Valorization of ricotta cheese exhausted whey (RCEW), a dairy by-product generated in large quantities worldwide, is essential to mitigate its environmental impact and unlock its economic potential. This study explores the use of RCEW as a substrate for polyhydroxyalkanoate (PHA) production by Azohydromonas lata DSM 1123. The substrate was characterized by low protein and fat contents and a relevant lactose concentration (3.81%, w/v). Due to A. lata’s inability to directly metabolize lactose, β-galactosidase supplementation was necessary. Mineral supplementation of pasteurized RCEW significantly improved both microbial biomass and PHA synthesis, achieving up to 25.94% intracellular PHA content, whereas pre-adaptation trials failed to enhance strain performance. Moderate nitrogen limitation in the substrate (C/N ratio 44) favored PHA synthesis (0.55 g/L) and 32.74% intracellular accumulation. Thermal treatments decreased initial microbial contamination, hence a balanced mixture of pasteurized–sterilized (75:25) substrate was used to modulate RCEW protein content without the inclusion of additional technological or chemical processing steps and without lactose loss or dilution. Bioreactor trials using optimized RCEW pre-treatment conditions led to a further increase in biomass (2.36 g/L) and PHA production (0.88 g/L), especially under fed-batch conditions. The extracted polymer was confirmed to be polyhydroxybutyrate (PHB), with high thermal stability and a molecular weight of 5.9 KDa.

The development of fully biodegradable poly(3-hydroxybutyrate) (PHB)-based materials with improved mechanical performance remains a major challenge due to the limited ductility and processability of this highly crystalline polymer. Blending and plasticisation are viable strategies to enhance PHB toughness; however, the interactions governing polymer–plasticiser compatibility and their impact on structure–property relationships remain not fully understood. In this work, the compatibility and plasticisation mechanisms of two citrate-based plasticisers, tributyl citrate (TBC) and acetyl tributyl citrate (ATBC), were systematically investigated in biodegradable blends based on PHB, polylactic acid (PLA), and poly(butylene adipate-co-terephthalate) (PBAT). Polymer–plasticiser affinity was evaluated through Hansen Solubility Parameters and interaction radius, which indicated good compatibility of PHB with both plasticisers and a stronger affinity for ATBC. Differential scanning calorimetry showed that citrate plasticisers reduced the glass transition temperature, modified crystallisation kinetics, and altered the crystalline morphology of the blends. Dynamic mechanical analysis confirmed the reduction in the glass transition temperature of PHB–PLA systems, which is in agreement with the DSC results. Migration experiments showed equilibrium after approximately 72 h, with PHB–PLA blends exhibiting better plasticiser retention than PHB–PBAT systems. TBC consistently showed higher migration than ATBC, in line with its lower molecular weight and higher volatility. Mechanical testing demonstrated that plasticisation efficiency strongly depended on blend composition: TBC was more effective in enhancing ductility in PHB–PLA blends, whereas ATBC performed better in PHB–PBAT systems. It was also highlighted that the plasticisers had a remarkable ability to substantially increase the ductility of the blends compared with their unplasticised counterparts, as reflected by the pronounced decrease in stiffness and the marked increase in elongation at break. SEM analysis of tensile fracture surfaces evidenced a brittle failure mode for PHB–PLA blends, whereas PHB–PBAT systems exhibited a ductile fracture mode with fibrillar features and clear signs of phase separation. Finally, thermogravimetric analysis showed no appreciable thermal degradation within the processing temperature window used for mixing and hot pressing, confirming the thermal stability of the materials under the selected conditions. These findings establish clear correlations between thermodynamic compatibility, migration behaviour, thermal properties, fracture mechanisms, and mechanical performance, providing useful guidelines for the design of citrate-plasticised PHB-based biodegradable materials.