alikocho wrote:I also wonder why so much of the literature, pro-brewers and hop processors talk about flashing off aroma compounds at temperatures if what you say is true. Could you point me at some references?
Perhaps we need to look at some basics. Suppose you have two liquids that are in solution together. That means they're inter-mixed at the molecular level, as opposed to simply oil floating on water. Lets take alcohol and water as an example, as it's a common situation, but the principle applies to all other solutions.
Alcohol (ethanol) boils at about 78C and water at 100C. Now suppose we heat a mixture of the two. If you've never studied this, I guess you'd expect that as you approach 78C, the alcohol in the mixture would "flash off" (to use your term). The temperature would hover around 78C until all the alcohol had gone, and then you'd be left with water. The temperature would then rise to 100C where the water would boil.
But that's not what happens at all. The mixture actually boils at some intermediate temperature between 78 and 100C, determined by the relative concentrations of alcohol and water and the detailed molecular interactions between them. Below this compromise temperature, the liquid doesn't boil, so losses to evaporation are very small, as with most non-boiling liquids. Some vapour is lost, but we're talking days rather than hours before much effect is noticed.
Once the boiling point is reached, the vapour that is lost is a mixture of alcohol and water. The relative proportion is determined by the vapour pressures at the boiling point, so with alcohol having a somewhat higher vapour pressure, somewhat more alcohol than water will be lost. This makes the alcohol concentration in the remaining solution fall slightly, and this raises the boiling point. So as the solution slowly boils away, its temperature rises and the concentration of alcohol falls. But the loss of alcohol is gradual. It doesn't all leave at a single temperature and a small amount persists until the entire liquid has boiled away. To have an appreciable effect on the alcohol concentration, you need to lose a significant volume of the solution to evaporation, even if the initial alcohol concentration is small. This takes some time to happen.
Turning to other solutions: in general, you expect the substance with the lower boiling point to be lost more rapidly once the solution boils. That's because a lower boiling point generally implies a higher vapour pesssure at any given temperature. As boiling continues, the concentration of this compound in the liquid will slowly fall. In our case, thanks to dcq1974, we know that the aroma compounds we're interested in all boil at much higher temperatures than water. So you'd expect that boiling a solution of them would actually *concentrate* the aroma in the wort rather than disperse it. Not what you might expect, perhaps, but I think that's where the science is pointing.
Of course, our wort isn't boiling. So although the composition of the lost vapour will be similar to if it were, the amount of lost vapour will be much smaller and things will happen even more slowly.
Most of this is a consequence of the well-known Dalton's Law. You can find a discussion of much of it in this Wikipedia article on distillation (in the Idealized Distillation Model paragraph in particular):
https://secure.wikimedia.org/wikipedia/ ... stillation
As to why there's so much reference to "flashing off" aroma compounds amongst brewers? Well, possibly because not many brewers have backgrounds in physics or physical chemistry and because the science behind distillation isn't really all that intuitive. Also, aroma compounds that aren't in solution can be lost far more easily, although I rather doubt that anyone processing hops ever gets them that hot.
Less, charitably, one might suspect that purveyors of fancy hops deliberately use the term and quote the "flash point" of certain compounds in the hope that people will conflate this with "boiling point", apply some bad science and reach the wrong conclusion (which is, of course, the right conclusion if you're selling fancy hops).
alikocho wrote:I'll buy into the notion that it stays in the can, but equilibrium still equals loss from the wort in this situation. And how long for that equilibrium to be achieved?
I don't see what's fanciful (Capn Ahab) about putting a lid on your boiler while steeping. It strikes me as eminently practical, if only to keep the flies out.
Equilibrium will probably be achieved within a few seconds (how long does it take to mist a piece of glass placed over the boiler?). And with typical vapour densities being around 0.001 of the liquid density, I don't think you'll be noticing the little bit that gets lost when you take the lid off.
But that's assuming you need to trap the aroma in the first place. I think the science says that you don't. Personally, I'd be worrying a lot more about what chemistry is going on in my wort. The effects of temperature on reactions involving flavour and aroma compounds seem much more relevant to me.
