Monday, 23 July 2012

Essential oil - steam

"Steam is the gas phase of water, which only occure at the boiling point. At 1 atmosphere (sea level, 1atm) thats 100 degrees C. At lower atmospheric pressure, its a bit less, (I'm no mathematician, but) I don't believe there's a single distillery producing atmospheric pressure essential oils anywhere in the world where water boils at less than 90 (probably 96) degrees C.

When you have a shower, you are not seeing steam; you are seeing visible droplets of water (called "mist") as a consequence of water vapour condensing with cooler air. In an atmospheric pressure still, it is impossible for mist to retrieve essential oils from plant material. This is because essential oil droplets, no matter how small, have to be boiled away from the surface of the plant material where they reside (sub-surface oils are reached by the steam breaking down the upper surface to expose the lower plant material, which effectively then becomes the surface). A mist does not have enough heat in its water droplets to boil; it is already cooler because of condenssation.

A mist created under a charge of plant material, not being a gas, has no pressure to speak of. It cannot force its way up through a charge into a condenser; it will condense on the lower layer of plant material in the charge and then fall back to the bottom of the charge chamber ("pot", "retort") as a liquid. This behaviour is called "reflux".

So how can steam (water gas) remove essential oils from plants? At boiling, the mechanical action of the water surface bubbling and popping throws water vapour into the surrounding gas. There has to be enough of this water vapour to gather on the surface of the plant material and form an "interface" with the oil droplet. It is at this interface where the vapour, gas and invisible (to the naked eye) miniscule water droplets carried by the water gas upward from the boiling water surface below, gathers. As the tempersature is still at the boiling point of water, this aggregation of water at the edge (interface) of the oil droplet, boils away the edges of the oil droplet until there is no oil left. The steam is then "carrying" small droplets of oil along with it up into the condenser, where cooling turns the steam back into "mist" (and, with more cooling, water liquid) which, along with the oil droplets, is now called "condensate".

So do I see "steam" when a kettle boils? NO. You see rapid condensation of water vapour with the surrounding cooler air; another "mist".

To wrap up then, there are only three ways that essential oils can be obtained from a still:

One: Use boiling water to generate steam at atmospheric pressure or above

Two: Use much lower than atmospheric pressure to make the water boil at a lower temperature

Three: Instead of water, use a liquid at atmospheric pressure that has a lower boiling point than water"

ref Chris Burder, Distiller 

Ian Brealey

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