Coupled Kinetic and Rheological Analysis of Lactic Acid Production from Whey by Kluyveromyces marxianus

Whey, a high-volume by-product of the dairy industry, is a valuable substrate for biotechnological valorization that presents an environmental challenge. This study aimed to investigate the kinetic and rheological behavior of Kluyveromyces marxianus during lactic acid fermentation using deproteinized whey by varying initial lactose (35 and 50 g L-1) and inoculum (3 × 106 and 10 × 106 cells mL-1) concentrations and working volumes (0.6 and 0.9 L), through a full factorial (23) experimental design. Growth kinetics, lactose consumption, and lactic acid production were modeled using the Gompertz, Luedeking-Piret, and Pirt models. Rheological behavior was obtained using the Ostwald-de Waele power-law model. A rheokinetic model that integrates biomass growth with fluid properties was proposed. The results showed that high lactose and low inoculum levels significantly improved lactic acid yield (0.285 g g-1) and productivity (0.116 g L-1 h-1). Strong statistical interactions were identified between inoculum and substrate levels. The rheokinetic model demonstrated strong correlations between the consistency coefficient and the rheological behavior index, as well as microbial growth (correlation coefficient (R2) > 0.92), providing a predictive tool that eliminates the need for destructive measurements.