Diageo Malting Plant: Silo Collapse

I was sad to learn that the Port Ellen malting facility on Islay was damaged yesterday. Luckily no one was hurt, but hundreds of tons of barley was in the silo when it collapsed.

Grain elevators in the US mid-west frequently explode because of airborne dust which is highly flammable, even explosive. Presumably we’ll know in a few weeks what the cause of this silo collapse was; it could have been explosive dust, or perhaps a structural failure.

What’s a malting plant? Malting is one part of the whisky production process that is centralizable. Malting is the process whereby the barley is sprouted by wetting it and letting it sit, historically it was spread out on a large malting floor, for several days. The need for a large floor was why malting benefited from economies of scale, however modern malting has improved upon the floor malting which can be adversely affected by weather (variations in humidity, primarily). Malting is still, for the most part, centralized even though floor malting has pretty much disappeared.

Malting is how the whisky producers crack open the barley to expose the sugars to the yeast that will be used in the next phase of production. If the barley seed were sprouting in a farmer’s field, the carbohydrates inside the barley seed would be used by the nascent plant to provide energy for its initial growth. Whisky producers need those carbohydrates (sugars), so the plant can’t be allowed to grow beyond its initial sprouting. The sprouting process exposes the tightly locked complex carbohydrates and enzymes. To stop the growth process before it goes too far, the barley is heated and dried which stops the growth and preserves the sugar for the yeast.

The malting process involves both sprouting (germinating), then drying the barley. The drying is facilitated by heat produced by burning coal and/or peat, which may impart desirable flavors to the malt. Once the malted barley has been produced, it can be shipped to a distillery for the next phase of the production process, wherein the malted barley is ground and mixed with hot water, which facilitates the action of the enzymes which convert the complex carbohydrates in the powdered malt into simpler sugars that are palatable to yeast. If you are familiar with the production of beer, you’ll recognize much of this process. Once the yeast is added, it produces, over the course of several days, a weakly alcoholic solution at about 6-7% ABV (as the alcohol concentration is increased, the alcohol kills the yeast, which puts an upper bound on the amount of alcohol that the yeast can produce). The next step, distillation, concentrates the alcohol.

How does distillation work? It’s not magic. Alcohol is more “volatile” than water. This is a term that has specific meaning for chemists, and it basically means that alcohol boils at a lower temperature than water. If you have a liquid that contains some alcohol and some water, and if you heat it in a precisely shaped container (e.g., a pot still), the alcohol vapor can be induced to condense back into liquid form.

If the condensation process were allowed to reach equilibrium, e.g., in a vertical tube, the alcohol would condense on the walls of the cylinder (if it were tall enough) and drain back down to the bottom, from where it would be heated enough to evaporate again. The reason it condenses is that the tube is cooler as one moves farther away from the heat. No matter how tall the vertical tube is, some alcohol vapor will escape over the top. My sense is that this is why the pot still is bent at the top, so the whisky producer can contain the alcohol and control the rate and amount of condensation within the still.

Also, keep in mind that the vapor that goes up the tube is a combination of water vapor and other volatile chemicals that boil at less than the boiling point of water. Even if the liquid is only kept at the boiling point of alcohol, there will still be water vapor present in the atmosphere above the liquid, since water evaporates even when the liquid is less than 100 °C.

Per wikipedia: “The boiling point of the alcohol ethanol is 78.29 °C, compared to 69 °C for the hydrocarbon Hexane (a common constituent of gasoline), and 34.6 °C for Diethyl ether.” The initial condensate, then, will contain lots of other volatile chemicals that may have foul odors or tastes (these odors and tastes come from fusel oils and other chemicals associated with alcohol production that may remind one of paint thinner, acetone, etc. — you wouldn’t want to drink them!). The tough part at this phase of production is that the foul odors and tastes will be soluble in water or alcohol, so they may be difficult to separate from the more desirable esters and phenolic compounds that whisky producers may want to retain in the finished product.

Once the pot still (actually, a pair of stills) has effectively burned off the more volatile components, the refined alcohol is allowed to freely flow to the “spirit safe” where the amount produced is measured very carefully for tax purposes. The operation of the still involves the careful attention of the stillman who determines when the proper product is ready to be collected, and who knows when to cut off the production before the still runs dry.

The spirit is usually distilled at least one more time before going into oak barrels for aging. Bruichladdich has produced “X4” that is quadruple-distilled and due to the concentration being increased at each stage of distillation, the ABV of the final product was well over 90%. Some Scotch is triple-distilled, as is most Irish whisky. The initial alcohol concentration of the spirit that goes into the oak barrels is about 70% ABV. Then the aging process reduces that concentration as the alcohol gradually escapes the semi-permeable membrane that is the wood while the wood works its magic of imparting its flavors to the liquid. As long as the alcohol concentration remains above 40% ABV, the liquid in the casks can be bottled and sold as Scotch whisky, provided it’s also at least 3 years old and meets a slew of other technical requirements.

Now that I have written some basics about distillation, I will go into some more detail about types of distillation and explain in a bit more detail why distillation works at all.


1 Comment»

[…] collapsed on 14-November (a rude awakening for the neighbors at 0600!) it damaged the other silo. I wrote about the first silo collapsing as soon as I heard about it. Ultimately the second silo was demolished a few days later. […]

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