Keep this in mind the next time you're trying to decide what to buy at the grocery store to stay healthy: Too much of what's sitting on shelves today isn't really food. It's nothing more than "edible substances." That's what best-selling author Michael Pollan told a sellout crowd during Wednesday night's California Lecture at Westminster Presbyterian Church, 1300 N St. The Knight Professor of Journalism at University of California, Berkeley, has won national acclaim for such books as "The Omnivore's Dilemma" and "In Defense of Food." "What is food?" was just one of the questions Pollan addressed in a question-and-answer format led by California's food and ag czar, A.G. Kawamura. That suc