Q. The strainer and mesh are discolored.

Soaking and cleaning with baking soda or oxygen bleach is effective.
Please try the following method.

Soak method:
1. Fill a bowl with hot or lukewarm water and add baking soda or oxygen bleach to dissolve.
(Beware of spills when using oxygen bleach)
2. Soak the lid and leave it until the hot water cools down.
3. When the hot water cools down, throw it away and rinse it well.
4. Clean the lid with dish soap and dry it well.
If you rub the mesh with a toothbrush etc., it will be easier to remove.