Volatile materials, like essential oils, evaporate in the presence of heat. Even compounds that are sensitive to high temperatures can withstand them for short periods of time. Steam distillation is a process that produces localized high temperatures for a short period of time, and very effectively extracts materials like essential oils.
The simplest way to practice steam distillation is to simply place the materials in a small pot (like a coffee carafe) with water, collect and condense the steam and vaporized oils and then skim the oils off the surface of the water. With some materials this will work quite well, but many delicate botanicals will not stand up to this treatment.
A more sophisticated method is "dry" steam distillation. In this process, steam is passed through a loosely packed mass of the plant material and a considerable amount of heat is liberated when the steam condenses to water. Every gram of water that condenses on the plant material releases 540 calories of heat at a temperature of 100° C (212° F), which is more than enough to vaporize most plant oils. Often those oils are bound inside the plant material in sealed pockets. When heated rapidly, the oils expand, break the walls of their prisons, and escape.
The released oils, in the form of aerosols and vapor (the oils boil at a lower temperature in the presence of steam than on their own - see Chapter 8, Raoult's Law), are swept up by the passing steam and carried to the condenser, where they're abruptly cooled down. The steam is condensed to water, and the extracted oils float on the surface of that water. Some of the extracted compounds are soluble in
We have designed a home-scale steam distillation apparatus that can be used for simple or "dry" steam distillation.
This diagram is not to scale, but the device is so small and simple that detailed plans should not be necessary. Let's examine how it works, section by section.
Water can be fed to the boiler from a reservoir. There are three reasons for this:
• To keep a small amount of water in the boiler, so it can come to a boil quickly.
• To assure that the boiler always has the same amount of water in it.
• To give you a visual check that there's enough water and that you won't run dry.
water, and these are collected as a hydrosol.
water, and these are collected as a hydrosol.
You will also note that there is a "safety valve". Hopefully, this will never be needed, but it is a prudent feature to have, particularly because it's so simple: a marble sitting on a rubber grommet placed in a hole through the boiler lid!
However, this reservoir feature is not a necessity. A pressure cooker with a hose fitted to the steam nozzle can make a fine boiler (just like the basic pot still), but it takes longer to heat up and you must to put enough water in it for the whole run. Fortunately, most small-scale steam distillation runs are quite short. Commercially available pressure cookers are equipped with safety valves, but these operate at quite high pressures. If using a pressure cooker that you don't want to drill holes in, it would be prudent to add to the delivery tube a little side-arm carrying a marble safety valve.
Steam passes to the plant material tube or chamber, which is the key to efficient extraction. This may be any size you like, but we find that between one and two liters is more than sufficient for most needs. Descriptions of industrial-scale steam distillation often speak of materials being "suspended in baskets" in the still. They do this so that they can change the depleted materials for new ones quickly, but some of the steam may flow around the basket. On the home scale, a chamber is easy to fill with plant material and ensures that all of the steam goes through it.
Pack the plant material loosely in the chamber, chopping it into small pieces if necessary. There is a perforated plate or screen at the bottom to retain the material, which has a thin rod or wire attached to it, making it easy to extract the spent material after a run. If you are processing leaves or materials that clump together when subjected to steam, you need to mix in some other material to keep the whole mass porous. Small, uneven pebbles, Raschig rings, wood chips, etc., are all good possibilities. Make sure that they have been boiled or steamed extensively, so that they will not contribute any odors or flavors of their own to your product!
A copper tube passing through the tightly-fastening lid of the chamber leads to a simple Liebig condenser. Do not use polypropylene tubing for this as essential oils can soak into plastics. The condensed steam and the volatile oils then drip into a Florentine collector.
Steam distillation produces a mixture of water and oils, and it is usually the oils that you're trying to collect. The Florentine collector is an ingenious solution to this problem, and lias been in use by the overflow perfume industry for ccnturics. Let's discuss how it operates with a couple of examples. The device is first filled with water (preferably distilled) until the bottom of the overflow pipe is covered (so no precious oil will block it). As the collector fills with condensed liquids, the oils and water separate out naturally, with lighter than water oils floating to the top and heavier than water oils sinking to the bottom. (Most oils are lighter than water, but a few, including anise, cinnamon, clove, nutmeg, and mace, are heavier than water and will sink.)
The overflow pipe is set up so only water enters it, and when the liquid level in the collector reaches the top of the pipe, it begins to overflow, keeping the level inside the collector constant. Since the density of the top oils is lower than that of water, the actual level in the collector tube may be slightly higher than the top of the outside arm. The water, or more correctly the hydrosol, is continually removed to a separate collection vessel. Depending upon the type of botanical, the hydrosol might contain substances you want. Hydrosols are widely used in making perfumes, soaps and cosmetics.
At the end of a run, collect the heavier than water oils from the collector through the stopcock at the bottom, then the small amount of hydrosol still trapped in the collector, and finally the lighter than water oils that float on top. You'll find that most of the oils will come across in the first half hour of steaming, but continuing to get the last bit for a further half to one and a half hours won't hurt. Oils will not get into the overflow tube, because it's always in water or hydrosol. Even when the product is withdrawn, hydrosol in the overflow tube will discharge back into the main collector, keeping the opening clear.
Steam-distilled essential oils are extremely concentrated and you should be careful not to let them touch your skin, and to handle them only in a well ventilated area. The yield from one batch (our botanicals chamber contains 2 liters of plant material) is only a few milliliters of oil, but these oils are extremely potent in their pure, undiluted form!
The simplicity and small size of the equipment make steam distillation an ideal first step into distillation in general, and it is a very interesting and rewarding pastime, too.
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