Chemical methods of preservation

Chemical methods of preservation

A wide range of chemical additives is available for food preservation and may be used to prevent microbial spoilage, chemical deterioration and mould growth. As high concentrations of some chemicals,  such as sulphur dioxide and sodium nitrate are poisonous, the use of additives is strictly controlled by legislation and maximum permitted levels are usually specified. Preservatives used include:

  • Salt:

Salt reduces the water available (aw) to bacteria. Its effectiveness depends on the concentration, contamination   levels pH, temperature, protein content and the presence of other inhibitory substance. In use, it may be rubbed into meat or, as brine, injected into muscular tissue. It can also be an ingredient in the manufacture of sausages and used to prevent fish.

Some micro-organisms are salt-tolerant but this tolerance is usually decreased by lowering the temperature or the pH. Moulds are less affected by salt. Staphylococci will grow in relatively high salt concentrations and are often associated with food poisoning from semi-preserved salted meats.

In preservation the use of salt, with the addition of other chemicals such as sodium nitrate, is termed curing. Its use for flavour or colouring is termed bringing.

  • Nitrates and nitrites:

Sodium nitrate and sodium nitrite salts are used in curing meat. They help retain colour and reduce spoilage. They are also essential in such products as pasteurised ham to stop the production of botulinum toxin, by preventing the germination of spores.

Traditionally, salt and nitrate solutions were injected into meat, which was then immersed in brine to enable salt tolerant bacteria to convert nitrate to nitrite. Currently, nitrites tend to be used directly as they are much more effective than nitrates.

The effectiveness of curing salts depends on various factors, including the pH of the meat, the number and types of micro-organisms present and the curing temperature.

  • Sugar:

Sugar acts in a similar manner to salt but concentrations need to be about six times higher. It is commonly used for jam and other preserves, candied fruit and condensed milk. Certain types of cakes have increased shelf life due to the effect of sugar. 

  • Sulphur dioxide/sulphite:

Sulphur dioxide may be used in gaseous or liquid from or as a salt. It is an antioxidant and also inhabits growth of bacteria and moulds. It is used in some foods to prevent enzymatic browning. Sulphur dioxide is also used in wine, beer, fruit juice and comminuted meat products. Apart from reducing the growth the spoilage organisms, sulphur dioxide also limits the growth of salmonellae. 

  • Picking/acidification:

This process involves using an acid such as acetic acid, i.e. vinegar, to acidify the food to create an environment in which micro-organisms will not multiply. The acidification process is controlled so that the pH of each part of the products drops below 4.5.

  • Sodium and calcium propionate:

Propionates are active in low-acid foods and very useful to prohibit mould growth. They are used in bread, cakes, cheese, grain and jellies.

  • Antibiotic:

These chemicals have a preservative role in addition to their normal function. Their use is strictly controlled by regulations to avoid the build-up of resistance by pathogenic organisms. An example of an antibiotic used for preservation is nisin, added to some cheese and canned foods. Nisin is heat-resistant but is destroyed during digestion and should not cause problems of pathogen drug resistance.