Chemical food preservation has been used since ancient times to prevent spoilage caused by microorganisms. Salt, sugar, spices, and nitrates are among the oldest preservatives, while organic acids, sulfites, and bacteriocins are more modern. Some chemicals are controversial due to their potential health risks, but they are still widely used in the food industry.
Chemical food preservation has been practiced ever since man began to preserve foods for later use. Food preservation has raised the issue of spoilage, usually caused by microorganisms such as bacteria, fungi and yeasts. In addition to making food inedible or unappetizing, microbial activity could lead to potentially fatal food poisoning. Throughout history, a wide variety of chemicals have been used to prevent food spoilage, and their use dates back to well before microorganisms were suspected to exist. 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 them to die or prevent growth. About six times as much sugar as salt needs to be used to get 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 food, particularly in the case of salt. They are therefore employed where their tastes will complement those of foods, so salt is used for meats and sugar for fruit. Many herbs and spices contain chemicals that can kill microorganisms and these have also been used as preservatives. Garlic, onion, allspice and oregano are particularly good at killing bacteria.
Sodium and potassium nitrate, also known as saltpeter, have been used to preserve and cure meat for centuries and are still used today. The bacteria reduce nitrates (NO3-) into nitrites (NO2-) and then into nitric oxide (NO), which destroys the ferredoxin enzyme, 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 meats, such as ham, bacon and sausages. However, this form of chemical food preservation is controversial; nitrites can react with chemicals in meat to form nitrosamines, which are carcinogenic. Nitrates and nitrites are not effective against microorganisms that do not use ferredoxin, for example Salmonella.
Among the chemical food preservation methods 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. By being absorbed into microbial cells, these chemicals interfere with the cellular transport system which allows for the uptake of nutrients across the cell membrane, preventing growth. They are effective against a wide range of bacteria and molds in low concentrations and usually do not affect the flavor of food; however, they generally perform best at a fairly low pH, making them more suitable for use in acidic foods such as fruit juice, carbonated beverages, and salad dressings. Parabens – esters of hydroxyparabenzoic acid – are effective over a wider pH range.
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 aflatoxin-producing fungi. These preservatives are often used in dried fruit and wine; however, their use is not permitted in meats as they redden the color and can mask the smell of decay, making spoiled meat appear fresh. Some asthma sufferers are very sensitive to sulfur dioxide and its derivatives and must avoid foods and drinks containing relatively high levels of sulfites.
The use of antibiotics for chemical food storage 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 because – unlike most bacteriocins – it is effective against a wide range of microorganisms. It is added to some processed dairy products and some canned foods.
Food and food containers are sometimes treated with certain chemicals before being packaged, instead of adding preservatives. For example, fruit is often fumigated with ethylene and propylene oxides to inhibit mold. Various flavorings and antioxidants added to food can also help preserve it.
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