At the Feeding the 5000 food festival in New York City, the first thing people walking by may notice is that all the food is free. It isn’t obvious, though, that all the offerings are made from food waste. A torte is made with trimmings and peelings from a food distributor; a “quick pickle salad” is made with surplus greens and vegetables. Dan Barber, head chef and co-owner of Manhattan’s Michelin-starred Blue Hill, will offer…