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Essential oils and steam distillation.

What is essential oil?

When I think about this it seems such an enigma. Wikipedia describes it as 'a concentrated hydrophobic liquid containing volatile aroma compounds from plants', a pragmatic answer: a concentrated substance that is repelled from a mass of water (hydrophobic) that has smell or odour (aroma compounds). Apparently they are 'essential' as well!

To feel a plant, reach out and pick a leaf or flower, crush it in the palm of your hands and inhale--this is the silent language of aroma and sensation which cannot be translated into academic words that make sense. Aroma is certainly a powerful conduit of subtle change, response and reaction. Essential oils are aroma.

To be able to steam distil, gathered fresh plants from the garden or wildcrafted is a profound experience. Even though the process is quite simple, it becomes part of a magical alchemy of transformation. The energy of steam changes the chemistry of a plant so that its constituents are released into the steam. That in itself is enough, however that liquid then releases millions of tiny bubbles of aromatic essential oil. They float to the surface of the water and gathers there as the volatile aromatics--the essential 'soul' of the plant.

The process of steam distillation

Steam distillation is the method mostly used to produce essential oils. The plant material is subjected to steam under pressure. It does not come into contact with the boiling water. Water boils at 100[degrees]C while steam has a higher temperature than boiling water; a steam burn is worse than a boiling water burn.

Distilling for essential oil by steam distillation

Following on from the previous article on hydrosols and hydro-distillation (AJHM 24:3;101), rosemary (Rosmarinus officinalis) is an excellent plant to use to illustrate the difference between the two distillation methods and the differences in the hydrosols that are produced. A copper alembic column still is used for steam distillation. The only difference between this and the one used for hydro-distillation, the traditional alembic pot still, is that the former has a column that sits on the pot with a sieve plate at the bottom. The herb is placed in the column and the column is placed on top of the pot of boiling water. The sieve plate keeps the herb material above the water so that only the steam can come into contact with it.

The column still is set up in a sheltered spot in the garden. It has already been cleaned by distilling rye flour slurry. To do this 450 g of rye flour is mixed in in 4 L of water and poured into the pot. The whole still is assembled and the rye flour mix is heated until it boils, becomes frothy and seeps out through the joins. Once it comes out of the condenser bucket as a distillate the heat is turned off. When the still is cool enough to handle, it is dismantled and scoured with Eco Cream Cleanser which it contains a citrus oil that cleans the copper. It is rinsed and dried thoroughly, leaving it in the sun to finish the drying process. Before starting a new distillation, I ensure the still is totally clean by distilling hot water and checking that the distillate is tasteless, odourless and clear.


The pot is 2/3 filled with water and placed on a gas ring to bring the water to the boil. It is important to have everything at hand for a distillation. The water pump is kept on a steady flow of cold water running through the condenser bucket and the sterile beakers and oil separators are ready to collect the distillate. A bowl of rye flour paste is ready to seal the joins in the still so it doesn't lose precious steam.

And of course the herb has been harvested and prepared. The leaves and flowers have been stripped from the stems of the rosemary and sit fragrantly waiting.

Five good handfuls of rosemary are placed into the column. When the water in the pot is boiling, the onion dome is carefully removed and the column is placed on the pot making sure it is square. The onion dome is placed on the column, checking that it is level. Because the rosemary is cold there is time before the steam works its way through. Once it is heating it is important to work quickly so as not to lose any steam or precious essential oil. The joins are quickly but carefully sealed with the rye flower paste between the column and the pot and between the column and the onion dome. As the copper heats it bakes the paste dry and seals the joins.

By the time this is done the onion dome will be hot which means the steam has spiralled up the column. The heat bursts open the cells of the plant and the volatile oils are released and carried in the steam. The steam spirals in the onion dome and any particles of dust or physical matter fall back. The steam flows down the bird's beak, through the connecting pipe and into the condenser coil.



Just as in the hydro-distillation it is vital to have a steady stream of cool water running around the condenser coil throughout the whole distillation. It is even more important when distilling with steam as it is a hotter, faster distillation. The water needs to be boiling vigorously to create a good body of steam under pressure to burst the cellulose of the plants and release the essentail oil.

The distillate will flow faster than a hydro-distillation. The essential oil will be released in the first 200 mL of distillate. A 10 L still should collect 500 mL of hydrosol. The oil has been expelled from the mass of water and floats to the top. The hydrosol is not as milky as it is in a hydro-distillation and of course there is significantly more oil released. The aroma is stronger, sharper and more intense. At this point the pH level is checked.

