Why do green onions turn green after washing?

'I recently saw on social media a picture of green onions that I bought and washed them in water, but they turned green. Can you tell me what this green color is? Is it harmful to your health?' (T.Huong, in Ho Chi Minh City).

Doctor - pharmacist Nguyen Thanh Triet, Deputy Head of the Department of Traditional Medicine - University of Medicine and Pharmacy, Ho Chi Minh City, replied: Green onions when bought and washed may be contaminated with copper sulfate (CuSO ₄ ). This type of salt exists in several different hydrated forms such as: CuSO ₄ (anhydrous salt, mineral chalcocyanite), CuSO ₄ .5H ₂ O (most common pentahydrate form, mineral chalcanthite), CuSO 4 .3H ₂ O (trihydrate form, mineral bonatite) and CuSO ₄ .7H ₂ O (heptahydrate form, mineral boothite).

Hành lá — Why do green onions turn green when soaked and washed?

Copper sulfate is a white powder that strongly absorbs moisture from the air to form the blue hydrate CuSO ₄ .5H ₂ O. If added in sufficient amounts, copper sulfate is not harmful to health.

However, like many other vitamins and minerals, if supplemented in excess of the required dose, copper sulfate can be harmful to the body, such as: causing oxidative stress, clinical hematology, changes in serum biochemistry... If in contact with the skin, _CuSO4 crystals can cause irritation and dermatitis. If swallowed in large amounts, CuSO₄ can cause symptoms such as abdominal pain, nausea, vomiting and diarrhea. More serious cases can cause damage to the liver, kidneys and nervous system.

Copper sulfate compounds are often used in fungal treatment for crops, so if green onions, chives... are sprayed and then harvested immediately, there may be residues on the leaves. Therefore, it is best to harvest at the right time after using the drug.

Dr. Triet noted that before using, vegetables containing copper sulfate must be thoroughly washed because this is a compound that can dissolve in water.

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Source: https://thanhnien.vn/bac-si-24-7-vi-sao-hanh-la-mua-ve-rua-nuoc-lai-co-mau-xanh-185250106200906973.htm