplastics. At the end of their life, these materials are sent to organic waste treatment facilities where they are usually processed into biogas and compost. However, these starch-containing materials can be converted into higher-value products before being processed to biogas and compost. This study evaluated enzymatic strategies to hydrolyse commercial starch-containing bioplastic bags and ferment the resulting sugars to ethanol. Among several candidates, α-amylases were more effective than glucoamylases, with the α-amylase from Apergillus terreus (AteA) resulting in the highest sugar release from pure ThermoPlastic Starch (TPS). The saccharification of commercial TPS-containing bioplastic bags required a cutinase-like enzyme (rCLE1) to hydrolyse the poly(butylene-adipate-co-terephthalate) (PBAT) fraction in the blend, allowing amylases to access the starch component. The combination of rCLE1 and a commercial amylase cocktail released 5.97 g/L total reducing sugars from 20 g/L bags, resulting in a 41 % weight loss. Subsequent fermentation by Saccharomyces cerevisiae yielded 1.36 g/L ethanol, representing 88 % of the theoretical yield. This proof-of-concept demonstrates the potential for producing bioethanol from starch-based bioplastics at the end of their life cycle.