An external substance that is added to avert putrefaction by microbes or any by chemical changes to food materials, pharma products, paints, etc. can be called a preservative. Physical and chemical preservation are usually present. Addition of chemicals externally to the products to prevent decay is chemical preservatives. Physical preservation includes dehydration, UV radiation, freezed drying, cooling and drying. Artificial preservatives are added in foods, cosmetics, etc. They help to lower the attack of microbes thereby preserving the freshness and nutritional value and also prevent the problem of infections.
Antimicrobial preservatives are used to prevent microbial growth by reducing the pH of the substance. Lactic acid is an example. Benzoic acid, sodium benzoate, nitrites, nitrates, sorbic acid, sodium sorbate, propionic acid, sulfur dioxide also comes under the group. Jams, juices, meat and meat products, fruits, cheese, wine, bakery, etc. uses these substances to act as preservatives. Antimicrobial preservatives added to foods can be grouped as;
1. Those added not defined as such by law; e.g. natural organic acids, vinegar, sugar, salt, wood-smoke, etc.
2. Substances generally recognized as safe (GRAS); e.g. propionic acid, caprylic acid, sulfur dioxide, etc.
3.  Chemicals considered to be food additives;
4. Chemicals proved safe and approved by the Food and Drug Administration.
Certain antioxidants are also used as preservatives. Ascorbic acid, propyl gallate, ethanol, tocopherols, sulfur dioxides, BHA, Gallic acid, BHT, etc. all comec under this group. These antioxidant preservatives are added mainly to prevent the rancidity of fatty products such as cheese, oils, etc. and also in beverages and wine. They slow down the oxidation pathway preventing the spoiling of such oily foods. Formation of hydro peroxides is also concealed. Smoking of fruits leads to release of phenols in them, which acts as antioxidants.
Some substances like citric acid, tartaric acid, lecithin, etc. are added to neutralize the action of metal ions if present. They are termed as sequestering agents.
Flavoring extracts such as vanilla and lemon extracts are preserved by their alcohol content. Propylene glycol is used as mold inhibitor and is sprayed to kill air borne microbes.
High sugar concentrations are used as preservatives in foods like condensed milk, jams, jellies and candies.Cinnamon and cloves containing cinnamic aldehyde and eugenol are bacteriostatic in nature. Oils of mustard help to inhibit yeast growth. Extracts of cabbage and turnip inhibits E.coli and Bacillus subtilis.

Certain non-synthetic substances like phenolase, ascorbic acid, tocopherols, citric acid. These naturally preserve fruits from enzyme action, color changes, etc.  Other natural preservatives include salt, sugar, alcohol, castor oil, rosemary oil, etc.Halogens are added to water for washing foods or equipment. Halogens kill organisms by oxidation. Iodine impregnated wrappers are used to lengthen the keeping time of fruits. Phosphoric acid is used in some soft drinks, e.g. colas. Powdered boric acid is dusted on foods such as meat as preservative. Sodium tetra borate is used to wash vegetables and fruits like orange.
The artificial preservatives mostly are responsible for many harmful effects also, even though they prove to preserve the foods for long periods of time. Certain substances like nitrites when heated react with proteins and become carcinogenic. Sodium benzoate added in, almost all ready to eat foods creates hyperactivity especially in children.