Sodium nitrite is mainly utilised for the preservation of meat. It is also used for developing the organoleptic quality of meat with respect to flavour and colour. Like sodium nitrate, it has the ability to suppress botulism activity (botulinal toxin). However sodium nitrite is believed to be more capable of reacting at low temperature while sodium nitrate may not able to be active at lower temperature. In addition to the organoleptic and preservative properties of nitrite in meat, it has some protective functions against the spoilage of meat like postponing rancidity, off-odour and off-flavour within the period of storage.
Nitrite (NO2-) is chemically decomposed in meat into nitric-oxide (NO) and subsequently reacts with iron atoms in myoglobin’s heme group. This can result in reducing the oxidation and developing the reddish-brown colour (associated with nitrosomyoglobin when the meat is still uncooked) or pink colour (referred to nitrosohemochrome or nitrosyl-heme when meat is cooked).
Application of sodium nitrate additives in food industry is almost controversial due to the potential activation of nitrosamines when the cured meat is cooked (nitrosamine is converted to diazonium ion in presence of heat) and the fact that extra nitrate is able to react with amino acids in protein after ingestion (nitrosamine is converted to diazonium ion in presence of acid in stomach). Some specific nitrosamines like N-nitrosodimethylamine and N-nitrosopyrrolidine produce carbocations which are able to react with biological nucleophiles including DNA or an enzyme present in the cell. If chemical reaction of nucleophilic substitution takes place at a susceptible area of biomolecule, a disruption of intact cell functioning may occur. This is likely to result in the cell death or cancer. Hence it is crucial to apply the correct / permitted amount of nitrate and nitrite additives in food products.
There are various types of nitrosamines. Predominantly, they are considered to be carcinogenic to human. Nitrosamine may be synthetised in some cured meats and its concentration is quite low. The main contributory factors involved in the level / synthesis of nitrosamine include the quantity of the added nitrite in processing / initial stage; percentage of amino acids in meat; physical/chemical characteristics of food; cooking methods and time; technical practice (to minimise nitrosamine production) and concentration / attributes of the other additives used in meat (e.g. ascorbic acid).
The formation of nitrosamine can be significantly reduced / inhibited by the addition of ascorbic acid (vitamin C) and erythorbic acid which are classified as antioxidants. Their functional properties involve reducing oxidation. For instance ascorbic acid is oxidised to dehydro-ascorbic acid and subsequently a powerful nitrosating agent known as nitrous anhydride is degraded to nitric oxide and as a result it is no longer a nitrosating agent.