Dried-bakery waste as a substrate for n-caproate and n-caprylate production via chain elongation

Bakery waste is a promising feedstock for the circular economy; however, its use for producing medium-chain carboxylates (MCCs) via microbial chain elongation remains unexplored. This study investigated pretreatment, pertraction, and chain elongation strategies to convert bakery waste into n-caproate and n-caprylate. Bakery waste was mechanically and enzymatically processed, and different inocula were tested to optimize the conversion of the resulting glucose-rich solution into lactate and ethanol (intermediates). These intermediates were fed into continuous chain-elongation systems that operated for over 386 days at 37°C and pH 5.5. Results showed that 30-35% of bakery waste carbon was converted into n-caproate and 10-15% into n-caprylate. Enzymatic starch hydrolysis proved essential, and lactate was a superior intermediate for chain elongation compared to ethanol. A maximum volumetric MCC production rate of 131 mmol C L^-¹ d^-¹ (0.1 g L^-¹h^-1) was achieved. This integrated approach demonstrates an efficient pathway for valorizing bakery waste.