Ethanol distillation in the biofuel industry is energetically expensive because ethanol is completely miscible in water. Upgrading ethanol into a hydrophobic chemical that is easier to separate would circumvent current fossil-fuel consumption for distillation. Here, we shaped a reactor microbiome to sequentially elongate carboxylic acids with 2-carbon units from dilute ethanol in yeast-fermentation beer. Our continuous bioprocess produced n-caproic acid, a 6-carbon-chain carboxylic acid that is more valuable than ethanol. No antimicrobials to inhibit methanogens were necessary. In-line product extraction achieved an n-caproic acid production rate exceeding 2 grams per liter of reactor volume per day, which is comparable to established bioenergy systems with microbiomes. Incorporation of other organics found in beer increased the mass of carbon in n-caproic acid by 10% compared to ethanol.