Why are food preservatives added to food? how does it effect the reaction?

Food preservatives are usually loaded to food to prevent decomposition by microbial growth. So preservatives interact with bacteria/mold and inhibit their growth. This interaction includes many biological and chemical reactions: reaction with bacteria's cell wall, proteins etc.

Food preservatives prevents the decomposers to grow in organic food so that food will last long.

It remains neutral without affecting the taste but it degrades quality as these chemical may be harmful to human.