Cultivation of methanotrophic bacteria in a novel bubble-free membrane bioreactor for microbial protein production

•Methane and oxygen diffused via hydrophobic membranes.•Explosive atmospheres were avoided in the fermenters.•Ammonia was fully assimilated by the methanotrophic enrichment.•Enrichments are more robust against methane supply variability. Microbial protein is proposed as an alternative protein source with low environmental impact. Methane oxidizing bacteria are already produced at commercial scale from natural gas. However, their productivity is limited because of the creation of explosive atmospheres in the fermenters during production. This work demonstrates the applicability of bioreactors with a membrane-based gas supply via diffusion. Methanotrophic bacteria were successfully cultivated, with growth yields from 0.26 to 0.43 g-VSS g-CH4−1, slightly below those observed in analogous fermenters relying on bubbling. However, ammonia yields ranged from 5.2 to 6.9 g-VSS g-NH3−1, indicating higher nitrogen assimilation than in conventional fermenters. Indeed, protein content increased during the operational period reaching up to 51% of dry weight. The amino acid profile included the majority of the essential amino acids, demonstrating suitability as feed ingredient. Never during the operational period was an explosive atmosphere established in the reactor. Thus, bubble-free membrane bioreactors are a promising technology for microbial protein production relying on explosive gas mixtures.