Steam distillation

Distillation, according to Encyclopædia Britannica, is a:

“… process involving the conversion of a liquid into vapour that is subsequently condensed back to liquid form. … (It) is used …in the separation of two or more liquids having different boiling points”

So in the general sense, it is a method of separating two liquids by exploiting their boiling points difference, or their difference in volatility. In this way it is possible to obtain a greater percentage of one of the two liquids in the condensate. You can repeat this operation time and time again to separate more and more liquid from the other one. A classic example of this operation is the production of Acquavite or Cognac from wine: wine (which we can define in general terms as being a mixture of perfectly miscible water and ethyl alcohol) is placed in a container and is heated until it reaches the boiling point.

In the case of two perfectly miscible liquids (a monophasic mixture), the boiling temperature is the weighted average of the boiling temperatures of two liquids. Wine vapors, due to the difference in boiling points between ethanol and water, enrich themselves with ethanol, and do not contain the non-volatile molecules present in the wine. The obtained distillate will therefore have a higher alcohol content and will be depleted in all non-volatile compounds.

This comparison is not, however, perfectly true to our case, and for two reasons: first, unlike in the case of wine, essential oil and water are two immiscible liquids (or rather, very little miscible ones) and do not therefore form monophasic mixture, but a biphasic mixture, which behaves differently from the standpoint of boiling temperature; second, in the distillation of aromatic plants, I do not put into the still a mixture of essential water and oil, but a load of plant material, immersed in water or invested by a stream of water vapor. So the problem breaks down into two distinct issues: first, how do I extract the aromatic molecules from the plant material so that they mix with water or water vapor, and second, how does a biphasic mixture occur once in distillation?

Steam distillation can be distinguished into three different processes: water distillation (or hydrodistillation), water and steam distillation (or direct, or wet, steam distillation) and steam distillation (or indirect steam distillation).

Hydrodistillation

The first type of distillation is the oldest and the simplest, so that with a minimum of preparation it is possible to self-build a distiller of this type and make it work in your own kitchen. If we were to make a comparison with the kitchen, it would be to boil plants and to collect the vapors. In fact, it is a matter of immersing the material to be distilled into water, in an still, connected with a condensation system. The water is heated either directly, by placing the pot on a heat source, or indirectly, for example by using a coil immersed in the water and heated by steam or electricity. In any case, the result is that the water heats up, and by heating it surrenders energy to the plant material, heating it in turn.

This supply of energy contributes to the extraction of essential oil from plant tissues, in a simpler way when essential oils are present on the surface of the plant (as in the case of thyme, rosemary, lavender, basil, etc.) because the structures are immediately in contact with boiling water, which can break the tissues and release essential oils, and/or allow the transfer of essential oils through the cell membrane or cuticle, thus dispersing essential oils in water. The dynamics are a bit more complex when the structures that store essential oils are deeper in tissues, in which case much more energy is needed to free them, or it is necessary to cut the material. Irrespective of this, once released from their structures, essential oils are found free in the water, forming a biphasic mixture. In such a mixture the boiling temperature is not an average but is always lower than that of the liquid that boils at lower temperatures. Since essential oils have a much higher boiling point than water, the mixture will start to boil at a temperature below 100 ° C.

The vapors released from the mixture are conveyed by a pip to a condenser apparatus, which is nothing other than a system for cooling the vapors to bring them back into liquid form. The distillate then drips and is collected in another container, which is called a separator or a Florentine flask. The separating vessel serves to separate the essential oils from the distillation water, which is now called aromatic water (or hydrolat, or hydrosol) because in the distillation process it is enriched by those (few) fragrant molecules that are soluble in water. Separation is due to the fact that essential oil and water are not miscible and have different specific densities (usually) and therefore stratify, almost always with the essential oil floating on top the aromatic water, apart from the few cases in which oil is heavier (cinnamon bark, clove, nutmeg, etc.), or has mixed densities (such as vetiver, which contains a lighter fraction of water and a heavier one). As you can Imagine if I put a certain amount of water in the still at the beginning of the distillation, it will gradually consume it, and the level will drop below that of the plant material. This process is dangerous because if the plant material is not completely immersed in water, essential oils are not extracted and vaporized, and the material can be damaged. If water finishes the heat in the still can increase much and burn the material apart from ruining the distillation. For this reason a number of precautions have to be taken.

The simplest and most applicable to any distiller is to measure exactly the relationship between plant material and water, so as to predict how much time to stop the process. Normally in a hydrodistillation, the still is never filled up more than 2/3 or 3/4 of the volume, and the ratio of the weight of the material to the weight of the water is 1/3 (one part by weight of the material for three parts by weight of water). When we calculate how much vegetable material and how much water to mix (say 10 kg of vegetable material and 30 liters of water), the process will be stopped once I have collected 10 liters of aromatic water.

