Chemical food preservation has been practiced ever since man first began to store food for later use. Food storage raised the problem of spoilage, usually caused by microorganisms such as bacteria, fungi and yeasts. As well as rendering food inedible or unappetizing, microbial activity could lead to potentially fatal food poisoning. Through history, a wide variety of chemicals have been employed to prevent food spoilage, and their use dates from well before the point when the existence of microorganisms was first suspected. These chemicals range from naturally occurring substances, such as salt, sugar, saltpeter and spices, to modern food additives such as benzoates, sorbates and sulfites.
Two of the oldest food preservatives are salt and sugar, which work by removing water from living microbial cells, causing death or preventing growth. Approximately six times as much sugar as salt needs to be used to achieve the desired effect, but both need to be used in relatively large quantities to be effective, and this obviously limits their use as they will impart a strong flavor to the food, especially in the case of salt. They are therefore employed where their tastes will compliment those of the foods, so salt is used for meats and sugar for fruit. Many herbs and spices contain chemicals which can kill microorganisms and these have also been used as preservatives. Garlic, onion, allspice and oregano are particularly effective in killing bacteria.
Sodium and potassium nitrate, also known as saltpeter, have been used for preserving and curing meat for centuries, and are still used today. Bacteria reduce nitrates (NO3-) to nitrites (NO2-) and then to nitric oxide (NO), which destroys the enzyme ferredoxin, used by some bacteria to obtain their energy, for example Clostridium botulinum, the bacterium that causes botulism. For this reason, nitrates or nitrites are considered essential in cured meat products, such as ham, bacon and sausages. This form of chemical food preservation is, however, controversial; nitrites can react with chemicals in the meat to form nitrosamines, which are carcinogenic. Nitrates and nitrites are not effective against microorganisms which do not use ferredoxin — for example Salmonella.
Among the methods of chemical food preservation introduced in the 20th century are the use of salts of some organic acids, such as benzoic, propionoic and sorbic acids — for example sodium benzoate, sodium propionate and potassium sorbate. On being absorbed into microbial cells, these chemicals interfere with the cell transport system which allows nutrients to be absorbed through the cell membrane, preventing growth. They are effective against a broad range of bacteria and molds at low concentrations and do not usually affect the flavor of food; however, they generally work best at fairly low pH, making them most suitable for use in acidic foods such as fruit juice, carbonated drinks and salad dressings. Parabens — esters of hydroxyparabenzoic acid — are effective over a wider range of pHs.
Sulfites and metabisulfites release sulfur dioxide, which dissolves in water to form sulfurous acid. This appears to work in a similar way to organic acids, and is particularly effective against yeasts and molds, including the fungi that produce aflatoxins. These preservatives are often used in dried fruit and wine; however, their use is not permitted in meat as they redden the color and can disguise the odor of decay, making spoiled meat appear fresh. Some asthma sufferers are very sensitive to sulfur dioxide and its derivatives and must avoid food and drink containing relatively high levels of sulfites.
The use of antibiotics for chemical food preservation is not permitted in most countries, as it can lead to the emergence of antibiotic-resistant strains of bacteria. The bacteriocin nisin, however, is sometimes used since — unlike most bacteriocins — it is effective against a wide range of microorganisms. It is added to some processed cheese products, and to some canned foods.
Food and food containers are sometimes treated with certain chemicals before being packaged, as opposed to having preservatives added. For example, fruit is often fumigated with oxides of ethylene and propylene to inhibit mold. Various flavorings and antioxidants added to food can also help to preserve it.