Borgmann Aquaponics Hydroponics
Steam Distillation
Steam distillation is the only extraction method recognized by the International Organization for Standardization (ISO) as the reference procedure for obtaining genuine essential oils. At the same time, it is the method where the expectations of home users most frequently and dramatically deviate from reality – because the underlying physics are intuitively difficult to grasp. Those who understand the method will have no false hopes and will still achieve surprising results.
Physical-Chemical Principles
The law of partial vapor pressures – core principle of the method
Steam distillation is occasionally described simply as "boiling with steam." This is misleading, because its actual operating principle is considerably more subtle and explains why substances with boiling points well above 200 °C can be distilled without issue at 100 °C.
The process is based on Dalton's law of partial pressures: In a mixture of two immiscible liquids – here water and essential oil – the vapor pressures of both components add up. The mixture boils when the sum of the individual vapor pressures reaches the external atmospheric pressure (approx. 1013 hPa). Since water at 100 °C has a vapor pressure of exactly 1013 hPa, it is sufficient that the essential oil contributes even a very small vapor pressure of its own – the boiling point of the mixture is always below 100 °C, in practice often at 95–99 °C. The volatile oil components are thus carried out of the plant together with the water vapor and condense in the condenser. Chemistry refers to this process as steam distillation or a form of azeotropic distillation.
The practical consequence is significant: A substance like eugenol (the main component of clove oil) boils at 254 °C in its pure form. In steam distillation, it transfers easily at just below 100 °C – without thermal degradation, without decomposition, without oxidation due to air contact. This is the crucial advantage over simple distillation and explains why this method has been used for centuries for the gentle isolation of essential oils.
What the method captures – and what it does not
The selectivity of steam distillation arises directly from Dalton's law: Only substances that have a measurable vapor pressure at 100 °C and are immiscible with water are transferred. This property is possessed almost exclusively by the volatile terpenoids and phenylpropanoids that make up essential oils – monoterpenes (C₁₀), sesquiterpenes (C₁₅), and their oxygen-containing derivatives (alcohols, aldehydes, esters, oxides like 1,8-cineole). In contrast, all non-volatile compounds are not captured: flavonoids, tannins, mucilages, bitters, alkaloids, glycosides, and fatty oils remain completely in the distillation residue. The distillate from a steam distillation therefore contains a fundamentally different spectrum of active compounds than an oil extract or an alcohol extract of the same plant – these three methods are not alternatives, but complementary methods with their own profiles of active compounds.
Hydrolat – the underestimated by-product
The condensed water that is produced together with the essential oil during steam distillation is by no means waste. This hydrolat (also called floral water or plant water) contains small amounts of water-soluble and slightly volatile oil components that have dissolved in the aqueous medium. Rose hydrolat (rose water), lavender hydrolat, and lemon balm hydrolat are traditionally used products in cosmetics and pharmacy with their own active profile. The concentration of active compounds in the hydrolat is significantly lower than in the essential oil, but it is directly skin-compatible and can be used without dilution. In home setups, one kilogram of plant material often yields one liter or more of hydrolat – and only a few milliliters of oil.
The yield – sober arithmetic
The expectation of many home users to obtain a significant amount of essential oil from a pot of herbs is the most common source of disappointment. The natural oil content of medicinal plants varies considerably but is in any case low:
- Lavender (Lavandula angustifolia): approx. 1.0–3.0 % essential oil based on the dry weight of the flowers. From 1 kg of fresh flower material (with high water content), realistically 3–8 ml of oil can be expected.
- Peppermint (Mentha × piperita): approx. 0.5–1.5 % in the leaves. From 1 kg of fresh herb about 2–5 ml.
- Lemon balm (Melissa officinalis): only about 0.05–0.15 % – one of the lowest-oil medicinal plants of all. From 1 kg of fresh herb, 0.3–1 ml is realistic. This explains the extremely high market price for genuine lemon balm oil (often several hundred euros per 10 ml).
- Cloves (Syzygium aromaticum, dried flower buds): approx. 14–20 % – an exceptional case with industrially relevant yield.
These figures make it clear: For the home user, steam distillation is primarily a learning method and a quality experience – you understand the method, smell and see the product being created – and a way to produce high-quality hydrolats yourself. Genuine essential oils in significant quantities require industrial scale.
Plant-Specific Characteristics – why not every plant is distilled the same way
The decision whether to use fresh or dried material, how long to distill, which apparatus variant to choose, and how to collect and separate the distillate depends heavily on the specific plant and its oil type.
