Distillation results in two different residues, which are technically referred to as co-products: pot ale from the first distillation, and spent lees from the second. Both represent significant volumes of liquid, which also play an additional role during the production process, saving a distillery significant amounts of time and money.
The quantity of pot ale remaining in the boil pot (ie. base) of the still is around two-thirds of the charge (ie. liquid being distilled). This means, for example, that a charge of 12,000 litres at Glenmorangie yields 4,000 litres of low wines, and leaves 8,000 litres of pot ale. Meanwhile, at Glenrothes a 12,500 litre charge results in around 7,500 litres of pot ale.
Pot ale resembles a pale yellow, golden liquid, with a malty, burnt cereal and yeast aroma. The key component is water, which is hardly surprising as one objective of distillation is to separate alcohol from water. Prior to the first distillation the wash (ie. fermented liquid) typically has an alcoholic strength of around 8-10% ABV, and consequently contains 92-90 per cent water. Pot ale also contains dead yeast, and proteins derived from the barley (effectively a ‘slurry’). The only trace of solids are fragments of barley husk, which drain from the mash tun. There’s also one other significant element.
“Pot ale contains alcohol at a strength of about 0.1% ABV, which is a standard figure in the industry. Trying to distill this remaining alcohol would use up too much energy for it to be commercially viable,” says Andy MacDonald, distillery manager, Glenmorangie.
The second distillation sees the low wines, which have an alcoholic strength of around 25% ABV (and consequently contain around 75 per cent water), distilled to produce new make spirit. This typically has an alcoholic strength of around 70% ABV.
The quantity of spent lees remaining in the pot still accounts for around one-third of the charge. This is significantly lower (proportionately) than pot ale, due to the low wines containing a lower level of water than the wash. At Glenmorangie for example, a charge of 7,500 litres results in 2,500 litres of spent lees. Meanwhile, at Glenrothes a charge of 15,100 litres results in approximately 6,380 litres of spent lees.
Spent lees is an oily, cloudy liquid, with a slightly oily aroma. Water accounts for the majority of the liquid, together with some ‘oily fats’ in liquid form (created during distillation), and the level of alcohol is around 0.1% ABV.
Meanwhile, the most significant characteristic of pot ale and spent lees at the end of a distillation run is the temperature, typically up to 98-99 degrees centigrade. This heat is utilised by draining pot ale and spent lees from the base of the pot still, and conducting each liquid to a plate heat exchanger, in order to increase the temperature of the wash and low wines prior to distillation, which saves time and the amount of energy used to heat the stills (scroll down to read about heat exchanges).
When pot ale and spent lees exit the heat exchangers, they each have a different route from the distillery, with an additional opportunity to utilise pot ale. This can be put through an evaporation unit and condensed into pot ale syrup, which is sold to farmers to feed cows.
“Pot ale has a ratio of 24 tonnes of water to 1 ton solids, and an evaporation process sees this ratio change to equal parts: 1 ton of water to 1 ton solids.
"The advantage of this is a smaller volume to deal with, which greatly reduces the number of tankers required to transport the pot ale syrup to farms,” says Alasdair Anderson, distillery manager, Glenrothes.
Opportunities to utilise spent lees are rather different, as is the eventual destination, but could this change?
“I’d love to see a further use for spent lees, and a lot of folks are researching this possibility, but so far no viable commercial use has been taken up. We send our spent lees to an effluent plant for processing, before being released into the environment, in accordance with regulations established by SEPA (Scottish Environmental Protection Agency),” adds Alasdair Anderson.
Plate heat exchangers operate on the basis that heat from hotter liquids (ie. pot ale and spent lees) transfers to cooler liquids (ie. wash and low wines).
Plate heat exchangers comprise a series of metal plates, with pot ale conducted on one side of the plates and wash on the other. Similarly, spent lees proceed on one side of the plates and low wines on the other. Heat transferring through the metal plates sees the pot ale and spent lees become progressively cooler, with the wash and low wines progressively hotter. Wash enters the heat exchanger at around 30 degrees centigrade, and low wines enter at around 15-20 degrees centigrade. Both exit the heat exchanger at around 70 degrees centigrade.
“Without a heat exchanger it would take 40-45 minutes to heat the still to 78 degrees centigrade, when the vapours begin to rise. With the heat exchanger this is reduced to 15 minutes. Saving energy is the biggest factor, but time is also important and adds up, enabling another distillation per week, which over a year is a lot. Within months the energy savings pay off the cost of a heat exchanger,” says Derek Sinclair, distilleries general manager, Inver House Distillers.
The stills at Glenmorangie