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

6 Potential for NMVOC emission mitigation from Scotch whisky maturation

Whisky maturation is a sensitive and complicated process where maturation in given environments produces a spirit unique to the location and barrel that it matures in. Maturation, as the name suggests, refers to a long period of time and any mitigation deemed for consideration would need to take the lifetime maturation of any spirit into account.

The reactions which improve the quality of the whisky in relation to a greater amount of time matured are not yet known (Conner, 2014). This is mainly due to the lack of easily observable activity in a laboratory over the time periods that high quality maturation requires. Attempts could be made to better understand the long-term reactions during the maturation process. This could then be used to provide evidence that a suggested mitigation technology will not negatively impact quality. Given the length of the maturation period, trials of any mitigation technology would need to take place for at least three years to ensure that quality is not impacted.

The MATIN (Maturation Innovation) project carried out by the Scotch Whisky Research Institute in collaboration with the Building Research Establishment (BRE), involved the monitoring of 17 warehouses in Scotland (Conner and Forrester, 2017). The team were unable to relate the parameters they monitored inside the warehouse to the rate of loss of ethanol. Alongside the monitoring, the project involved the development of a building energy model for maturation warehouses, based upon the conditions inside two warehouses. The conditions were recorded using 36 loggers recording temperature and humidity in the warehouse. From the model created, the team determined that, in a controlled environment, more than 98% of the variation in losses was due to temperature. The greater the temperature, the greater the loss in ethanol per year. Measurements of ethanol loss were taken from various casks in two warehouses over a two-year period, one warehouse in North Scotland and the other in South Scotland. However, amounts of ethanol loss were found to have varied considerably between casks, even within the same warehouse where the warehouse temperature was constant. The rate of loss is unlikely to be random, and it will most likely be driven by physical processes with quantifiable parameters related to the environment the whisky is in and others relating to variation in barrels that is not easily defined. If these parameters are studied in detail, then mitigation measures can be developed to control the parameters and therefore reduce the evaporative loss of ethanol.

The MATIN study determined that reducing warehouse temperature reduced the evaporative loss but also has an impact on the speed of the maturation process. In particular, the study determined that reduced temperatures slowed the extraction of colour and reduced the colour of the whisky. Therefore, reducing warehouse temperatures may result in casks requiring longer maturation periods to reach the same level of quality. It is not clear from the study, however, if the reduction in temperature reduces the NMVOC emissions over the entire length of the maturation process when taking into account any additional time that may be required for maturation due to the slowing of the maturation process. It is possible that controlling other parameters not determined by this study driving the rate of ethanol loss may not reduce the length of the maturation process, and thus the quality of the product produced.

The Central Valley of California has challenging air quality issues. There, the San Joaquin Valley Unified Air Pollution Control District, in its Rule 4695 requires VOC emissions from brandy aging and wine aging operations above a certain size to be controlled (San Joaquin Valley Unified Air Pollution Control District, 2009a). The rule applies to brandy aging and wine aging operations that have the potential to emit at least 10 tons (9,072 kg) of VOCs per year. A storage warehouse should be sealed so that it qualifies as a permanent total enclosure (PTE) pursuant to US EPA Method 204, although exception is made for moving product in and out of the warehouse and for maintenance. Emissions should be captured by the PTE and vented to a control device such that the total control efficiency is 90%.

A number of control technologies and devices can be used. The brandy aging industry in the district has universally selected the use of a regenerative thermal oxidizer to burn off the VOC emissions due to its low annual maintenance costs (San Joaquin Valley Unified Air Pollution Control District, 2009b). However, other technological feasible approaches include catalytic thermal oxidation; adsorption vapour recovery; wet scrubbing (absorption); condensation, refrigeration and cryogenic systems; and biological oxidation (San Joaquin Valley Unified Air Pollution Control District, 2009b).

In the Final Draft Staff Report for Rule 4695, San Joaquin Valley Unified Air Pollution Control District staff noted that the nature of whiskey aging operations differs from wine and brandy aging, with ambient conditions, such as storage temperature and humidity, as well as seasonal variations, being important factors in the whiskey aging process (San Joaquin Valley Unified Air Pollution Control District, 2009b). Consequently, whiskey aging was not included in the scope of Rule 4695.

The key importance of ambient conditions in the warehouse on product quality was discussed in a US EPA study of the control of VOC emissions from whiskey warehousing (US EPA, 1978). The barrel environment is extremely critical in whisky maturation and varies considerably by distillery and warehouse. In line with tradition and experience, distillers may alter the barrel environment to produce a product with the distinctive characteristics of its brand; this may include moving barrels between different locations in a warehouse or altering the amount of natural ventilation.

A collection system to capture gaseous emissions in a warehouse, as required for brandy aging operations in the California Central Valley, could be expected to significantly disrupt ventilation and air flow patterns around barrels in warehouses and so adversely impact product quality. The only known full-scale test of a control system, where a warehouse was closed and emissions ducted to a carbon adsorption unit, was run between 1960 and 1968, but product quality was adversely affected, ultimately resulting in termination of the test (US EPA, 1978).

In principle, instead of operating continuously an extraction system could be designed to fully replicate natural conditions as these affect air flow around barrels. This might include intermittent changes to ambient temperature and humidity, as well as replicating the formation of stagnant layers and temperature variations in different parts of the warehouse. Another concern with altering ventilation patterns in a warehouse is that ethanol concentrations could be raised above safe levels.

Because of the crucial importance of the unique taste and smell characteristics of each whisky brand for marketability and consumer acceptance, any measure that could affect the maturation process would have to be considered very carefully. Any future full-scale demonstration of potential mitigation measures will require a substantial investment in product and time to show that product quality is not discernibly affected over the full maturation process.

6.1 Voluntary testing of mitigation solutions

Given the importance of the relationship between maturation and the quality of spirit, it is inevitable that whisky industry stakeholders will be cautious towards any proposed mitigation processes. A proposed approach could involve establishing a voluntary testing group with the Scotch Whisky Association (SWA) and Scotch Whisky Research Institute (SWRI). This working group could identify potential distilleries or companies to trial maturation mitigation systems over prolonged periods of time. Engagement of stakeholders will be crucially important for deriving effective solutions.