CHICAGO - Buying organic milk these days - or organic apples, eggs, or beef - no longer has to mean an extra trip to a Whole Foods supermarket or the local co-op. Organic products now line the shelves at Safeway and Costco. And Wal-Mart - already the nation's largest organic-milk seller - says it wants to sell more organic food. Large companies including Kraft, General Mills, and Kellogg own sizable organic- and natural-food brands. Now, they are developing organic versions of their own products, too. Still, while some organic-food fans welcome its broadening appeal and availability, others worry that the...