Yeast is the magic fermenter, the kitchen activist that awakens the gluten in flour and causes it to swell and stretch in a network of gaseous pockets. Of the several sorts of yeast on the market, I prefer active dry yeast, a camel-colored, granulated powder that has a wonderful and distinctive smell. You can buy active dry yeast in bulk in health food stores, but I have taken to buying it in foil packets in the supermarket, where it should be kept in the refrigerator case and marked with a clear expiration date. Each of these packets contains 2 teaspoons yeast, not 1 tablespoon as many people suppose. When you bring yeast home, store it in the refrigerator and use it up within the date of expiration, while it is still strong and healthy.