In conventional fermentation, yeasts turn sugars into ethanol. Ethanol, as everyone knows, is promising as an alternative fuel. The problem is that yeasts die at concentrations higher than about 10% ethanol by weight, and so the fermentation process can at best produce alcohol that is 90% water. Obviously, this "beer" cannot be burned as fuel, and the excess water must be removed somehow, by distillation or adsorbtion, which adds a significant energy cost to each unit alcohol produced. At the earth's equator, solar energy can be relied upon to make up this energy cost. At the more extreme latitudes, that's not necessarily the case.

Ethanol is not the only alcohol produced in fermentation. Higher alcohols such as butyl, amyl, isoamyl, and 1-hexyl are also produced, albeit in trace concentrations. As a fuel alcohol, 1-hexanol has a lot going for it compared to ethanol. Firstly, it's much "greasier" than ethanol, having a thrice-longer hydrocarbon tail, and thus will handle and burn much more like the hydrocarbon fuels we're already using. Second and most importantly, however, unlike ethanol, 1-hexanol is *not* infinitely soluble in water, meaning that at some concentration the fuel and the water will simply phase-separate. Now, instead of having to spend energy to dry the alcohol, you just tap it straight out of the bioreactor at burnable concentrations.

The only reason we're not doing this already is that (known) yeasts don't produce useful concentrations of 1-hexanol. But because they're microorganisms and they reproduce rapidly and in huge numbers it's not inconceivable that they could be bred to do so. What's needed is a rapid, colorimetric, quantitative assay for hexanol concentration so that thousands of individual yeast cultures can be rapidly screened in high-throughput equipment like plate readers. Without such an assay, chromatography of some sort is required, slowing the process of screening down by many orders of magnitude. With the right indicator, though, it would be possible to screen yeast cultures almost as fast as they could be selected and grown. A rate of 10000 generations per year is entirely reasonable. Note that 10000 generations is approximately the same "distance" that separates homo sapiens from neanderthals.
-- iamanangelchaser, Jan 31 2007

Formation of so-called byproducts from ethanol by microorganisms http://www.ncbi.nlm...37567&dopt=Abstract
[iamanangelchaser, Jan 31 2007]

Bioreacted Hexanol Production http://aem.asm.org/...t/abstract/39/4/720
[daseva, Jan 31 2007]

More work on this topic. http://www.actahort...ooks/388/388_36.htm
[daseva, Jan 31 2007]

Biology Could Be Used To Turn Sugar Into Diesel http://science.slas...0219248&threshold=1
discussed today on Slashdot. [xaviergisz, Feb 01 2007]

A good general source http://journeytoforever.org/ethanol.html
[Ned_Ludd, Feb 02 2007]

//Note that 10000 generations is approximately the same time that separates homo sapiens from neanderthals.// And in all that time, I doubt the output of our bowels has changed a great deal.

[I'm no archaeological coprologist (no really!) and while available outsourcing outfits (i.e. intesinal bacteria) might exist in different areas (and foodstuffs) - I'd be surprised if the end product today is largely different to what it might have been 5000, 50,000 or 50,000,000 years ago. Similarly, despite all manner of study on fossils, I don't remember hearing anything suggesting that dinosaur droppings were particularly outlandish (but I may have missed any such announcements) on account of my rich and satisfying social life.

All of this conjecture further stresses the point (since those providing the outsourcing services have significantly shorter generational time spans) that waste products (poo and alcohol alike) can (and apparently tend to) remain remarkably the same after innumerable hops down the evolutionary ladder. That being said, this is in an undriven environment - targeted breeding may yield different results.]

If we're going to breed yeast for energy production, might we also find varieties that can survive in higher ethanol concentrations?

