This is because purple sweet potatoes contain anthocyanins, which are responsible for turning blue and green. Anthocyanin is a water-soluble natural pigment, which is characterized by being red when it is acidic, usually turning blue when it is alkaline, and there may be transition colors such as purple and green in the middle. If the color is dark, a purple-black state may appear.

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Besides anthocyanin, purple sweet potato also contains many active ingredients such as β-carotene, dehydroepiandrosterone (DHEA), sugar egg white, dietary fiber, polyphenols and so on. It is reported that β -carotene in purple sweet potato can reduce human active oxygen and eliminate free radicals, and has physiological effects such as cancer prevention, anti-tumor and immunity improvement.

In addition, β -carotene also has the functions of sweating and beauty. Professor A. Sevez, a biochemist from Philadelphia Medical College, isolated DHEA from sweet potato, which is a precursor of sex hormone, and can regulate obesity, diabetes, anti-cancer and virus infection, improve memory, immune response and stress response, relieve tension, resist cortical diseases, delay aging and promote protein synthesis.

Glycoprotein in purple sweet potato can prevent fat accumulation in cardiovascular system, protect arterial elasticity, prevent connective tissue atrophy of liver and kidney, and promote lubrication of digestive tract, respiratory tract and joint cavity. Dietary fiber has the functions of preventing constipation, colon cancer and cardiovascular diseases, treating obesity and eliminating exogenous harmful substances. Polyphenols in purple sweet potato selectively combine with protein in the outer layer of HIV to form immune function and hinder HIV infection, which is considered as an important substance to inhibit the onset of AIDS.

People's Daily Online-Why do purple potatoes turn green (form Q&A)