Why does iodine, saliva and water mixed together turn yellow? please help!!!!!!!?

When starch (white/clear) is mixed with iodine (yellow), the two react and appear dark purple. When simple sugars are mixed with iodine, there is no reaction. If you mixed pure starch with water, let it sit for 1/2 hour at body temperature then added iodine, the solution would turn purple. If you mixed pure starch with a little water and some saliva, let it sit for 1/2 hour at body temperature then added iodine, the solution would appear yellow. Explain why, naming all products, reactants, and any other substances that are involved.