Non-methane volatile organic compound emissions from malt whisky maturation: final report

2 Properties of NMVOC emissions specific to Scotch whisky production

2.1 Properties of NMVOCs

NMVOCs are a group of organic chemicals which cover a wide range of compounds. They essentially include all volatile organic compounds (VOCs) apart from methane, which is excluded from this grouping due to its lack of toxicity and low reactivity compared to other VOCs. NMVOCs are characterised by their low boiling point and their high vapour pressure at room temperature. NMVOCs are highly volatile, existing as a gas at room temperature and pressure.

As well as their high volatility, NMVOCs are also characterised by their relative ease of reaction with other compounds. They are involved in the photochemical production of ozone and secondary organic aerosols in the atmosphere. The category of NMVOCs includes between 10,000 and 100,000 different species (Goldstein & Galbally, 2007) and the reactivity of each NMVOC differs. Consequently, some species have a greater propensity to react to form other compounds than others.

The Air Quality Directive 2008/50/EC (European Parliament and Council, 2008) includes the following definition of VOCs:

"’Volatile organic compounds' (VOC) shall mean organic compounds from anthropogenic and biogenic sources, other than methane, that are capable of producing photochemical oxidants by reactions with nitrogen oxides in the presence of sunlight".

This is broader than the definition provided by the European Monitoring and Evaluation Programme (EMEP)/ European Environment Agency (EEA) guidelines for the reporting of emissions inventories by countries (EEA, 2019):

"NMVOCs compromise all organic compounds except methane which at 293.15 K show a vapour pressure of at least 0.01 kPa (i.e. 10 Pa) or which show a comparable volatility under the given application conditions."

Due to their ease of reactivity, NMVOCs can be involved in reactions with other pollutants in the atmosphere to produce 'secondary' pollutants. These include the formation of ozone, through the reaction between NMVOCs and nitrogen oxides (NOx), catalysed by sunlight.

In addition, NMVOCs can influence the formation of secondary organic aerosols (SOAs) and secondary inorganic aerosols (SIAs), particularly ammonium nitrate and ammonium sulphate. This is either by reactions involving NMVOCs directly, or by reactions involving secondary pollutants, particularly ozone, formed through other reactions involving NMVOCs. Both SIAs and SOAs are considered to be particulate matter which is typically categorised as PM₁₀ or PM_2.5 where the aerodynamic diameter is less than 10 µm or 2.5 µm, respectively. SIAs and SOAs can belong to both of these fractions of particulate matter.

2.2 Properties of NMVOC emissions from whisky production

Over 90% of NMVOC emissions from whisky making occur during the maturation stage. This occurs by first the saturation of the wooden barrel and then evaporation through the wood into the atmosphere. Surface area of the barrel, temperature, and humidity can all affect the rate of emission of NMVOCs during the maturation process (Conner, 2014). Emissions also occur during fermentation and when the whisky is drained and pumped from the barrels, but these are minor compared to the maturation stage.

Greater than 99% of all NMVOCs produced in whisky production is ethanol (Passant et al., 1993). Other volatile compounds are produced, such as acetaldehyde, ethyl acetate, glycerol, for example, but only in trace quantities. Ethanol, like other NMVOCs, helps to form ground-level ozone and particulate matter in the presence of sunlight.

Across the UK, ethanol is now the largest NMVOC emitted by mass at approximately 16.8% of total UK VOC emissions which is partly due to the recent growth of the whisky industry (AQEG, 2020). However, ethanol is currently not monitored by the Defra Automated Hydrocarbon Network, operated throughout the UK, nor in other countries.

At the start of the maturation process, approximately 50% of the cask will contain ethanol. A typical cask will lose around 2% of its ethanol content each year (Conner and Forrester, 2017). Therefore, if whisky is matured in a typical 190 litre barrel for 6 years before being bottled, it will lose 14 litres of ethanol. More premium products with a longer maturation time will lose a greater amount.

In Kentucky, USA, the rate of evaporation of ethanol in whisky production is nearly twice that measured in Scotland (Conner and Forrester, 2017). This may not be purely due to climatic factors, but also due to warehouse design and layout, and the impact this may have on internal conditions within the warehouse. For example, stacking maturation barrels more densely may restrict airflow between barrels, maintaining more constant temperatures of the barrel surface and liquid.