Richard Hebron, 41, was driving along an anonymous stretch of highway near Ann Arbor, Mich., last October when state cops pulled him over, ordered him to put his hands on the hood of his mud-splattered truck and seized its contents: 453 gal. of milk. Yes, milk. Raw, unpasteurized milk. To supply a small but growing market among health-conscious city and suburban dwellers for milk taken straight from the udder, Hebron was dealing the stuff on behalf of a farming cooperative he runs in southwestern Michigan. An undercover agricultural investigator had infiltrated the co-op as part of a sting operation that...