What is a food preservative?why is food preservation needed?how does preservative preserve food?some examples of food preservative chemicals?what is the role of potassium bisulfite as a food preservative?give some reactions that occur during the process of food preservation when potassium bisulfite is added?

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Thanks!

Bisulfate (the cation isn't important. It can be sodium bisulfite, potassium bisulfite, or whatever) is a reducing agent. It counteracts the effects of oxygen on food. Oxygen reacts with certain foods (some of the most reactive are ketchup and applesauce) and discolors them and degrades them. When a reducing agent is present, the oxygen will react with it instead of reacting with the food.