Sake, although called 'rice wine', is not wine at all; it's actually made by a fermentation process similar to beer. Grapes have a high sugar content, so if they're left in a bucket in the sun for awhile, yeast from the atmosphere will convert the sugar into wine. But if you leave rice in a bucket in the sun, the result is nothing more than warm, parched rice. Rice and other grains are largely starch, so a starter (bacteria or fungi) is needed to first break down the starch into sugar to later be converted into alcohol. Before the use of koji mold around the 8th century, the Japanese simply used spit: the amylase enzymes in saliva acted as the starter. (If you chew a savory cracker for a while, you'll notice it becoming sweet. That's the work of analyze breaking down the starch to sugar).