FIGURE 7.

Simplified illustration of the cellular mechanisms involved in lactic acid production and self-inhibition in Lachancea fermentati in anaerobic wort fermentations under the assumption of fundamental homology to Saccharomyces cerevisiae. Adapted from Sauer et al. (2010) and Bellut et al. (2019a). (1) Glucose transport into the cell by facilitated diffusion. The net ATP yield per glucose molecule is 2. (2) Glycolysis, yielding one molecule of ATP per molecule pyruvate formed and one molecule of NADH which has to be recycled to NAD⁺. (3) Ethanol production via pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH). The ethanol can leave the cell by passive diffusion. (4) Lactic acid production via lactic acid dehydrogenase (LDH). At high intracellular pH, the lactic acid dissociates into lactate and H⁺. (5) Both, ethanol formation and lactic acid formation are a means to recycle NADH to NAD⁺. (6) At the very least, the H⁺ has to be exported out of the cell at the expense of one molecule of ATP. In the worst case scenario, ATP-dependent mechanisms may be involved in both proton and anion export (Abbott et al., 2009; Mans et al., 2017). (7) At low extracellular pH, lactic acid is present in its protonated form and can enter the cell again via passive diffusion, creating an energy-requiring cycle with 6. (8) Maltose transport into the cell is facilitated via proton symport. Consequently, the proton has to be exported out of the cell at the expense of ATP. For that reason, the net ATP yield per glucose molecule from maltose is 1.5 instead of 2.