Beef-free burgers, egg-free omelettes and milk-free ice cream — meat and dairy alternatives have shot to the top of the menu, and producers are facing up to the challenges of going mainstream. “Progress has been stunning,” said Bruce Friedrich, director of the Good Food Institute, which hosted a conference this week in San Francisco for an industry abuzz with ambition and technological advances. A new generation of vegetarian products has re-invigorated the sector, with fast-food giant Burger King now offering the meat-free “Impossible Whopper” and McDonald’s working with Nestle on its own plant-based burgers.