The problem with active dry yeast is that it is temperamental and not super shelf stable. If your liquid is too cold (under 90 degrees), it may not work.

The LeSaffre company started selling instant yeast in the 1970s, which means it is a newer product than active dry yeast. It is less expensive (per ounce), works faster, and is the most reliable.

To a large mixing bowl, add the active dry yeast, sugar and warm buttermilk. Mix, and let stand for 10 minutes. Stir the sourdough starter and egg into the yeast mixture.

Fresh yeast. Yeast comes in two forms: fresh yeast, and dry yeast. Fresh yeast is mainly used by professionals. It's soft, moist, and will go off quickly.