Is there a right and a wrong way to distill?
Let’s give an example: distillation in water is often considered an obsolete method because it tends to give a lower yield in essential oil, to “lose” many oxygenated compounds and to facilitate hydrolysis processes. That being said, if distillers are interested in distilling very delicate parts of the plant like petals, or they need to distill dry spices or woods reduced to powders, or resins, or even small dry fruits and seeds, then water distillation is the right and only option, because those materials cannot be properly distilled in steam. Another example is the distillation of very fragile and heat-sensitive fragrances (heat-sensitive). In this case lowering the temperature is desirable. One method to lower it is to distillate under reduced pressure, but to do this, a special still is required, completely isolated from the environment and with a pump to lower the pressure inside. But even without going so far away, we can still reduce the steam temperature by some degree if we distill into water and steam rather than distillate with indirect steam. This is because the indirect steam, generated from an external boiler at pressure usually between 2 and 7 bars, is in the high pressure and temperature range. As soon as it enters the still, the pressure is equalized and the temperature drops, but tends to remain a couple of degrees Celsius higher than that of steam generated in the water and steam distillate. In addition, the vapor of an external generator always tends to be dryer than the others, and in some cases (i.e. already dried plant material) this can be a negative factor.
With these things in mind, however, there are general fairly clear parameters that indicate when a distillation was badly conducted. Generally if my plant is very rich in esters, which are very fragrant molecules, (such as in the case of a high mountain lavender, with a very high linalyl acetate content) distilling in water for a very prolonged period is much more harmful than distilling in water and steam or in indirect steam. This is because the hydrolysis phenomena in the water distillation breaks down the molecules of linalyl acetate (highly odoriferous) in the molecules of acetic acid and linalol (less valuable), moving the high quality lavender chemical profile to that of a humble lavandin, a much less valuable essential oil.
It is equally wrong to load the distiller without paying enough attention to how the material is compacted. If the material does not have a homogeneous density and is not compressed well, steam will tend to find preferential pathways between the material instead of going through the material, creating chimneys and bags of un-extracted material.
Another classic mistake is to distill for too little time. When this happens, not only is there a quantitative loss because part of the essential oil will remain in the plant material but also a qualitative loss, because those in the plant material are the heaviest molecules, often sesquiterpenes, and this means that the obtained oil will not have all the spectrum of molecules present, with an impoverishment of the perfume and possibly a reduction in its effectiveness.
This post is also available in: Italian