When Whisky Was (Possibly) (Slightly) (More) Carcinogenic

Let's be honest with ourselves. When we drink alcohol, especially distilled spirits, we are drinking poison.

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

As many of these nitrosamines have boiling points comparable to phenolic compounds (~150º C vs ~180º C) that also find their way into malt whisky, they can be carried through the production process and ended up in the final product.

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

However, some maltings still use direct heat in the form of burning peat. While peat fires are generally used to generate smoke rather than heat, per se, they can still produce nitrogen oxides. This makes it important to avoid flaring while burning peat, as that will increase the production of nitrogen oxides. To prevent this from happening, sulfur is burned alongside the peat, which forms sulfur dioxide, which reduces the pH of the malt and inhibits the formation of nitrosamines. This process also occurs naturally in kilns heated by coal and oil burners, as these fuels contain fairly high levels of sulfur. In comparison, natural gas contains very low levels of sulfur, which likely contributed to the formation of nitrogen oxides when it was used to dry malt.

The kiln at Springbank distillery

Alongside the changes to production techniques, analytical techniques have also improved over the decades since this issue was first brought to light, which makes the routine analysis of malt for nitrosamine content relatively simple. This ensures that quantity of nitrosamines in malt used for brewing and distilling are far below the level that would do you any harm. So you only have to worry about what the alcohol itself is doing to you.