But overall, and apart from these asides, assuming that hexanol can be produced biologically, this really could be a great idea.
-- zen_tom, Jan 31 2007


Well, given that there are already 1-hexanol variations in outputs of different yeast mixes (for example, Pichia fermentans makes more hexanol than common beer yeast), it sounds realistic - but the strains that people know a lot about seem to be studied as flavor, not fuel, so there are a few orders of magnitude to overcome here.
-- jutta, Jan 31 2007


//the same time that separates homo sapiens from neanderthals// I thought I was just separated by my cubicle wall.

Samuel Adams brewing company has developed a high-alcohol-tolerance yeast. They are making 25% alcohol Utopias beer with it. I can't recall much more, but it was on the National Geographic Channel, IIRC. If they sell some of that to the ethanol folks, this idea may never be needed.

But, if they can grow a special yeast, so can other folks. A good idea, a worthy goal and quite feasible. Sounds like a good crackpot project for a homebrewer to work on in his basement, then sell for great wodges of cash.

Massively yeasty croissant.
-- baconbrain, Jan 31 2007


Selective breeding? Come on, Gregor! These are yeasts we are talking about! Genetic engineering is what you want!

That said I am puzzled over the hexanol "production". I wonder if this is something the yeast is actually producing, or if it is a secondary reaction product between ethanol and other things in the fermentation chamber. My hazy recollection of anaerobic glycolysis has no cameo by hexanol. [Chaser], if you know the equations or can link them up, please do so.

It would seem that there would already be a considerable selective advantage for wild yeasts to be able to convert to hexanol rather than ethanol: one can produce more offspring if one can avoid being killed by ones waste products.
-- bungston, Jan 31 2007


I don't know about balanced equations, but I posted a relevant PubMed abstract to the effect that higher alcohols and carboxylates are actually produced from ethanol.
-- iamanangelchaser, Jan 31 2007


They are pushing ethanol here in farming country, but a lot of people think it is all hype by big businesses to get government money (God bless those conservatives). We may see.

//killed by ones waste products// It seems to me that the best way to start breeding hexanol yeasts is to put a variety of yeasts in test tubes, and start adding hexanol. Whatever lives is probably good at making hexanol.
-- baconbrain, Jan 31 2007


[bigsleep], most analysts agree that ethanol from biomass would be a viable alternative to gasoline if the energy costs associated with removing the water could be overcome. Hence the desirability of hexanol fermentation, which could provide an essentially equivalent fuel and eliminate the costly drying step.
-- iamanangelchaser, Jan 31 2007


Sugar has only 5 carbons. Where are you going to get the sixth?
-- daseva, Jan 31 2007


[GB] - not sure what sort of sugar you are talking about. Glucose has 6.

But your point re # of carbons is good. The hexanol must be produced synthetically, probably starting from shorter molecules.

The comments about whether ethanol is a good biofuel miss the mark here.
-- bungston, Jan 31 2007


Oh, right. I forgot about that little one hanging off the end there, was only thinking of the ring. Pentoses have five carbons, but that's not a very good point. Well, then. Good thinking!
-- daseva, Feb 01 2007


This does have potential. Personally, I've wondered why the government wants to rot corn to fuel cars, when they should be able to just as well rot wood to make methanol...

All the same, both of those alcohol fuels have problems, and I must admit your hexanol sounds like a better candidate... IF yeast really produces it, and IF it is not just a byproduct of lots of ethanol in water, and IF it separates out at concentrations which are not also lethal to the yeats.

I would admit that the complexities of selectively breeding yeast are not as great as some believe, but some GM might be useful here. Of course, using that in an idea is often a request for fishbones...

I suspect you'll need to breed your yeast for more than just hexanol production. As bungston points out, you might want them to be able to live in hexanol better than current yeast can... not because this indicates that they produce hexanol, but because if they did, they might have to live in some pretty high concentrations. All of this MIGHT be within the ability of a dedicated home brewer, but I don't think most home brewers want a lot of hexanol in their drink. As I recall, methanol, and many other alcohols are far more dangerous then ethanol as far as consuption goes.

Burining plant products directly might indeed be a more reasonable energy producing method, but that is an old idea, whereas yours is new, and apparently original.
-- ye_river_xiv, Feb 01 2007


... for instance, entire systems of economy, politics, culture, and ecology.

