Final answer:
Iodine may not show starch presence in all plant foods due to lack of amylose, presence of goitrogenic substances, or iodine's low solubility in water.
Step-by-step explanation:
Iodine will not show the presence of starch in all plant-based foods because not all plant-based foods contain amylose, which is necessary for the iodine-starch test to work. Starch is a mixture of two polymers: amylose and amylopectin. The iodine molecules fit into the coiled structure of amylose, causing a characteristic blue-violet color change.
However, foods with goitrogenic substances like vinyl-2-thiooxazolidone can make iodine unavailable, preventing this reaction. Additionally, the iodine solubility in water is low; it prefers to react with iodides forming brown solutions which could also interfere with the starch detection.