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Author
Peregrin Soler, Anna
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Abstract
Under anaerobic conditions Lactococci produce lactate as primary fermentation product, often leading to a by-product inhibition, which limits the achievable biomass yield. Nevertheless, some Lactococci express a NADH oxidase (Nox) which, under microaerobic conditions, ena-bles the mixed-acid fermentation, thus potentially lowering the lactate inhibition and improving biomass yield. A conventional DO probe is not suited to control microaerobic conditions in cultivations. Instead, oxidation-reduction potential (ORP) probes are used. They determine the sum of electrons acceptors (oxidizing agents) and donators (reducing agents) and can precisely sense very small oxygen concentrations.
Selected Nox-positive Lactococci strains were grown at lab scale under microaerobic condi-tions and evaluated based on the cell yield (flow cytometry, CFU and DCW). To this end, a PI controller was first tuned for the newly established ORP sensor actuating on the air flow rate and stirrer speed. Microaerobic cultivations were achieved, where the entire carbon source present in the media was consumed for the first time. The strain LACla γ exhibited higher biomass yields compared to the non-aerated control at EOF and the specific lactate produc-tion was 50 % lower. Higher production of acetic acid, acetoin and diacetyl, mixed acid fer-mentation by-products, were detected showing the potential of less lactate inhibition.
During the growth under a controlled ORP of – 150 mV, the metabolic flux towards a mixed-acids products was accomplished demonstrating that a reduced lactate production leads to a biomass yield improvement of around 30 % without affecting the acidification activity of the final product.
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