A preservative is a natural or synthetic chemical that is added to products such as foods, pharmaceuticals, paints, biological samples, etc. to retard spoilage, whether from microbial growth, or undesirable chemical changes.
Preservative food additives are often used alone, or in conjunction with other methods of food preservation. A distinction is sometimes made between anti-microbial preservatives which function by inhibiting the growth of insects, bacteria and fungi, and antioxidants, which inhibit the oxidation of food constituents. Common anti-microbial preservatives include sodium nitrate, sodium nitrite, sulfites, (sulphur dioxide, sodium bisulfate, potassium bisulfate, etc.) and disodium EDTA. Antioxidants include BHA and BHT. Other preservatives include formaldehyde (usually in solution), glutaraldehyde, diotmaceous earth (kills insects), ethanol and methylchloroisothiazolinone. The benefits and safety of many artificial food additives (including preservatives) are the subject of debate.