Introduction
More consultant-baloney that needs to be addressed. I read an article that “researched” distilling at altitude. The consultant’s conclusion was that craft distillers located at higher altitudes have the amazing benefit of “gentler boils”. And the article implied that these craft distillers have a benefit to others, located closer to sea level, as these low-altitude distilleries are confronted with more aggressive boils in their stills.
Now, everyone is entitled to their own opinion. But no one is entitled to their own facts. The right attitude to distilling at altitude is not based on an opinion, but on science. Science perfectly explains how distilling at altitude differs from distilling at sea level. Spoiler alert: the consultant that claims high-altitude distilling is favored by a more gentle boil in his still is completely wrong. Science teaches us it is the exact opposite.
Air pressure
The earth is surrounded by a layer of air. This air asserts a pressure downward, on the earths crust. Or on the people, distilleries, and stills on that crust. A lower location results in a bigger layer of air above your distillery. A bigger layer of air above your “lowland” distillery results in high air pressure.
A distillery that’s located at 2,000 meters or 6,000 feet could be called a “highland” distillery, or a “distillery at altitude”. The layer of air above this location is smaller, resulting in lower air pressures at these higher altitude distilleries. Why is this important? Because air pressure is one-on-one related to boiling temperature.
Pure ethanol boils at 78.3 Celsius. At sea level, that is. In Utah and Colorado, visiting distilleries at altitudes of 2,000 meters and more, I have seen boiling points drop to as low as 72 degrees Celsius. How come that high-altitude distilleries are faced with lower boiling points of pure ethanol? Because there is less air pressure.
A boiling liquid uses all the energy input to form gasses. The bigger the air blanket, the higher the air pressure, the more back-pressure those gasses are met with. Higher pressure, including higher air pressure, results in higher boiling points of the liquid that generates the gasses. Contrary, at high altitude, in a low air pressure situation, gasses are not pushed back as hard. They find less resistance and the boil starts earlier because of it.
Slightly more scientifically?
Liquid boils in order to remove a surplus of energy that is introduced in the system. The way the substance can cope with the surplus is by getting molecules in a gaseous phase. The bigger the air blanket, the higher the air pressure, the higher the vapor pressure is, resulting in higher boiling points of the liquid that generates the vapor. Contrary, at high altitude, in a low air pressure situation, vapors are not pushed back as hard. They find less resistance and the boil starts at a lower temperature because of it.
Gentle boil, really?
As air pressure creates “resistance” to the boil, it slows down both the boil and the vapor speeds of the resulting gasses. More air pressure equals a slower boil and slower vapor speeds in the column. Less air pressure results in less resistance in the column, and thus in a more aggressive boil!
There you have it: the consultant was wrong. Completely wrong. And there is a risk that many craft distillers read his article and thought they needed to be afraid of not being high up in the mountains. They shouldn’t be afraid. It is 180 degrees different from how the consultant presented his conclusions. It is the sea level distilleries that are faced with the gentler boils, because the higher air pressure dampens both the boil and the speeds of the gasses exiting the liquids because of that boil.
The right attitude
You distill in a location, because you live there. Or because you expect it to be a city full of commercial opportunities. But, please, don’t move up the mountain, because of this consultant’s completely wrong assessment that it would lead to more gentle boils, implying “better product”.
Here’s what is really going on, when we compare a sea level distillery to a high-altitude location. The high-altitude distillery is faced with a more aggressive boil and higher vapor speeds in his column. This results in more smearing of heads and tails into hearts. All things equal, a higher location results in less control and a more contaminated product, that needs more aging.
There is an additional disadvantage for high-altitude distilleries. Air pressure changes and their relative influence. Sea level distilleries have a big air blanket above them. If the air pressure changes (different weather fronts coming in, for example), it changes a little bit. For distilleries at altitude, even small changes are RELATIVELY bigger, resulting in more variability in vapor speeds and more variability in smearing and flavor (like the “bumping” issue in vacuum distillation, discussed in an earlier iStill Blog post).
How to manage altitude
In order to compensate for the lower air pressure at higher altitudes, the distiller needs to power down. A lower power input results in a less aggressive boil and lower associated vapor speeds. Sometimes as much as a 50% reduction in power is needed, but this obviously depends on still type, spirit, and location.
To compensate for the higher variability in vapor speeds, in distilleries at altitude, the distiller needs to invest in a still that has power management, air pressure sensors, and automated cuts management. Power management allows the distiller to compensate for low air pressure conditions. An air pressure sensor can detect changes in air pressure, a computer can translate these changes into new cut point temperatures, and an automated cuts selector can translate these outcomes into perfect cuts, each and every time.
Hmmm … if only there were such a still. A still with 1-100% power management. A still with seven digit accurate air pressure measurements on a per second basis. If only there is a still with a computer and software to translate air pressure changes into new cut points, instantly and automatically … wouldn’t that be an amazing innovation? If only …
iStill’s air pressure sensor measures every second with seven digit accuracy …
The iStill Blog 
I just read an article named “The Benefits of Distilling at Altitude” posted by an American Rum distillery and they claim that:
“A LOWER BOILING POINT LEADS TO BETTER FLAVORS”.
“EASIER TO CONTROL TEMPERATURES OF FERMENTATION TANKS”
They further claim that: “At altitude, the rum naturally moves in and out of the barrel pores more often.”
and
“If we made rum at sea level and put it into an empty oak barrel, 20 to 25% of the rum would move into the pores of the barrel on day one and stay there. The other 80 to 85% of the rum would stay in the center of the barrel and never actually come into contact with the wood.”
🙂
Debunked that lower bp leads to better flavors. How temps during ferments can be easier controlled, baffles me. In fact, scientific research shows yeast performs better in a slightly higher pressure environment (Brakenhoff e.a., 2020). Finally, these people seem to think wooden barrels are … pressure vessels? 🙂
Anyhow, Daniel, thanks for sharing! SO much BS going around …
Regards, Odin.
