A sustainable pH shift control strategy for efficient production of β-poly(L-malic acid) with CaCO₃ addition by...

β-poly(L-malic acid) (PMLA) has attracted industrial interest for its potential applications in medicine and other industries. For a sustainable PMLA production, it requires replacing/reducing the CaCO₃ usage, since the residual CaCO₃ impeded the cells' utilization, and a large amount of commercially useless gypsum was accumulated. In this study, it was found that more glucose was converted into CO₂ using soluble alkalis compared with CaCO₃ usage. Moreover, since the high ion strength and respiration effect of soluble alkalis also inhibited PMLA production, they could not effectively replace CaCO₃. Furthermore, comparing the fermentations with different neutralizers (soluble alkali vs. CaCO₃), it was found that the differential genes are mainly involved in the pathway of starch and sucrose metabolism, pentose and glucuronate interconversions, histidine metabolism, ascorbate and aldarate metabolism, and phagosome. In detail, in the case with CaCO₃, 562 genes were downregulated and 262 genes were upregulated, and especially, those genes involved in energy production and conversion were downregulated by 26.7%. Therefore, the irreplaceability of CaCO₃ was caused by its effect on the PMLA metabolic pathway rather than its usage as neutralizer. Finally, a combined pH shift control strategy with CaCO₃ addition was developed. After the fermentation, 64.8 g/L PMLA and 38.9 g/L biomass were obtained with undetectable CaCO₃ and less CO₂ emission. KEY POINTS: • The effect of CaCO₃ on PMLA metabolic pathway resulted in its irreplaceability. • A pH shift control strategy with CaCO₃ addition was developed. • Undetectable CaCO₃ and less CO₂ emission were detected with the new strategy. Graphical abstract.