Six hop acids are common to hops and beer: alpha acid, beta acids, isoalpha acids, rho-isoalpha acids, tetrahydro-isoalpha acids, and hexahydro-isoalpha acids. The six hop acids were tested to determine which were the most effective in inhibiting the growth of bacteria common to fuel ethanol production. The bacteria used in the tests were Lactobacillus brevis and Lactobacillus fermentum. The minimum inhibitory concentrations (MIC) of the hop acids were determined using MRS-broth. Molasses mash and wheat mashes were used as the growth media for the fermentations. In all cases the hop acids controlled the growth of these two lactobacillus bacteria with tetrahydroisoalpha acid, hexahydroisoalpha acid, and isoalpha acid killing the most bacteria at the lowest MIC. Treating yeast propagators, steep tanks, and fermenters with a minimum inhibitory concentration of hop acids will stop bacteria growth, increase ethanol yields and avoid the need for antibiotics.