Lavender (Lavandula angustifolia)
Lavender is the classic distillation plant and is ideally suited for beginners because the oil content is comparatively high and the oil floats visibly on the hydrolat. The fresh or slightly wilted flowers are distilled, ideally harvested shortly before full bloom (highest oil content). Dried material can also be used but yields less oil, as volatile components are lost during drying. The distillation time for lavender is typically 60–90 minutes; after that, oil transfer decreases sharply. The oil floats on top and can be separated with a pipette or a separating funnel. Lavandin oil (L. × intermedia) contains more camphor and has a sharper profile – it is known to be confused with true lavender oil and is often encountered in trade as a cheaper alternative.
Peppermint (Mentha × piperita)
Peppermint oil consists of 30–55 % menthol, which is solid at room temperature (melting point approx. 43 °C) and can crystallize in the condenser as a white solid if cooling is incorrect – which can block the condenser. A common mistake in home distillation. Remedy: do not cool the condenser too strongly, aim for a cooling water temperature of approx. 15–20 °C. The oil is lighter than water and floats on top. Fresh leaves shortly before flowering contain the most oil; after flowering, the menthol content decreases and the menthone proportion increases. Distillation should be carried out quickly with a high steam flow rate, as menthol transfers relatively quickly, and long distillation times hardly improve the profile.
Rose petals (Rosa damascena)
Rose oil is one of the most valuable essential oils in the world – and at the same time the most impressive example of sober yield arithmetic: For 1 ml of genuine rose oil (Otto), 3–5 tons of rose petals are required, depending on the estimate. The oil contains up to 300 individual substances, including citronellol, geraniol, and solid rose paraffin (stearoptene), which precipitates at room temperature and makes the oil appear milky-cloudy – a sign of authenticity, not a flaw. Petals should be harvested in the morning shortly after opening and processed immediately; the fragrance and oil content are volatile and decrease drastically within hours. The "rose oil absolute" available commercially, on the other hand, is obtained through solvent extraction and is not a distillation product.
Resins, needles, and barks (general)
Coniferous trees (spruce, pine, fir, cedar) yield oils from needles, young twigs, and resins. The resin itself must be coarsely crushed or placed in small pieces before distillation – large resin pieces are hardly penetrated by steam. The distillation time is significantly longer (2–4 h) because the dense tissue structures require more time for steam passage. Barks often yield heavier sesquiterpene oils, which require a higher vapor pressure and only transfer completely towards the end of distillation – stopping after one hour would yield a qualitatively incomplete oil. The distillation of cinnamon bark (Cinnamomum verum) produces cinnamaldehyde (trans-cinnamaldehyde), which can attack metallic components of the apparatus – use only glass and stainless steel components here.
Apparatus and Procedure
For the home user, there are two practicable variants: the simple cooking pot setup and the compact distillation set made of glass or copper (available from specialist suppliers). The following steps apply analogously to both.
Coarsely chop fresh material (leaves, needles, flowers: directly; roots and barks: grind or crush in a mortar). Briefly moisten dried material with some water (soak for 30 min) – dry material conducts steam poorly and tends to burn during water distillation. Ratio: 100–500 g plant material per batch, depending on oil content.
All connection points must be vapor-tight – even small leaks mean loss of volatile oil components. For the cooking pot setup: place plant material on a sieve insert above the water (never directly in the water, except for simple water distillation). Cool the condenser with running cold water or an ice bath. Choose a collection vessel as narrow as possible to minimize evaporation of the distillate.
Bring water to a boil, then reduce heat: A steady, calm steam flow is more efficient than vigorous bubbling, which uses unnecessary amounts of water and overheats the apparatus. Observe the distillate dripping in the condenser. The initial distillate is often cloudy (oil-water emulsion) and clears as the water proportion increases. Distillation time: 60–90 min (flowers, leaves), 2–4 h (resins, barks, seeds).
Transfer the distillate (oil + water) into a separating funnel and let it stand for at least 2–4 hours, preferably overnight – phase separation takes time. Most essential oils float on top (density < 1 g/ml); exceptions: clove oil, cinnamon oil, thyme oil – these sink (density > 1 g/ml). Collect the hydrolat separately and do not discard it.
Fill essential oil into small dark glass vials (1–5 ml). Store sealed, cool, and dark. Shelf life depends on the oil: Oils rich in monoterpenes like turpentine or pine oil oxidize rapidly (6–12 months); oils rich in sesquiterpenes like sandalwood or patchouli are significantly more stable (several years). Store the hydrolat in dark bottles in a cool place (shelf life: 3–6 months, with preservative up to 12 months).
