About the possible chemical transformations in the still By Benoit Roger, Ph.D
- Aromatika Magazine
- Dec 3, 2025
- 2 min read
Pg. 30-34 - 4-minute read
When I distill plants, I always see how the most polar volatile compounds prefer to remain in the hydrolate, while the essential oil only holds what doesn’t dissolve in water. A classic example is Rosa × damascena: its phenyl ethyl alcohol largely ends up in the hydrolate, making up about half of its aromatic profile, while only around 1% appears in the essential oil. Distillation is also far from gentle—boiling or steaming plant material for hours at nearly 100°C inevitably causes chemical changes, such as the hydrolysis of esters into their corresponding alcohols and acids. This means that even if the plant naturally produced a specific ester/alcohol ratio, distillation can shift that balance.
Other transformations during distillation can actually be beneficial. For instance, the conversion of matricin into chamazulene is a well-known degradation that gives certain essential oils their deep blue color and contributes valuable antioxidant and anti-inflammatory properties. Achieving maximum conversion often requires extending steam distillation to five or six hours. However, many other reactions—like isomerisation, oxidation, and the formation of trace “still note” compounds—also occur, varying depending on the plant, the acidity, the equipment, and the distillation method. These reactions can introduce new molecules, sometimes desirable and sometimes not, from terpinene isomers to limonene oxides and sulfurous or aldehydic off-notes.
Distillers often underestimate the complexity behind these changes. Even materials like copper don’t simply “remove” sulfurous still notes—they actually catalyze transformations of thiols into other sulfur-containing molecules. Ultimately, the composition of both essential oils and hydrolates is shaped by countless factors: heat, water exposure, acidity, reflux, steam flow, and equipment. Adjusting these parameters—or distilling under vacuum—can help minimize unwanted degradations. But despite the challenges, mastering distillation is incredibly rewarding. It demands patience, precision, and respect for the plant, sitting at the intersection of art and science.
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