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Comment by mirimir | Hacker News Reader

So guaiacol boils at 205°C. How does it end up in the whisky, after distillation? I mean, the water should come over before the guaiacol does.

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Guaiacol is present in wood smoke. It's maybe being leached out of the charred barrels into the whiskey during the aging process.

Right - it's the barrels that infuse a lot of the flavor.

That makes sense. But the paper seems to claim that the guaiacol comes from the smoked malted barley:

> Many whiskies, especially those that are made on the Scottish island of Isley, have a typical smoky taste that develops when malted barley is smoked on peat fire. Chemically, the smoky flavour is attributed to phenols, and in particular guaiacol, which is much more common in Scottish whiskies than in American or Irish ones.

I agree that this is where the smoky flavours are supposed to come from. So your question remains unanswered. Perhaps guaiacol is generated from intermediate compounds after distillation? From chemical reactions with the barrel?

I wasn't able to find a source in a quick search, but I wonder if (traditionally) the charring of the barrel is also done with peat.

No, it isn't. Scotch whisky can only be aged in barrels that have already been used to age liquor. Conveniently, bourbon must be aged in NEW barrels, so used bourbon barrels are the primary source used for scotch. So, the barrels are sourced for the US and thus not charred with peat.

Other barrels used for scotch are typically fortified wine products, like sherry or port. Occasionally limited releases will be aged in things like ex-rum or ex-brandy casks.

It does mention "It is likely that the concentration of guaiacol in Isley whiskies is even higher.", since Islay whisky often uses peated malt, won't at least some of it be coming from the malt.

As far as I understand, they don't treat the casks differently to other distilleries.

I believe it is present in both the original distillate and also is leached from barrels. Some info here on the leaching from barrels:

AGING OF WHISKEY SPIRITS IN BARRELS OF NON-TRADITIONAL VOLUME

http://etd.lib.msu.edu/islandora/object/etd:1059/datastream/...

Scottish (and most, although not all, other types of) whisky is produced using a "pot still" rather than a modern column, so you don't get an exact separation by boiling points. I mean, generally you're running the still at under 100C, but you get a substantial fraction of water (~30%) in the output as well.

I suspect that if you plotted the still's throughput as a function of boiling point of the inputs, what you'd get is a fairly broad normal distribution centered on the temperature of the still "head" during the distillation run, with the width of the curve dependent on the design of the still. The upper tail of that curve could easily (must, really) be catching small amounts of substances that boil at 200+C.

OK, thanks. So they use a still with low BP resolution, and so there's appreciable higher BP stuff in the product. But doesn't that also make it harder to exclude methanol and other low BP stuff?

Liquids partially vaporise below their boiling point (eg water vapour in humid air). See http://pubs.acs.org/doi/abs/10.1021/jf60134a022?journalCode=...

Neat, that's very interesting as it does mention it's present in the ferment.

There's also the possibility of forming an azeotrope (like with ethanol/water at 96% ethanol). Azeotropes are cool, they're exploited a lot e.g. as working fluids in heat pumps.

Interactions between ethanol, water and guaiacol are a key concept in the article. So there could be an azeotrope, I suppose. There's one for anisole aka methoxybenzene. Its BP is 153.8°C, and the anisole-water azeotrope (40.5% by weight) boils at 95.5°C.[0] Guaiacol is 2-hydroxyanisole, so maybe there's one for it too.

0) Horsley (1947) Table of Azeotropes and Nonazeotropes http://sci-hub.io/10.1021/ac60008a002

Well, the water DOES come over. Whiskey is distilled to at most 80% AL so you get 20% water with other things coming along for the ride. If you go much higher then 80% you start loosing so much flavor that you might as well be aging vodka.

That said, most of the flavor still comes from the barrel.

Water boils at 100C, so shouldn't the guaiacol come over first? Or am I misunderstanding how distillation works?

Nope, it will come out last.

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