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

• Pretreatment of starch in bakery waste was necessary to form intermediates. • Lactate was superior to ethanol as an intermediate for microbial chain elongation. • Maintaining mash particles ≤ 362 µm was important for long-term product extraction. • Continuous bioreactors operated stably for 386 days at 37°C and a pH of 5.5. • Bakery-waste carbon conversion reached 35% for n-caproate and 15% for n-caprylate. 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 − 1 h −1 ) was achieved. This integrated approach demonstrates an efficient pathway for valorizing bakery waste.