What are the Chemical methods used for food preservatives?

Food preservatives help to control the spread of bacteria which can cause life threatening illnesses such as salmonellosis or botulism.
They are added to food to enhance appearance and taste. Depending on the purpose, there are preservatives used for different purpose. For example, Antioxidants are used to absorbing oxygen in the food and thereby arresting oxidation of the constituents in the food.

benzoic acid is a good agent.

benzoic acid

Benzoic acid, proionic acid and many acids use as preservatives in foods.

The most commonly used are the acids, such as sorbic acid, benzoic acid and propionic acid. These check mainly the growth of yeasts and molds. Sorbic acid is used for preservation of syrups, salads jellies and some cakes. Benzoic acid is used for beverages, margarine, apple cider etc.

Propionic acid is an ingredient of bread and bakery products. Sulphur dioxide, as gas or liquid is also used for dried fruits, molasses and juice concentrates. Ethylene oxide is used for spices, nuts and dried fruits.

chemical methods used include :-

1. use of nitrites

2.use of sulphites

3.use of benzoic acid- due to its antibacterial activity

4.use of ascorbic acid- due to their antioxidant properties 

There are many chemical methods to preserve food and few of them are :
Antioxidants preserve freshness in meats. Emulsifiers re used to maintain the consistency of French dressing. and lecithin are used in ice-cream.