Material-to-compost ratio is key for biodegradation of microbial polyester-based composite in home-composting conditions

In the context of increasing plastic pollution, the potential widespread use of biodegradable packaging requires clarification of the little-studied impact of material loading ratios on biodegradation performance in home-composting environments. The biodegradability of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and PHBV/cellulose composite, either shredded or as whole thin films, was studied for up to 12 months under home-composting conditions using EN NF ISO 14855-1 respirometry protocols. The material-to-compost ratios tested were 1/3, 1/30, and 1/50, simulating 100%, 10% and 6% food packaging substitution by the tested biodegradable materials, respectively. Regardless of material nature and form, a high loading ratio of 1/3 (100% substitution) decreased the biodegradation rate by up to tenfold compared to a loading ratio of 1/50. Such high loading ratios create inhibitory conditions that can overshadow the increased surface area gained through grinding. Meanwhile, at low loading ratios (1/50 and 1/30), film grinding increases biodegradation rates by up to 3.2-fold for all materials. The ultimate biodegradation of shredded materials significantly exceeds 100%, reaching up to 131% for PHBV powdered film at a loading ratio of 1/30. This suggests an 'overactivation' (or priming) effect of the inoculum induced by the materials, leading to CO 2 overproduction that does not derive directly from material biodegradation. This priming effect appears limited by dilution effect at the low 1/50 loading ratio and by substrate inhibition at the high 1/3 loading ratio.