How can the starch test show that food has been prepared bin the leaf?

Starch/glucose is the food prepared during photosynthesis. if a plant has photosynthesised, it means it has produced its products. you can check this by doing the starch test. boil the leaf which will kill the cells and then dip it in ethanol to extract the green colour. then add the iodine. if the leaf turns blue/black after adding the iodine, this means starch is present and starch is the extra glucose produced (product)