Purple potato can be dyed blue. Cut into pieces and cook in water, and the water will dye blue. Purple fruits and vegetables are mostly sensitive to acid and alkali, which turn red in acidity and blue or green in alkalinity. Chop the purple cabbage and boil it with low fire to get a blue-purple soup. After cooling, add baking soda to bring out the blue color, but be careful not to heat it, which will change color.

Or you can use butterfly bean curd, also called blue butterfly, English iris or butterfly pea. Rich in vitamins A, C and E, it can improve immunity, help and promote skin elasticity and collagen production. People in Southeast Asian countries often use it to make tea or dye food.

Butterfly bean curd contains natural anthocyanins. Soak in water after drying, and the water is blue. Add lemon juice and it will turn purple. It can be used as food coloring agent or cocktail. It is pure natural, and the dyed color is perfect blue.

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Butterfly bean curd, as a natural antioxidant plant, not only helps to improve immunity, but also reduces blood lipid and blood pressure to some extent. Butterfly bean curd has the functions of strengthening brain, preventing stomachache and resisting stress, and is a natural health drink to relieve mood. Although it is good for health, it is not suitable for everyone to drink.

Pregnant women and menstruating women can't drink butterfly bean curd, which contains 10 times more anthocyanins than ordinary plants. Anthocyanin has a strong inhibitory component on platelet coagulation. If pregnant women drink it, it is likely to cause bleeding.