What is the secret ingredient why siopao white?

There is no secret ingredient that makes siopao white. The whiteness of siopao comes from the flour used to make the dough, which is usually all-purpose flour. All-purpose flour is made from a blend of hard and soft wheat, and it has a high gluten content. Gluten is a protein that gives bread its chewy texture, and it also helps to hold the dough together. When all-purpose flour is mixed with water, the gluten forms a network of bonds that traps the air bubbles created by the yeast. This network of bonds also prevents the dough from collapsing, which is why siopao is able to maintain its shape while it is being steamed.