I think due to oxidation and reduction they change the color with passage of time.

I think due to oxidation and reduction they change the color with passage of time.

Agree with fazal.

Thanks @Wajid Zaman

Salam,

There is a simple test to do: get a cyclic voltamogram of your sample in water.

Yeah it's possible to reduce the rate of oxidation in these products. You can use antioxidant food additives. You can find a list FDA approved antioxidants in the link below; hope it will help.

https://www.fda.gov/food/ingredientspackaginglabeling/foodadditivesingredients/ucm091048.htm