Atmospheric pressure isn’t the only force to consider in boiling. Water is polar-covalent so the molecules are attracted to each other. Alcohol does not have that attractive force and so while the alcohol becomes a gas more easily with lower atmospheric pressure, water still fights to not boil. Gentler boil? I would think so if kept at a lower temp.
Hi Eric,
Thanks for reaching out and contributing. Correct, atmospheric pressure is not the only force to consider if we focus on the boil. Water is polar-covalent, but not sure that’s an argument for water molecules being attracted/attached to each other. I’d call that a result of the Van der Waal forces. As you know, those Van der Waal forces also apply to the alcohol molecules.
The temperature in the boiler, while distilling, is – in general – the result of the exact alcohol/water mixture. Lower power inputs do not result in lower temperature boils. Further, the water does not “fight” the boil.
What is at stake here are 1. the vapor speeds; 2. the variability of those vapor speeds. In a boil any excess energy input is translated to gas formation. Higher power input won’t change the temperature in the boiler, but will result in more gases being formed. Secondly, since more gases are released from a specific surface area, at higher power settings we’ll see a more aggressive boil with much more liquids (bubbling up) disturbing the gas bed above the liquids, from which the column or riser draws. So more variability in vapor speeds as well.
Higher vapor speeds translate into more smearing. More variability in vapor speeds result in less control over smearing.
Now let’s expand the above to the high altitude distillery. The liquid boils sooner and with more ease, and gases meet less (air) resistance. So at equal power inputs to a lowland distillery, we’ll see more gasses being produced AND a more aggressive boil at the high altitude location. That’s more smearing and less control over smearing, so by definition bad news for the craft distiller. Okay, so they need to compensate by turning down the power, to make those boils LESS aggressive. Great. Lower vapor speeds, so equal smearing as a lowland location. But still, variability in vapor speeds (as air resistance is more volatile and changes more) will still be an issue.
All the outcomes, from whatever way you look at it, make the definition that high altitude distilleries are favored by more gentle boils wrong. Their boils are more aggressive and the vapor speeds are less predictable, therefore less controlable.
Regards, Odin.
Hi again Odin,
This is all really interesting to me because I know virtually nothing of distilling and love Scotch!
That said, and I’m not arguing because again I know so little that I had to look up “smearing”, the Van der Waal force is very weak and applies to non-charged particles (like alcohol). Hydrogen bonding, on the other hand is a fairly strong influence on the energy required to allow changes of state.
My thinking is that by slowly turning up the energy added to a mix of alcohol and water, the alcohol will change states from liquid to gas at a much lower temperature and in so doing will pull energy from the mix, keeping the water from changing states and remaining a liquid longer as the temperature is slowly added to the mix. As the alcohol continues to change states, pulling heat energy from the mix, the water will continue to resist, by way of the hydrogen bonds that form between the slightly positive side of one water molecule to the slightly negative side of its neighboring water molecule.
Once most of the alcohol is released from the liquid mix, I would think that the water would, in fact, boil rather aggressively.
Thanks for the explanation of the process. It really is fascinating all that goes into the science/art of distilling.
Eric
Thanks for sharing your thoughts as well as motivation, Eric. It takes a curious mind to learn a lot, and I know one when I see one. Please understand that the boiling point isn’t lower or higher when more or less energy is added. The bp is the result of the mixture of alcohol and water. More alcohol? Lower bp. Lots and lots of water? A bp closer to the bp of water. So … unfortunately your approach would not work.
Where more or less energy input does make a difference is in the make-up of the gasses produced. More energy and a more aggressive boil means higher boiling point alcohols and water make it over into the vapors. So lower ABV and more smearing. Less energy results in a slower distillation process, and relatively more low bp alcohols and less water coming over. But in the vapors, that’s key.
You might want to register for the iStill University course if the distilling bug is starting to bite you. Lots of great info there!
This is a fun conversation to wander into through my phone’s newsfeed! Apparently, my phone knows my very specific interests. 🙂
I was curious why we aren’t considering the latent heat of vaporization (the amount of energy it takes to convert liquid into vapor)? The boilup rate (flow rate of vapor) is the product of the power applied and the latent heat of vaporization. For most materials, the heat of vaporization is slightly higher with lower pressures.
Higher altitudes = Lower pressure = Higher heat of vaporization = Slower vapor flow rate at a given power setting
Going against that trend, the lower boiling point means the still is cooler during operation. It would also take less energy to heat it up at the start of a run, and we’re losing less energy to the environment. With less heat loss, more of the energy goes to converting liquid to vapor, leading to a higher vapor flow rate.
Any other trends we should be considering?
Unless you have an underpowered or poorly controlled power supply, I agree that these atmospheric pressure effects are not going to make a huge difference on your still design or product. Thanks for all of your insight, and I appreciate you publicizing these interesting topics!
Thanks Brad, for such an insightful post & reply. Could you elaborate a bit on higher/stronger bonding at lower pressure? Especially as we see more vapor formation at lower pressure?
It’s more accurate to frame the conversation around energy to vaporize rather than just bond strength. Bond strength ignores the motion of the molecules based on their temperature. Grossly oversimplified: at lower pressure, the boiling point temperature is lower. The molecules in the liquid have less energy at this lower temperatures. So you need to add more energy for them to make the jump into the vapor phase. Again, grossly oversimplified.
If you want data on the energy to vaporize water at different pressures, a web search for steam tables should get you that information in any units you prefer. There should be a column labeled enthalpy of vaporization or heat of vaporization. Always happy to chat if you want to dig deeper on this or any spirits related topic.