A method of controlling microorganisms such as lactobacilli metabolism in mash in an ethanol production facility includes adding to the mash an effective amount to control such microorganisms of one or more of a substantially water insoluble pristinamycin-type antimicrobial agent, a substantially water insoluble polyether ionophore antimicrobial agent, or both, wherein the term "substantially water insoluble" means the antimicrobial agent has a solubility in pure water at 20.degree. C. of 0.1 grams per liter or less, and wherein at least a portion of the substantially water insoluble antimicrobial agent(s) is added to the mash in the form of particles comprising said substantially water insoluble antimicrobial agent(s) and having a weight mean average diameter of less than 5 microns. Particles having a weight mean diameter between 0.1 and 1 microns are preferred.