Now, as Paracelsus noted, "The dose makes the poison" and in moderate amounts, alcohol may have beneficial effects that outweigh its downsides. But there was a brief period in the late 1970s and early 1980s when there was worry that beverages made from malt, including beer and whisky, contained dangers above and beyond their standard risks.
The fear was caused by a compound called N-nitrosodimethylamine (NDMA). Nitrosamine compounds form when nitrogen oxides react with amines. For instance, nitrosamine levels used to be fairly high in meats preserved with sodium nitrite, such as bacon, and are still rather high in tobacco products.
Beginning in the late 1950s and into the 1960s, evidence began to accumulate that nitrosamines could lead to cancer. Studies in rats showed that administration of NDMA led to liver cancer and there was an incident in Norway where pigs fed herring (fish tends to have high levels of amines - hence its smell) preserved with sodium nitrite developed liver diseases, including cancer. At this point, while it was known that nitrosamines were dangerous, analytical techniques were unable to detect it in human foodstuffs.
That made it very alarming when studies released in 1979 found that beer and malt whisky contained detectable levels of NDMA. While concentrations were low, as little as 0.4 to 0.7 parts per billion (PPB), this was still unsettling as some studies on rats had concluded that even 10 PPB were enough to triple lung cancer rates. It is known that nitrosamines can react with DNA to form adducts, which is a plausible mechanism for much of their carcinogenicity.
How did this happen? While nitrosamines were likely always present in malt to one degree or another, increasing levels came about from the advancing technology used in the process of drying malt. Heat is used to arrest germination and dry the malt to preserve it in a stable form. Though peat and coal had been historically used all over Scotland, they were being phased out in favor of gas burners, which are more flavor-neutral sources of heat. These appeared to burn cleanly and dry the malt without imparting any flavor, making it easier to produce the unpeated malts distillers needed for the making lighter, more cleanly flavored whisky.
However, the temperatures produced by gas flames were significantly higher than those of peat or even coal and oil, which increased the formation of nitrogen oxides (primarily dinitrogen trioxide and dinitrogen tetroxide) from the nitrogen present in air. Those nitrogen oxides would then react with nitrogen-containing compounds in the malt to produce nitrosamines. As noted, the most common nitrosamine is NDMA. This is formed primarily from hordenine, a dimethyl derivative of tyramine (itself a derivative of the amino acid tyrosine), which is at its peak concentration early in the kilning process. The nitrogen oxides in the hot air act both to cleave dimethylamine from hordenine and convert it into NDMA.
|Formation of NDMA from hordenine|
Thankfully solutions to this problem were found fairly rapidly. The simplest was to heat the malt indirectly rather than directly. Heat from a gas burner is fed into an exchanger, which transfers that heat to clean air, which is passed through the malt. This is the process now used in almost all maltings, especially larger ones.
|From Shimadzu News 3/2005|
|The kiln at Springbank distillery|