$k_La$ (h⁻¹) quantifies the rate of oxygen transfer from gas bubble to liquid per unit driving force. It increases with: (1) higher agitation speed (breaks bubbles, increases interfacial area $a$), (2) higher aeration rate (more gas hold-up), and (3) lower broth viscosity (faster liquid-film renewal). Adding antifoam reduces bubble coalescence resistance but can also reduce $k_La$ by coating bubble surfaces. Temperature increase raises $k_La$ but lowers $O_2$ solubility ($C^*$).