There is no doubt that yeast must be put in the hair cake, because yeast reproduction produces carbon dioxide, which makes the hair cake fluffy. Many people question why yeast is not put in the cake. The cake is filled with gas by external force, and it does not need to be expanded by yeast fermentation at all. Moreover, yeast is acidic, eggs are alkaline, and yeast put in the cake will be inactivated. So the cake doesn't need yeast at all.