The blue residue on the onion is blue alum, that is, anhydrous copper sulfate. Some vegetable farmers spray a medicament called "blue alum" on shallots, commonly known as blue powder. It is a common chemical, which is mostly used for vegetable preservation. The copper ion of copper sulfate can replace the magnesium ion in chlorophyll, so that the vegetable leaves are not easy to turn yellow and appear bright green. At the same time, it can react with fungi and keep fresh.
Three no blue powder pesticides cannot be used and are not allowed to be added. Article 16 of the regulations of the people's Republic of China on the administration of pesticides stipulates that the packaging of pesticide products must be labeled or attached with instructions. If there is no information such as composition, content, registration number and production license number, it can be judged that it is a "three noes" product and cannot be added to food.
The label or instruction manual shall indicate the pesticide name, enterprise name, product batch number, pesticide registration certificate number or temporary pesticide registration certificate number, pesticide production license number or pesticide production approval document number, as well as the effective ingredients, content, weight, product performance, toxicity, purpose, use technology, use method, production date, validity period and precautions of the pesticide.