Fermentation, for example, depends on yeast converting all the sugars in the wort into alcohol before it’s too late. The temperature of the wort rises during fermentation, and heat is one reason why yeast expires. Consequently, adapting the pitching temperature (the temperature of wort when adding yeast) to the ambient temperature creates the conditions for a successful, and consistent result. Higher summer temperatures would see yeast getting off to a quicker start, so a relatively cooler pitching temperature slows it down. Correspondingly, a higher pitching temperature in winter gets yeast off to a faster start than the ambient temperature would allow.
“Fettercairn Distillery lies at the base of the Cairngorm mountains, by the Cairn O Mount, so our daily ambient temperature is relatively stable. However, maintaining consistency is our priority, regardless of time of year. This means that in the summer our pitching temperature is 19 degrees centigrade, and in winter we adjust it to 21 degrees centigrade to arrive at a maximum of 34 degrees. We also maintain our consistency with good air ventilation in our Tun Room, which is managed to ensure consistent conditions for the 11 traditional wooden washbacks it houses. On warm days we’ll also open the windows for extra ventilation,” says Stewart Walker, distillery manager, Fettercairn.
Outside Scotland, using temperature-controlled fermentation vessels is hardly unusual. “Winter can be minus 12 degrees centigrade or even colder, and summer can exceed 38 degrees centigrade. It would be impossible to adjust the pitching temperature to achieve consistency, so we use temperature-controlled fermentation, which enables us to maintain a consistent temperature all year round,” says John Rempe, head distiller and master blender, Lux Row Distillers.
Stainless steel fermenters are fitted with stainless steel coils that encircle the interior surface. Conducting cold water through the coils keeps the wort at a particular temperature.
“Fermentation lasts 96 hours, and 12 hours into fermentation the wort reaches the temperature the cooling coils are set to. This is an optimum temperature for the yeast to work efficiently, and is maintained until the end of fermentation,” says John, but controlling temperature is not solely about making yeast comfortable, as yeast behaves differently when the wort temperature changes.
“Changing the temperature set by the cooling coils, even by a degree or two, produces different flavour compounds. A slightly higher temperature produces more banana and fruit notes, for example,” John continues.
Water is also used to modify the temperature during distillation. Dalmore has a spirit still fitted with a water ring that conducts cold water through a copper jacket encircling the neck of the still. The jacket contains around 40 pipes through which alcohol vapours leaving the top of the neck are conducted. Water cools the exterior of the pipes, and as heavier flavour compounds require higher temperatures to remain in vapour form, some condense within the cooler temperature of the water jacket, and are conducted back into the pot still for re-distillation. Meanwhile, the temperature is high enough for lighter flavour compounds to remain in vapour form, and proceed to the condenser.
This process, known as reflux, determines the proportion of lighter and richer flavour compounds in the resulting spirit. The colder the temperature the greater the degree of reflux, and the higher the proportion of lighter flavour compounds.
“We’ve done experiments that show spirit produced without using a water jacket contains higher levels of sulphur compounds. Being pungent and assertive, higher levels of sulphur compounds conceal lighter fruit notes and sweetness, so the water jacket must be in operation during the spirit run as it has a significant influence on spirit quality,” says Stuart Robertson, Dalmore’s distillery manager.
At Fettercairn, reflux is increased by spraying cold water onto the external surface of the spirit still using a cooling ring fitted around the top of the still. Spray holes ensure the still is evenly sprayed and cooled. Water takes about five seconds to reach a trough at the base of the still, from which it drains.
“Spraying water onto the still produces new make spirit with floral, fruity notes, and background nuttiness that increases the complexity of the sweetness. When we experimented without the cooling ring, the resulting spirit had more prominent nuttiness and less sweetness, with the floral notes also relegated,” says Stewart Walker.
Scotland’s mild climate promotes gentle, gradual maturation, while hotter climates increase the rate of reactions occurring in the cask, including evaporation. However, this should not be considered as achieving the same result in a longer/shorter time-frame, as the resulting whisky will show differences.
In the US state of Iowa, for example, where the Templeton distillery is located, the temperature can reach 7 degrees centigrade in winter, and 27 degrees centigrade in summer. This raises the question of whether to allow maturing whiskey to experience such fluctuations, or create a managed environment.
“The external temperature has no bearing on our maturation warehouse, which is not like a rickhouse, it’s state-of-the-art and fully insulated, with the temperature maintained at a constant 18-21 degrees centigrade which in turn controls the humidity levels. Additionally, barrels are stored six high, to ensure that they all experience the same conditions within the ageing warehouse,” says Shane Fitzharris, executive vice-president, global sales, Templeton.