Irish grain whiskey
In common with the Scottish whiskey industry, most of the Irish grain whiskey produced is used to produce Irish blended whiskeys such as Jameson, Powers and Tullamore Dew. However, there are fine bottlings of what are termed "single grain Irish whiskey" and grain whiskey plays an important, and arguably more important, role in blends as pot still whiskey does.
Typically, Irish grain whiskey comprises both malted barley and maize. The malted barley is ground into a coarse flour called 'grist' and separately the maze is also ground into fine flour.
The maize flour is mixed with water and heated in a 'high-temperature cooking' process by continuously pumping through a loop where the maize is heated to 150°C. This bursts the starch granules so liquefying the starch. This liquefaction takes place in just two to three minutes, the time it takes for the maize to pass through the loop. As it emerges from the loop it is flashed cooled to 63°C.
Separately the malt grist is mixed with water and then added to the cooled liquefied maize in a flash tank. When mixed, the malt's enzymes start to breakdown the liquefied starch into fermentable sugar. The percentage of malted barley used is usually around 5%. (If the malt was added earlier the high-temperature cooking process would de-nature the malt's natural enzymes so unable to convert starch to fermentable sugars.)
The malt and maize mixture is pumped from the flash tank to a cooker converter where further conversion takes place. The liquid flows like a plug down through this converter with a slow-moving agitator stopping material adhering to the sides. The agitator is there to prevent vertical mixing to ensure that each part of the mixture spends the same amount of time in the vessel – around 45 minutes.
A starch iodine test at this stage would show an incomplete conversion i.e. that some of the starch is yet to be converted into fermentable sugar. The mash (milled grain and liquid) is pumped from the converter through a cooler where it is cooled to 28°C and pumped into a fermenter. A key difference to the pot still brewing process is that the whole mash (grains and liquids)are pumped to the fermenter as opposed to just the liquid. This is because not all the conversion has taken place prior to fermentation starting so the grains still contain starch which can be turned into fermentable sugars.
A proportion of the sugary malt and maze mixture is put into a 'bub-tank' where the yeast is added. This allows the yeast to become accustomed to the sugary environment and start to multiply its cell numbers before being added to the main fermenter. The 'bubbed' yeasty mixture is added to the fermenter with the rest of the malt and maize.
Fermentation begins immediately with the already converted fermentable sugars being turned into alcohol, water and carbon dioxide by the yeast. Meanwhile, conversion of the remaining starch to fermentable sugars continues for the first 24 to 30 hours so there is a slow release of more fermentable sugars during this period. This delayed release of fermentable sugars actually helps the fermentation process as if all the sugars were available at the start the yeast could be stressed by the overly sugary environment.
Fermentation lasts around 90 to 100 hours. The beer-like liquid produced has a relatively high alcohol concentration of 13 to 15% alc./vol.. The 'beer' is pumped to the column stills.
After fermentation the beer is pumped to the 'beer-well', the column still's feedstock holding tank. The alcohol and desirable flavours in this beer are then extracted using three distillation columns - firstly the beer column, then the extractive distillation column and lastly the rectifying column. Each of these columns performs a different role.
The beer column strips all the alcohol and flavour from the beer. When the beer enters this first column it still contains grain solids. During distillation grain particles fall to the bottom of the still from where it is removed for incorporation into animal feed. The distillate produced by this column is known as 'beer column high wines' and is typically around 74% alc./vol..
The beer-column high-wines, which contain impurities such as heavier alcohols and fusel oils are pumped to the extractive distillation column where cleaning of the spirit begins. The high wines enter this second column halfway up while water is pumped into the top of the column. The interaction between the rising vapour from the high-wines and the water falling through the column's plates causes impurities to concentrate near the top of the column because they have a greater affinity or bonding mechanism in an alcohol rich environment as opposed to a water-rich environment. These top vapours condense and reflux down to a decanter where the fusel oils are extracted before the bulk of the condensate returns to the top of the column.
The ethanol alcohol accumulates lower in the second still in a weaker strength area known as the pinch point. From here the alcohol is drawn off and transferred to the rectifying column. Once the column is correctly set up a balance is achieved which allows the desired level of alcohol and flavour to be extracted from the pinch point. This distillate (the 'pinch') is typically 25% alcohol by volume when transferred to the rectifying column.
The last column, the rectifying column, removes additional fusel oils and some volatile alcohols known as overheads. These alcohols are recycled back into the extractive distillation column so no alcohol is lost from the system. The final strength of the distillate produced by the rectifier is typically 94.4% alcohol by volume.
This three-column process operates continuously, only being stopped for cleaning and maintenance. The first distillation column which strips the fermented mash needs cleaning once a week but the second and third columns can run for a couple of months without having to be cleaned.
Maturation, blending & bottling
The maturation process (see Irish whiskey production) that follows distillation tends to be shorter than for pot still of malt whiskeys but is a minimum of three years. Various styles of grain whiskey are produced with different proportions of 5% malted barley to 95% maize using the three-column process described above, each intended as a component to produce various brands of blended Irish whiskey. However, increasingly bottlings of single Irish grain whiskey are becoming available.
Single grain Irish whisky typically has flavours of toffee, vanilla and apple with toffee popcorn often used as a descriptor.