Critical Parameters & Typical Mistakes
Coherence of the steam flow
A steam flow that is too weak unnecessarily prolongs the distillation time and allows the back-diffusion of already transferred oil into the plant material. A flow that is too strong carries water into the condenser (mechanical entrainment, not a distillation process) and cools the plant material. The goal is a steady, calm steam flow that uniformly permeates the entire plant bed – "hot spots" and dry areas are the most common cause of incomplete extraction.
Distillation beyond the endpoint
Essential oils are not a homogeneous substance but consist of dozens to hundreds of individual substances with varying volatility. Lightly volatile monoterpenes transfer first; less volatile sesquiterpenes and their derivatives follow later. Stopping distillation too early yields an oil that is rich in monoterpenes but poor in the characteristic heavy terpenes – often with a flatter, less rounded fragrance profile than the industrial reference product. Anyone aiming for a qualitatively complete oil distills until the oil transfer measurably decreases.
Artifact formation through hydrolysis
Ester-rich essential oils (e.g., linalyl acetate in lavender, bornyl acetate in spruce) can partially hydrolyze under the conditions of steam distillation. This produces the corresponding alcohol and a carboxylic acid – which slightly alters the fragrance profile of the obtained oil compared to freshly crushed plant material. This effect is minimal with short distillation times and moderate temperatures but increases with excessively long runs.
Material selection for the apparatus
Copper is a traditional material for distillation helmets and cooling coils; it has antimicrobial properties and binds sulfur-containing compounds. However, copper can react with aldehyde-rich oils (cinnamon, lemongrass) and introduce trace amounts of copper into the distillate. For sensitive applications: Borosilicate glass or stainless steel (V2A/V4A) are chemically inert and suitable for all plant oils.
Typical Oil Contents and Home Yields at a Glance
The information refers to fresh plant material and a well-functioning apparatus. Realistic home yields are often 20–40 % below the theoretical maximum due to apparatus losses.
| Plant | Plant part | Oil content (dry weight) | Realistic yield / kg fresh material | Main components |
|---|---|---|---|---|
| Lavender Lavandula angustifolia | Flowers (fresh) | 1.0–3.0 % | 3–8 ml | Linalool, linalyl acetate |
| Peppermint Mentha × piperita | Leaves (fresh) | 0.5–1.5 % | 2–5 ml | Menthol, menthone |
| Thyme Thymus vulgaris | Flowering herb | 1.0–2.5 % | 3–7 ml | Thymol, carvacrol |
| Rosemary Salvia rosmarinus | Twigs (fresh) | 0.5–2.0 % | 2–5 ml | 1,8-Cineole, camphor, borneol |
| Lemon balm Melissa officinalis | Leaves (fresh) | 0.05–0.15 % | 0.3–1 ml | Citral (neral + geranial), citronellal |
| Cloves Syzygium aromaticum | Dried buds | 14–20 % | 80–120 ml | Eugenol (> 70 %), β-caryophyllene |
| Spruce twigs Picea abies | Young needles & twigs | 0.2–0.8 % | 1–3 ml | Bornyl acetate, limonene, pinene |
Further Specialized Literature
The following works are also accessible electronically (SpringerLink / German National Library):
- Steinegger E., Hänsel R.: Lehrbuch der Pharmakognosie und Phytopharmazie. 4th Edition, Springer-Verlag Berlin Heidelberg, 1988. DOI 10.1007/978-3-662-08318-5. — Contains the chapter "Essential Oils and Drugs" with detailed treatment of steam distillation as a production method.
- Hänsel R., Hölzl J.: Lehrbuch der pharmazeutischen Biologie. Springer-Verlag Berlin Heidelberg, 1996. DOI 10.1007/978-3-642-60958-9. — The chapter "Essential Oils" covers biosynthesis, ingredients, and distillation methods.
- Carle R. (Ed.): Ätherische Öle – Anspruch und Wirklichkeit. Wissenschaftliche Verlagsgesellschaft Stuttgart, 1993. — Practical presentation of the production, quality control, and analysis of essential oils; explicitly cited as a source in SpringerLink chapters.
- Bavarian State Research Center for Agriculture (LfL): Information sheet "Steam distillation of essential oils from fresh or wilted plants". Status 1998. URL: www.lfl.bayern.de/publikationen/merkblaetter/040574 — Publicly accessible, reviewed information sheet with yield reference values for home users.
- European Pharmacopoeia (Ph. Eur.): Monographs on lavender, peppermint, and thyme oils; continuously updated. EDQM Strasbourg. — Binding quality standards and identity characteristics for distilled essential oils.
The yield information in the table is based on published literature values and own estimates, considering typical home apparatus losses. They are to be understood as reference values; actual results depend on plant variety, harvest time, preparation, and apparatus.
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