Enhanced propionic acid production from whey lactose with immobilized Propionibacterium acidipropionici and the role of trehalose synthesis in acid tolerance†
Whey lactose is a major by-product of the cheese and casein manufacturing industry. Utilization of this cheap and renewable resource is an area of interest among biotechnologists and bulk chemists. The objective of the present work was to determine the suitability of whey lactose for propionic acid production by a food microbe, Propionibacterium acidipropionici. Batch and fed-batch fermentations of whey lactose with both free cells and immobilized cells in a fibrous bed bioreactor (FBB) were examined and compared. It was observed that cells immobilized on polyethylenimine-treated Poraver (PEI-Poraver) in FBB favored higher productivity and yield of propionic acid as compared to free-cell fermentation. Interestingly, P. acidipropionici accumulated high levels of trehalose, especially in response to acid stress. An analysis of the P. acidipropionici genome sequence revealed the presence of two putative trehalose biosynthesis pathways (OtsA–OtsB and TreY–TreZ), and their functions in response to acid stress were subsequently studied. To further improve propionic acid production, an enhanced trehalose synthesis mutant was obtained by over-expression of an otsA gene encoding enzyme belonging to the OtsA–OtsB pathway. In this mutant, the maximum concentration of propionic acid reached 135 ± 6.5 g L−1 in a fed-batch fermentation approach, which to our knowledge is among the highest propionic acid concentration ever produced in the traditional fermentation process. In view of these results it was concluded that the FBB fed-batch fermentation with an enhanced trehalose synthesis mutant from whey lactose can therefore be considered as a good candidate for the large-scale production of propionic acid in the near future.