Determination of residual levels of nitrites and nitrates in meat products.

Abstract

Nitrite and nitrate occur naturally in food but may also be present as a result of the use of fertilisers on crops or from their use as preservatives. They are primarily added to processed meat products (in the form of curing preparations) to provide protection against microorganisms, particularly Clostridium botulinium, that can cause food poisoning. Nitrite is considered as one of the most important food additives, both from an economical as well as from a technical point of view. Apart from exerting antibotulinal and other antimicrobial functions, addition of nitrite converts perishable meats, fish and poultry into unique cured products such as bacon and ham, having desirable sensory characteristics and a longer shelf-life. Nitrate, when added, serves mostly as a reservoir from which nitrite is derived. It is also added to certain foods (some meats, fish and dairy products) as a preservative but only becomes active when it is converted to nitrite via processing or microbial activity. Both nitrite and nitrate are monitored regularly because of their toxicity. Their presence in food (especially meat products) may cause methaemoglobinaemia and favour the formation of carcinogenic nitrosamines. Although nitrate is more stable and less toxic, it gives rise to concern because it is easily converted to nitrite by microbial action. The use of nitrite and nitrate in meat products is locally controlled by the Permitted Food Additives Regulations, 1998 which virtually conforms with the European Directive 95/2/E( on food additives and various other international standards. Nitrite can be partially recovered in the finished meat product as "residual nitrite" which represents the measurable/detectable part of the curing salt in the complex meat matrix. The remaining part of added nitrite disappears by combining with pigments or by undergoing other reactions.

This project includes a survey to determine the residual level of nitrite and nitrate in 50 samples of meat products available on the local retail market. Nitrites were determined spectrophotometrically by the diazotisation-coupling technique (modified Griess-Illosvay reaction) while nitrates were reduced to nitrites by the cadmium reduction technique before being assayed spectrophotometrically as nitrites. The amount of residual sodium nitrite found in the samples varied from 1.8 to 10.2 mg/kg in canned products, <0.3 to 11.0 mg/kg in refrigerated products and <0.3 to 55.9 mg/kg in frozen products (with respective means of 7.3, 5.3 and 8.8 mg/kg). On the other hand, nitrate levels varied from 18.4 to 57.7 mg/kg in canned products, 13.6 to 162.7 mg/kg in refrigerated products and 22.6 to 487.4 mg/kg in frozen products (corresponding to mean values of 31.8, 72.7 and 93.2 mg/kg respectively). All samples analysed were found to be within the maximum permitted level for nitrite; however, one product (namely locally preserved collar bacon) was found to exceed considerably the current regulatory limit for nitrate. The case however merits further investigations and legal considerations given that the period of the survey happened to coincide with a transition phase in local regulations on the use of preservatives in foodstuffs, involving a change in maximum level permitted from a total of 500 mg/kg (nitrite + nitrate), in the case of bacon & ham products, to an absolute maximum of 250 mg/kg of nitrate (as from 01.01.99). Comparing the findings from this survey with those from similar surveys, it becomes evident that amounts of added nitrite have decreased over the years (reducing risk from volatile nitrosamines), while nitrate has been practically phased out/substituted. In fact, it is believed that current residual nitrite is about one tenth the level present about 25 years ago. However, studies show that there is no complete and suitable alternative for nitrite, in spite of the continuously changing processing techniques and formulations for the vast selection of cured meats produced. On the other hand, a total lack of nitrite may indicate potential risk to the consumer's health if the food in question is infected with Cl. botulinium ..