After the Egyptians crushed cut cane and captured the juice, they boiled and strained the liquid, let it settle, then strained it again. The cane juice was now poured into molds with holes in the bottom, so that all the liquid could drain out, leaving only a powder. That powder was then mixed with milk and boiled again. After one round of these steps, the process was repeated all over again. As a result of all this effort and care, Egypt was known for the "whitest and purest" sugar.What is the central idea of the passage?