What is often missed in the ethanol debate is the question of scale. To my mind the beauty of traditional hooch-style ethanol is that the economic viability relative to petroleum fuels improves as total systemic demand diminishes. Ethanol must be considered in conjunction with a vast reduction in overall consumption of liquid fuels.

The hexanol thing sounds good, but only if it doesn't require processes that are extremely sensitive to economies of scale.
-- Ned_Ludd, Feb 02 2007


I am in agreement with bungston. Get in there and find the gene.
-- Cuit_au_Four, Feb 02 2007


That's actually a very good idea. Selective breeding and genetic engineering would probably involve a roughly equivalent amount of time and effort, for yeast, at this point.

A race?

Update: It looks like a bacteria called clostridia produces n=4-5 carbon solvents at the end of its' life cycle, and Science is hard up on it already.
-- GutPunchLullabies, Feb 02 2007


Do you have a reference, Lullaby? I notice you capitalize "Science"--is that the magazine or the institution you refer to?

It seems to me the easiest way to find the gene is to compare sequence data from high-hexanol and low-hexanol or no-hexanol (if such a thing exists) yeast cultivars. Maybe there's some other way that I don't know about, but if not, you're committed to a selective breeding program before you can isolate the gene, anyway.

To the argument that cultures should be subjected to high hexanol concentrations as a condition of selection, I would point out that the limit of solubility of hexanol in water is 0.6% by weight, which is a heck of a lot less than the 10% at which ethanol starts to kill yeast. I would be surprised if any yeast which was viable for ethanol brewing would not survive totally saturated aqueous hexanol. So, certainly that condition could be applied, but I don't think it will be very difficult to meet.
-- iamanangelchaser, Feb 03 2007


I did a study of the thermodynamics of potential fuels a few years back, and I thought I'd add some of my findings. While you're correct that increasing the chain length of an alcohol increases its alkane character, it also decreases the vapor pressure. n = 6 to 8 is great for alkanes because they're volatile liquids; n < 6 must be stored under pressure, and n > 8 will start having problems with higher flashpoints, low combustion efficiency, and, consequently, more hazardous byproducts such as carbon monoxide, formaldehyde, etc.

The advantages of using an alcohol instead of a hydrocarbon are based upon the fact that n < 6 still gets you a liquid that can be dispensed without dangerous and expensive pressurization. If the same were true of alkanes, then they, too, would boast higher efficiency and lower emissions because these are almost entirely a function of n, given identical combustion conditions. Thus, the real advantage of alcohol fuels is that we CAN use ethanol safely.

Using n > 4 alcohols runs into precisely the same problems as using n > 8 alkanes. They do become oil-like, but mostly to the effect that they are insoluble in water. They also become less volatile and suffer from inefficient combustion with greater emissions.

The main advantage I can see to your idea is that having a hexanol layer separate from the culture might draw ethanol out of the aqueous phase. I'm not certain, but I would guess ethanol and hexanol are miscible. This would allow the bioreactor to continue producing ethanol without killing itself off.

Good thoughts.
-- sqykly, Feb 04 2007


[sqykly] - that comparison of alkanes with alcohols is very interesting. The advantage of ethanol is that we can use it - well said. Thanks for making an account to post it. As regards the possible benefits of hexanol, ethanol is so soluble in water it is hard to imagine in would be more soluble in hexanol.
-- bungston, Feb 04 2007


Right, but I think he's saying that maybe we can feed fresh hexanol into the bioreactor to remove the ethanol and keep the cells in a friendly environment.
-- daseva, Feb 04 2007


That probably would work, to some degree. Even if less soluble in hexanol, some ethanol would go into a hexanol layer,and so not be in the aqueous layer with the organisms. This is a good idea, but a different idea than the one initially proposed - one could accomplish it by simply adding hexanol to the vat. No need for special yeasts.
-- bungston, Feb 04 2007



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