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John Fraser McArthur
Dec 6, 2013
Scotch Whiskey: Microbiology of my Scottish background
True Scotch Whiskey (or just ‘Scotch’) is a very well defined product that legally must be made in
accordance with UK Law. Dating back to the late 15th century, Scotch has been the hallmark alcoholic
drink in Scotland and desired around the world. As both sides of my family can trace their roots
back to their Scottish roots, Clan Fraser and Clan McArthur, this drink is important to my
cultural roots.
There are two sources of Scotch whiskey; malt and grain. The only permitted cereal
(carbohydrate source) for malt whisky is malted barley. Grain distilleries may use any cereal
carbohydrate source provided it is hydrolyzed by enzymes from the malted barley, and the
alcohol may be recovered by continuous distillation (Production of Scotch Whiskey). The
majority of Scotch whisky brands are blends of malt and grain whiskies.
Malt whisky is distilled from the fermented hot-water extract of malted barley, with either a
pure culture of distiller's patented yeast or a mixture of distilling and brewing yeast being used.
Since temperatures less than 60C are used in this process, bacteria may survive the initial
fermentation process (though they can hardly compete with the yeast) and can be encouraged
by the distillery to grow after fermentation to add to the unique flavor of the
brand(http://www.ncbi.nlm.nih.gov/pmc/articles/PMC126549/). Many different species of
Lactobacillus can be found during these processes.
For grain whiskey, a different process is used for the preparation of fermentable wort from
unmalted grains. Grains (either milled to a pulp or whole) are cooked under pressure with
steam for approximately two hours, which quite effectively kills any bacteria. The cooked grain
is then cooled to 62C, and ground malt. The most important characteristic of the malt at this
point is that it has a high enzyme activity – that is, able to break down the complex
carbohydrates into simple sugars for the yeast to ferment.
Yeast used in fermentation is usually a strain of Saccharomyces cerevisiae, either a wild type or
a genetically modified strain. There are some key requirements for yeasts used in whisky
fermentation namely a fast rate of fermentation, efficient substrate utilization, a high ethanol
yield (and tolerance to ethanol), as well as a resistance to infection from invasive bacteria.
Scotch whiskey is then distilled to an ABV of <94.8% in order to ensure that the essence of the
flavors from the malts, grains, yeasts and bacteria are still present in the final product
(http://www.legislation.gov.uk/uksi/2009/2890/contents/made).