Salted white noodles arose in northern China and are now most widely known in their Japanese version, udon (below). Yellow noodles, which are made with alkaline salts, appear to have originated in southeast China sometime before 1600, and then spread with Chinese migrants to Indonesia, Malaysia, and Thailand. The yellowness of the traditional noodles (modern ones are sometimes colored with egg yolks) is caused by phenolic compounds in the flour called flavones, which are normally colorless but become yellow in alkaline conditions. The flavones are especially concentrated in the bran and germ, so less refined flours develop a deeper color. Because they’re based on harder wheats, southern yellow noodles have a firmer texture than white salted noodles, and alkalinity (pH 9–11, the equivalent of old egg whites) increases this firmness. The alkaline salts (sodium and potassium carbonates at 0.5–1% of noodle weight) also cause the noodles to take longer to cook and absorb more water, and they contribute a characteristic aroma and taste.