Experiment:
You were using metal knives and ceramic knives also cut with a piece of fruit-all cut by half after peeling place 5-10minutes(potatoes,apples,etc.).
Metal sliced half of the fruit,usually you’ll soon find:fruit surface quickly becomes dark it! 

---It is the fruit of acid and metal materials(iron)and air (oxygen)from a chemical reaction is oxidation of reason.  

Ceramic sliced the other half of the fruit,you will find:fruit surface simply do not see any color,just cut off the fruit will naturally dry skin,this shows that the ceramic knife and the fruit knife is not from a chemical reaction,and everything is naturally.
   Cut By Ceramic Knives  Cut By Metal Knives