Lactococcus lactis metabolizes glucose homofermentatively to lactate. However, after disruption of the gene coding for lactate dehydrogenase, LDH, a key enzyme in NAD+ regeneration, the glycolytic flux shifts from homolactic to mixed-acid fermentation with the redirection of pyruvate towards production of formate, acetate, ethanol and CO2. A mathematical model of the pyruvate metabolism pathway that enhances ethanol production was developed from in vivo Nuclear Magnetic Resonance (NMR) time-series measurements that describe the dynamics of the metabolites in L. lactis. Both Michaelis–Menten and S-system models capture the observed in vivo dynamics of the glycolysis pathway in L. lactis, while prior models describe only the in vitro dynamics. The models provide insight into the maximization of selectivity of ethanol with respect to acetate and CO2 as undesired products in multiple reactions. High concentrations of NADH and acetyl-CoA and low concentrations of pyruvate and NAD appear to maximize ethanol selectivity.