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My interest is primarily food-related here but I think this is a fairly general chemistry question. Let's say I took some liquid like lemon juice and ran it though a rotary evaporator to get just the volatile compounds. In the case of lemon juice I'd assume I'd wind up with mostly water, and some other volatile compounds (that would smell exactly like lemon juice does). To increase the intensity of this we'd need to somehow remove water.

Similarly a common technique for making extracts is to dissolve volatile compounds of common aromatic foods into alcohol. In case this process left any non-volatile compounds you could certainly use a rotary evaporator. You wind up with a somewhat similar issue where alcohol is the primary liquid.

I'm wondering if it would be possible to concentrate such mixtures so that they contain higher ratios of smell-able volatile compounds to water/alcohol. One thing I've noticed is that most of the characteristic volatile compounds in say lemons have a much higher molecular weight than water or even alcohol. Is there someway to utilize this to like centrifuge out a more concentrated liquid? Are there good methods for either water or ethyl-alcohol specifically?

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The simplest method would be fractional distillation: if you have a mixture of miscible liquids with different boiling points, as is the case in your question, then you can heat them slowly. When the boiling point of the most volatile compound in the mixture is reached, it begins to boil and the temperature does not increase until it has all boiled off. The vapour formed is cooled with a condenser and collected as a liquid. As soon as the temperature begins to increase again, all of this compound has evaporated, so it can be removed. Provided the boiling points aren't too close, you could use this to separate out all the compounds in the mixture. A diagram of the set-up (from Wikipedia) is below.

Fractional distillation

This would be easier with water than ethanol, since it has a higher boiling point so is less likely to boil at a similar temperature to your desired compound. (The most common way to separate out mixtures in chemistry is chromatography - for example, HPLC or GC - but I'm not sure you'd want to put the results in food! HPLC, which stands for high performance liquid chromatography, involves the use of a solvent mixture which normally includes chemicals like acetonitrile or methanol, neither of which are safe to ingest.)

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  • $\begingroup$ hmm, so something seems slightly off here. The most preventable volatile compound with an order in lemons is limonene (go figure). I know from personall experience that if you heat lemon juice up a bit (to say 160F, for 10 seconds) it loses almost all aroma. The boiling point of limonene is supposedly like 349F however. It seems likely to me that there must be some other mechanism causing these low concentration volatile compounds to evaporate off. Any idea what's happening? I suspect whatever mechanism is occurring here would make this technique invalid. $\endgroup$
    – Jake
    Commented Feb 12, 2020 at 19:17
  • $\begingroup$ OP: Of course it is off, it should be downvoted. Go back to the rotovapor that is basically steam distillation under reduced pressure. Except that contrary to your expectation you have to find limonene in the condensing phase, and get it off from there. Limonene is not soluble in water. $\endgroup$
    – Alchimista
    Commented Feb 13, 2020 at 8:46
  • $\begingroup$ Fair enough - the OP described it as a general chemistry question so I gave a fairly general answer. I'm interested by what you say about lemon juice, because my experience is totally different! If you put lemon tea with boiling water, for example, it smells very strongly. Limonene actually has a boiling point of 176 oC, which I found surprisingly high. $\endgroup$
    – atbm
    Commented Feb 13, 2020 at 17:14
  • $\begingroup$ It could break down at high temperature, of course - vitamin C does - but from what I could find online, I don't think it does break down at that temperature (160F is only 71 oC). $\endgroup$
    – atbm
    Commented Feb 13, 2020 at 17:16

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