Precursors nitrosating agents

When solutions of sodium nitrite (NaN02) aie acidified, a number of species ar e formed that act as nitrosating agents. That is, they react as sources of nitrosyl cation, N=0 . For simplicity, organic chemists group all these species together and speak of the chemistry of one of them, nitrous acid, as a generalized precursor to nitrosyl cation. 

A source for the NDMA was not found in this tannery. It was reported that they did use dimethylamine sulfate on an experimental basis and nitrosation of three of the bulk process water samples did result in the formation of 0.0015 to 0.0025 yg/ml of NDMA. We also collected three air samples using acid pH traps and after nitrosating the content, found twice the level of NDMA, thus indicating the presence of the NDMA precursor amine in the air. In a further experiment we sampled air over magnesium silicate coated with morpholine and found that from 3-20% of the 30 yg of morpholine had been converted to N-nitroso morpholine thus indicating an airborne nitrosating agent. 

Since this review deals mainly with the analysis of preformed NOC in foods, the chemistry of formation of these compounds is not dealt with in this chapter. It might just suffice to say that the formation of NOC in foods is dependent on a variety of factors, such as the nature (basicity) and concentration of the precursor amines, that of the nitrosating agents, the presence or absence of catalysts or inhibitors, pH, and the temperature of the processing or cooking conditions. The details have been discussed in recent reviews (5,6,9,10). 

The A-nitrosamines are formed from different precursors and under a wide variety of conditions. They can be formed by the reaction of secondary amines with nitrosating agents, such as... 

The possibility of the formation of nitrosamines and their concentrations in foods depend on many factors, such as the presence and quantity of amines and their precursors, type and quantity of nitrosation agents, pH, temperature, reaction time, composition of food (such as fat content), method of heat treatment, presence of substances that catalyse nitrosation reaction (e.g. rhodanides and to a lesser extent chlorides), and the presence of inhibitors (e.g. ascorbic acid, its esters and salts, sulfur dioxide, phenolic antioxidants, such as gallates and melanoidins). 

N-Nitrosamine inhibitors Ascorbic acid and its derivatives, andDC-tocopherol have been widely studied as inhibitors of the N-nitrosation reactions in bacon (33,48-51). The effect of sodium ascorbate on NPYR formation is variable, complete inhibition is not achieved, and although results indicate lower levels of NPYR in ascorbate-containing bacon, there are examples of increases (52). Recently, it has been concluded (29) that the essential but probably not the only requirement for a potential anti-N-nitrosamine agent in bacon are its (a) ability to trap NO radicals, (b) lipophilicity, (c) non-steam volatility and (d) heat stability up to 174 C (maximum frying temperature). These appear important requirements since the precursors of NPYR have been associated with bacon adipose tissue (15). Consequently, ascorbyl paImitate has been found to be more effective than sodium ascorbate in reducing N-nitrosamine formation (33), while long chain acetals of ascorbic acid, when used at the 500 and lOOO mg/kg levels have been reported to be capable of reducing the formation of N-nitrosamines in the cooked-out fat by 92 and 97%, respectively (49). 

Methylating agents can be generated by chemical ni-trosation of endogenous metabolites. For example, methylamine produced by the decomposition of organic matter can condense with carbamyl phosphate, a precursor of pyrimidines, to form methylurea, which in turn can be nitrosated to yield methylnitrosourea (MNU). Such nitrosation reactions can be catalyzed by bacterial enzymes (35). 

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