Positive effect of reduced aeration rate on growth and stereospecificity of dl-malic acid consumption by Azospirillum brasilense: Improving the shelf life of a liquid inoculant formulation

•Lower aeration rates elicit d-malic acid consumption in two strains of A. brasilense.•Increased shelf life of a based liquid inoculant in pneumatic bioreactors.•The positive effect of decreased aeration was observed in biomass growth.•A new strategy of process to produce a based liquid inoculant formulation.

Azospirillum brasilense has significance as a growth promoter in plants of commercial interest. Two industrial native strains (Start and Calf), used as a part of an inoculant formulation in Mexico during the last 15 years, were incubated in laboratory-scale pneumatic bioreactors at different aeration rates. In both strains, the positive effect of decreased aeration was observed. At the lowest (0.1 vvm, air volume/liquid volume × minute), the highest biomass were obtained for Calf (7.8 × 1010 CFU/ml), and Start (2.9 × 109 CFU/ml). These were higher in one magnitude order compared to cultures carried out at 0.5 vvm, and two compared to those at 1.0 vvm. At lower aeration, both stereoisomeric forms of malic acid were consumed, but at higher aeration, just l-malate was consumed. A reduction in aeration allows an increase of the shelf life and the microorganism saved higher concentrations of polyhydroxybutyrate. The selected fermentation conditions are closely related to those prevalent in large-scale bioreactors and offer the possibility of achieving high biomass titles with high shelf life at a reduced costs, due to the complete use of a carbon source at low aeration of a low cost raw material as dl-malic acid mixture in comparison with the l-malic acid stereoisomer.