Depending on the purpose, distilling may be continued for more hydrosol. There will be very little oil in the next 500 mL but often the hydrosol is still of a good aroma, flavour and pH level. As soon as the pH increases the distillation is stopped.

This is the best part! The two oil separators, one 500 mL and the other 60 mL, are clamped on to a retort stand. The larger separator is used when distilling botanicals that will yield between 3 and 10 mL of oil from 500 mL, such as rosemary, eucalyptus, lavender, clary sage, manuka, thyme or peppermint. The smaller one is used to collect precious drops of plants such as rose geranium, balm, rose, lemon verbena, chamomile or yarrow.


The hydrosol is poured into the large oil separator. As this is done the oil mixes with the hydrosol again. This is really something to watch as the millions of miniscule bubbles stream to the surface, tiny drops of essential oil released from the water like a sponge being squeezed. The oil collects at the top, often a golden colour, and the hydrosol settles and clears.

A clean beaker is placed at the bottom of the oil separator. The tap is carefully turned on to release the hydrosol, leaving the oil to collect at the bottom of the separator. Once the hydrosol has drained off completely, the tap is turned off. The reward is 3 mL of precious essential oil! It will need to be distilled another 3 times to get the desired 10 mL, but with this will come 2 litres of fragrant rosemary hydrosol.

At this stage there will still be some moisture in the oil. If left, bacterial growth and degradation of the oil could result. The test tube with the oil is put into the freezer where it dries the oil out and freezes any moisture. The oil is then poured off into an amber dripulator bottle and becomes part of the precious collection of my own distilled essential oils.

By this time next year I hope to have 25 of my own distilled oils. Already in stock is 50 mL eucalyptus, 50 mL peppermint, 10 mL rosemary, 3 mL lemon verbena, 10 mL lime, 10 mL manuka, 10 mL ginger and 10 mL thyme. Spring is the time for harvesting German chamomile, orange, grapefruit and lime flowers, spring manuka and kanuka, tarata, rose geranium, bay leaves and balm. In summer it will be lavender, clary sage, lemon verbena, basil, peppermint, Helicrysum (immortelle or everlasting oil), yarrow, angelica and some trimming of the citrus as the fruits form tiny balls for petitgrain.

Every distillation is different depending on the season, soil and climate. The table below shows an approximate guide to volumes and quantities expected; for example the harvest of three mature flowering tops of Lavendula augustifolia weighs approximately 1 kg and will yield approximately 25-30 mL of oil. Most other plants will yield only a quarter to half that amount.

Jill Mulvaney

Alembics NZ,


Jill Mulvaney set up and ran a natural skincare business for many years which found her importing raw materials, manufacturing and teaching. Jill and her partner are both avid distillers of hydrosol, essential oils and spirits. They run workshops and demonstrations throughout NZ and sell alembic stills worldwide. They share the knowledge of this ancient process, using natural organic seasonal botanicals and beautiful handcrafted copper.
Approximate guide to volumes and expected quantities

Size        Plant        Example of    Approx. yield      Hydrosol
           quantity      Lavandula       of essential
                        augustifolia         oil

5 L        250 g or       1 mature         1-2 mL        250-400 mL
column   2-3 handfuls      plant

10 L       500 g or     1.5-2 mature       3-5 mL        500-1.5 L
column   5-6 handfuls      plants

20 L       1.5-2 kg      4-6 mature        5-15 mL         3-5 L
column                     plants

40 L         5 kg       8-10 mature       20-50 mL         5-10 L
column                     plants

150 L      20-30 kg     60-90 mature     200-500 mL       20-50 L
column                     plants

Comparison of hydro-distillation and steam distillation

Hydro-distillation                  Steam distillation

Plant is subjected to boiling       Plant is subjected to pressure
water                               and steam

Distillation is slower and cooler   Distillation is fast and hot

Hydrosol is often milky             Hydrosol is mostly clear

The distillate shows little         Essential oil is evident on top
essential oil floating on the       of the hydrosol and is removed
surface, most remains in

Aroma is complex as are the         When the oil has been removed the
flavours                            aroma of the hydrosol is light
                                    and delicate, the flavour less

Nothing has been separated from     Has a dual result with both
the distillate, it remains          essential oil and the hydrosol,
complete                            however the hydrosol has lost
                                    the element of the essential oil
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Article Details
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Title Annotation:Growing and manufacturing
Author:Mulvaney, Jill
Publication:Australian Journal of Herbal Medicine
Article Type:Report
Geographic Code:8NEWZ
Date:Dec 1, 2012
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