What makes a bourbon "hot"?

[Flyfish]

You might assume that alcohol is what gives bourbon its heat. But veteran tasters know you would be wrong. PHC at barrel strength, for example, has a lot less burn than Benchmark Old #8 at 80pf. So, there must be other chemical and/or esthetic factors that contribute to our perception of heat.
A corrolary: I think wheated bourbons tend to taste sweet because the underlying corn is not masked the way it is by the more pronounced spice of rye.
What accounts for heat if it isn't the alcohol? Any of you chemists or other bourbon geeks know what might mask the burn? (Unless, of course, I'm totally wrong and it is the alcohol.)

[Brisko]

I think tannin and other related wood compounds may play a role. If I'm not mistaken you can get a lot of tannin in immature whiskey as well as in over-aged whiskey. But that's just my guess. I'd love to hear an educated opinion.

[callmeox]

A lack of balance makes it hot.

[deathevocation]

I'm probably wrong but I generally identify "hot" with "cheap".

[HighHorse]

This weekend I had the pleasure of attending a function with David Perkins of High West. He brought with him some samples of the "heads" and "tails" from the distilling process. The "heads" in particular have powerful benzene-like tastes .. so strong that they're darn difficult to approach. A lot of bourbon and rye distillers cut the heads and tails from the process saving only the sweeter "middle" for the barrel. David - and others - utilize the entire run to capture the entire essence of the mashbill. When we watered down the head and got past the "heat", you could find some flavors in the heads and tails that otherwise would have been cut from the barrel. After that experiment, I'm pretty well convinced that these full-flavored runs carry a punch that's telegraphed immediately by the inclusion of "heads". That and Alcohol content.

[soonami]

What makes a bourbon "hot" is usually fusel alcohols, the congeners that are fermentation by-products which condensate with the alcohol. The reason the "heads" and "tails" from a distillate is culled is because these contain the isopropanol, acetone, butanol, benzenes, etc that have similar boiling points that ethanol. These are also the compounds that are much more likely to make you sick or hung-over the next day from over-imbibing.

[OscarV]

Good questio0n but I don't understand what HighHorse is saying.
You would think that hotness equals alcohol content but it's not.
I have had 80 proofers burn my tounge and 130 proofers that taste as sweet as caramel.

[Young Blacksmith]

Think of the distillation process as a bell curve. As the mash heats up, it releases chemicals, one of which happens to be alcohol. The heads and tails are what are at the up and down ends of the bell curve.

The idea sounds right to me, about the heads being more hot, having different chemicals in the distillate. Could explain some of the older dusty bottles that are variable, as they used more of a human hand back then VS. now.

[p_elliott]

I could be wrong on this so Chuck or John Hansell or someone like that jump in and correct me if I am. But aren't the heads and tails methanol? That's the stuff that's makes you go blind or can kill you?

[soonami]

Methanol has a boiling point of about 65 C, Ethanol is about 78C, so the heads likely contain some methanol, but the tails probably have very little of it.

Nice little article about how cuts are made by distillers to separate the heads, tails, and the desirable hearts:

In distilling parlance, the compounds in the wash that are not ethanol or water are called congeners. Some congeners such as acetaldehyde, methanol, and certain esters and aldehydes, have boiling points lower than ethanol, while certain other esters, the higher alcohols (fusel alcohols), and water, have higher boiling points than ethanol. This means the lower-boiling-point congeners come out in high concentration at the beginning of the distillation run, and the higher-boiling-point ones come out in high concentration towards the end of the run, leaving the ethanol as the most abundant compound during the middle of the run.

So, when distillation takes place in an artisan still, such as the reflux stills discussed above, the distillate that comes out is divided into three phases called: heads, hearts and, tails. The heads contain the unwanted lower-boiling-point congeners that come out at the beginning of the run, and the tails contain the unwanted higher-boiling-point congeners that come out at the end of the run. And, the hearts are the desired spirit.

Since whiskey is not distilled at a high-separation level, it means that each phase bleeds into the adjacent phase. That is to say, there‘s a considerable amount of ethanol in the heads phase, and there are late heads congeners at the beginning of the hearts phase. Similarly, there's a significant amount of early tails congeners at the end of the hearts, and there‘s a considerable amount of ethanol in the tails phase. The hearts are the whiskey, and whilethey are comprised mostly of ethanol and water, they have a delicate balance of late-heads and early-tails congeners that make up the flavor profile of the whiskey.

So yeah, there's stuff in the heads and tails that can make the whiskey taste hot, but these also make bourbon taste different than just vodka poured in to a barrel. These "congeners" or contaminants provide a lot of the flavors we like in bourbon like graininess, yeast fermentation by products, the fruity esters, spicy phenols from rye, etc. The problem is that in order to include the good stuff you want, you'll sometimes have to increase the bad.

As for the variability in quality of dusties, I think there are a couple reasons. There was just a glut of good stuff back then. When the brown liquor market was shrinking, a lot of the older quality stock that wasn't selling so they started trickling down to the lower shelf brands. For example, say Very Old Fitzgerald wasn't selling well. Then there's more 12 year old bourbon that isn't getting bottled that is taking up space in the warehouse and losing volume due to Angel's Share, so maybe to free up space, some of the good stuff was blended into OF Prime or Rebel Yell. So then there's be some great bottlings of OF Prime with the VOF stock and some of the regular stuff. In addition, although technology is better now, I think most distillers probably trust their palettes and noses more than a gas chromatograph. So maybe now with the whiskey industry a bit more stable in this country we are seeing better trained distillers that stick around and keep working rather than crews moving in and out.