THE CHEMISTRY OF NITROSAMINE FORMATION: RELEVANCE TO MALTING AND BREWING

This paper reviews the chemistry of nitrosamine formation with special reference to malting and brewing. Whilst the main emphasis is on the formation of nitrosodimethylamine (NDMA), the chemistry is also related to the formation of other nitrosamines and certain other nitrocompounds. It seems likely that in malting and brewing the only important source of the nitroso group is the mixture of oxides of nitrogen (NOx) in the hot air in contact with malt during kilning. Under normal commercial conditions there appears to be little if any formation of nitrosamines at other stages of malting or brewing. It is suggested that much of the nitrosation is due to NOx which exists temporarily in the form of nitrogen trioxide (N2O3) and nitrogen tetroxide (N2O4) as it dissolves in water in the moist malt or in malt lipids. The dissolved NOx subsequently reaches equilibrium with nitrite and nitrate ions formed from it. The origin of the dimethylamine portion of the NDMA is still not clear. The most likely sources are free DMA, gramine and hordenine, all of which are produced in germinating barley. On balance it seems that hordenine is the most important source of NDMA in the malt at the end of kilning. The reasons for the low yield of nitrosamines from the precursors and NOx present during kilning are discussed and related to the formation of other nitroso- and nitro-compounds. The occurrence of competing reactions which reduce nitrosamine formation is also described.