Direct steam distillation

The raw material is packed into the still on a grid, not soaked in water. Steam from the separate boiler rises through the material, breaking the cells and carrying off the molecules light enough to travel with it.

The mixture of steam and essential oil passes through the condenser, is cooled and returns to liquid. Because essential oil does not dissolve in water, the two layers separate by themselves in the receiver. For most materials the oil is lighter than water and floats on top.

Not always. Clove, cinnamon bark and wintergreen give oils heavier than water that sit in the lower layer — the distiller has to know in advance so as not to draw off the wrong layer.

The remaining water is called hydrosol or floral water. It contains water-soluble molecules that the essential oil does not hold, and for some species the hydrosol smells quite different from the oil of the same batch.

Something seldom mentioned: the composition of the distillate changes over time. The early fraction is rich in light monoterpenes, the late fraction in heavier oxygenated compounds and sesquiterpenes. This is exactly what you see on a smelling strip, except that here it unfolds over hours instead of minutes. Experienced distillers cut the head and tail fractions off separately instead of combining the whole batch.

Because the steam only passes through rather than soaking, the material undergoes the least chemical change of the three distillation methods. Esters are better preserved, and the analysis is closer to the fresh material.

But heat is still heat. Light esters and some aldehydes are still lost, so the result is always drier and less waxy than the fresh material. Smell a flower, then smell the distilled oil of the same species: the gap between the two is exactly what the heat has taken away.

It needs a separate boiler, so the investment is higher than for the other two methods. For small producers that is a real barrier.

Powdery materials, or materials that compact into a dense mass, block the steam path and create zones the steam cannot pass through. For such materials water distillation is more suitable.

And heat still destroys the most fragile molecules. Jasmine, tuberose and most white flowers do not survive the still — what comes out is not the smell of the flower but its remains.

Distilled products are usually clear and thin, and leave no oily mark on paper after a few hours. If a label says pure essential oil but the liquid is thick, dark and leaves a lasting oily mark, it is most likely an absolute or has been diluted in a carrier oil.

On a GC/MS analysis, distilled products rarely contain very heavy molecules such as plant waxes, because these do not evaporate with steam. The absence of heavy compounds is an indirect sign of the method used.

Distillation is the oldest aromatic technique still used at industrial scale. Persian and Arab alchemists refined distillation apparatus from around the tenth century, mainly to make rose water; the technique followed the trade routes into Europe and became the foundation of the Grasse perfume industry centuries later.

But the apparatus of that time was water distillation, not direct steam. The separate boiler appeared only with the industrial age, and it was a change of principle, not just of scale.

In Vietnam, direct steam distillation is found mainly in newly built facilities. Most traditional distilling — lemongrass, cajuput, mint, star anise — still uses directly fired copper stills, that is, water distillation.