When President Trump recently griped about Europe's distaste for buying American chicken, his comments touched on a long-running and divisive trade spat that's flared up from time to time. Europeans disparage U.S. poultry as "chlorinated chicken," or "Chlorhünchen" in the German press, and see it as possibly unsafe. The phrase refers to the use of chlorine in poultry processing plants after the birds have been slaughtered in order to cut down on harmful bacteria that are frequent sources of food-borne illness like Salmonella and Campylobacter. Rinsing poultry in chlorine was common practice in the U.S. when the European Union first...