This method works well for the distillation of aromatic waters, but much less if we want to obtain the essential oil. To get the essential oil I have to distill for a much longer time, and I find myself with the problem of the water inside the still. The possible solutions are two:

    • in the first case I will have a tank containing always hot water, external to the still but connected to it via a faucet controlled by a float. The boiling water will then keep the internal level always stable, allowing me to distill for long periods.
    • The second solution is used when the essential oil you want to get is particularly expensive and valuable, and it is called cohobation. In this case, the overflow of the Florentine bottle, the one that automatically draws aromatic water, is connected to the still. The aromatic water continues to come back inside the still, avoiding the risk material being dried and ruined but, above all, carrying partially saturated water of essential oil within the distillation cycle. This partially saturated water will, at each step, increase its saturation until it reaches the critical point beyond which it will no longer be able to solubilize other essential oil, which will then be harvested in Florentine, increasing the final yield of the process.

Distillation In water is now scarcely used commercially, for high energy costs, long times and degradation of oils. However, it is still very much used when distilling certain specific materials: dry plant powders, flower petals, seeds, or many small fruits, resins. These materials, when invested by steam, tend to form compacted lumps, where the stam does not penetrate or penetrate very little, thus not allowing good distillation. If they are immersed in water and constantly stirred, they can be extracted much more effectively. While less used in the past, this distillation is still one of the preferred methods of small distillation distillers, or particularly interested in aromatic waters. This is because the characteristics that are considered negative in certain contexts, that is, very intimate contact with water and long distillation times, are good for other professionals because optimum distillation parameters to obtain the most essential oil are often negative if the ultimate goal is aromatic water. To put it simply, the “wrong” distillation for essential oils is often correct for aromatic waters, and vice versa. In addition, distillation in water, and water and steam distillation, can keep the vapor temperature a few degrees lower, a favorite feature of some distillers.

Water and steam distillation

The second type of distillation is derived immediately from the former, and is also quite ancient. It has been developed in part to solve some distillation problems in water, which forces it to keep the plant material in contact with water for a long time, facilitating degradation processes (eg hydrolysis) which can reduce the quality of essential oil. In addition, distillation in water limits the amount of plant material that can be used at one time.

One way to reduce these problems is to not immerse the plant in water. Keeping with the culinary imagery, water and steam distillation is like steam cooking. The plant material is not immersed in water, but is suspended, thanks to a net or a basket, above the water in the bottom of the still. The water is heated and when it boils, the steam rises upward and encounter the vegetal material.

As in the previous case, there will be two technically separate moments, the transfer of energy from steam to plant material, and the release of essential oils from inside the structures that stored them. Once released outside the plant material, these molecules are “caught” by the vapor flow, vaporized and come back, in the form of mixed steam of essential water and water, to the head of the lamb, where they are picked up again by the pipe which leads them to the condenser.

This distillation technique is preferred in handicraft or small-sized productive areas, and more generally by distillers preferring a smaller vapor flow operation and with a higher vapor humidity, for instance distilling always naturally-dry material such as lavender.

Steam distillation.

The third type is the one that has been developed most recently to solve some of the problems of the first two methods and to adapt the distillation to a scenario that has become industrial. But what are these issues that limit the utility of the first two methods? One we have already seen is the intimate and prolonged contact of water in liquid form with the plant material. Also, from the distillation in water to the water and steam distillation, the problem is not resolved completely. To understand why we have to stop a second talking about water steam. Technically, with this term we indicate the water gas, that is, gas-phase water. As such the steam should be colorless and invisible. When, however, I produce water vapor by simple boiling at ambient pressure, such as when boiling water to cook pasta, vapor appears to me as a fog, clearly visible. This fog effect is due to the fact that water vapor conveys with liquid water in the form of suspended microdrops, which are then visible. This is a case of wet steam, which can carry with it 3 or 4% of liquid water. However, by distilling in water and steam I am using a very wet steam, and therefore the vegetal material that the vapor meets will be wet by liquid water. In the case of steam distillation, I use an external vapor source, which allows me to inject the vapor into the vegetal material without the water of the still. In this way, I can use a “dryer” steam, that is, with less liquid water, and I could theoretically use a perfectly dry steam (not advisable!). This gives me an interesting advantage: I can change the humidity of my steam, making the system more ductile and adaptable to various circumstances; and allows me to wet the plant material less, reducing the degradation phenomena. It also makes it easier to inject large quantities of steam into the still, accelerating distillation (speeding up distillation is not always a positive or always negative factor, it depends on the context, but has the ability to make me more freedom of action).

In an indirect vapor distillation I have a still with a grate or a basket that allows me to suspend the plant material about 20 cm from the bottom of the still. Below the grate I have a steam distribution system (a kind of large inverse shower tray, or a perforated flute system, or concentric circular hubs), connected by an external hose coming from the steam generator. Steam enters the still, is distributed and comes in contact with the plant material where it performs the work described above. Indirect steam distillation is the one that allows a greater operational ductility, in the sense that it is easier to intervene both on steam humidity and on the intensity of steam flow. In addition, the use of an external steam generator usually allows a greater industrialization of the process because it is easier to operate a very large distillate, while with distillation in water or in water and steam operating large stills becomes more and more difficult.

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