Borgmann Aquaponics Hydroponics
Oil Extract – Maceration
Oil as a solvent, time as a tool: Maceration is one of the oldest extraction methods in pharmacy. The result is a plant oil loaded with active compounds, which can be used directly for skin care, ointments, and topical applications.
Physical-Chemical Principles
Diffusion as the driving force
The oil extract utilizes physical diffusion as its sole driving force. Dissolved molecules migrate according to Fick's first law along a concentration gradient – from a location of high concentration (the plant tissue) to a location of low concentration (the carrier oil) – until a thermodynamic equilibrium is reached. The diffusion rate is proportional to the concentration difference and the exchange surface area, and inversely proportional to the path length a molecule must travel. In practical terms, this means: the finer the plant material is chopped, the larger the surface area and the faster the extraction.
Lipophilicity and the log P value
Oil is a non-polar solvent. Following the principle "similia similibus solvuntur" (like dissolves like), it preferentially dissolves lipophilic – i.e., fat-soluble – compounds. The decisive factor for this is the so-called octanol-water partition coefficient (log P): substances with a log P value significantly greater than 1 preferentially transfer into the oil phase. This group includes, among others, essential oil components (terpenoids, phenylpropanoids), carotenoids like the β-carotene in calendula, fat-soluble vitamins (A, D, E, K), and a number of flavonoids and diterpenes. Water-soluble substances such as tannins, mucilage, polysaccharides, or water-soluble minerals, however, largely remain in the plant material and are not captured by an oil extract – a point that must be considered regarding the expected active compounds in the finished oil.
The cell wall as a barrier – and how to overcome it
The actual hurdle in maceration is the plant's cell wall. Intact plant cells release their contents only slowly. Moist, fresh plant material swells in the oil, making the cell wall more permeable – but carries the significant risk of water inclusions in the oil (see section "Critical Parameters"). With dried material, the cells have already collapsed, and the cell walls are pre-damaged by water loss, which significantly reduces the diffusion barrier. Through mechanical size reduction (mortar, grinding), the cell wall is mechanically broken open, and the effective extraction surface area is multiplied. For hard plant materials like roots, barks, or seeds, breaking them down by size reduction is therefore not optional, but mandatory, as otherwise, hardly any active compounds transfer into the oil.
Temperature: opportunity and risk
Heat significantly accelerates diffusion processes: According to the Arrhenius equation, the reaction or diffusion speed roughly doubles with every increase of 10 °C (Q₁₀ rule). Controlled warm maceration at 35–40 °C can therefore reduce the extraction time from several weeks to a few days. At the same time, the risk of undesirable processes increases with temperature: Above 50 °C, many polyunsaturated fatty acids in the carrier oil begin to oxidize; sensitive active compounds like hypericin in St. John's wort oil or volatile terpenoids can decompose or evaporate. The "water bath at 40 °C for several hours" is therefore an acceptable compromise for robust plant parts – for delicate flowers and aromatic herbs, cold maceration at room temperature is the safer choice.
Completeness of extraction and remaceration
A single maceration cycle never achieves complete extraction. The equilibrium between plant material and oil establishes itself at a specific, plant-dependent concentration; after that, no further transfer occurs – even if maceration continues for weeks. If aiming for the most complete yield possible, one should perform a remaceration: The pressed-out plant material is set up a second (or third) time with fresh carrier oil. The extracts are combined at the end. This method is standard in pharmaceutical manufacturing practice and nearly doubles the yield in many cases. Alternatively, percolation (continuous flow of fresh solvent through the plant material), used industrially, allows for an almost complete yield in a significantly shorter time – however, it is equipment-intensive for home use.
Values below 15 MPa½ (e.g., n-pentane at 14.4 MPa½) indicate very weak interactions. Values above 18 MPa½ (e.g., ethanol at 26.5 MPa½) suggest strong polar or hydrogen-bonding properties that are immiscible with nonpolar oils.
The unit MPa½ corresponds to the Pascal½ (also written as √MPa) and is the SI unit for this parameter. However, it is important to note that the Hildebrand parameter, as a single scalar value, is reliable only for nonpolar or slightly polar systems; for strongly polar substances or hydrogen-bonding systems, the more detailed Hansen solubility parameter is often required to correctly predict miscibility.
Plant-Specific Characteristics – why "one method" doesn't fit all
Maceration is not a universally applicable method. Cellular structure, water content, localization of active compounds, and the chemical nature of the ingredients vary significantly from plant to plant and require adapted approaches.
Calendula (Calendula officinalis)
The flowers contain flavonoids, triterpene saponins (oleanolic acid glycosides), carotenoids, and essential oil components. Saponins are polar and barely transfer into the oil – the skin-care properties of calendula oil are therefore primarily based on the carotenoid and flavonoid fractions, as well as the carrier oil itself. Freshly picked flowers must first wilt for 12–24 hours to reduce their high water content. Fresh flowers placed directly into the oil will develop mold within a few days and ruin the entire preparation. The progress of the extraction is easily visible: the oil takes on an intense orange-yellow color.
St. John's Wort (Hypericum perforatum)
St. John's wort oil is a special case because it is classically made from fresh flowers and flower buds – not from dried material. The characteristic hypericin (a naphthodianthrone) is a photoactive dye that colors the oil an intense dark red. This color change serves as a visible quality indicator. Important: Hypericin is a photosensitizer; users with fair skin should avoid direct sun exposure after applying the oil. Since fresh plants are used, wilting for 24–48 hours is particularly critical. Some traditional recipes call for maceration in sunlight – the slight heat generated supports diffusion, but UV radiation is not a relevant factor with closed jars.
Arnica (Arnica montana)
The flowers contain sesquiterpene lactones (mainly helenalin and its derivatives) as well as flavonoids. Sesquiterpene lactones are moderately lipophilic and transfer well into oil. Since A. montana is strictly protected, preparations available commercially are mostly made from Mexican arnica (Heterotheca inuloides) or Arnica chamissonis – where the active compound profile differs slightly, a factor to consider when purchasing starting material. For home production from dried flowers, a drug-to-oil ratio of at least 1:5 (weight to volume) is recommended.
Roots and barks (general)
Hard plant parts like roots (e.g., valerian, lovage, echinacea) or barks pose the greatest challenge for maceration. The dense, lignin-rich cell wall and the lack of spongy tissue like in flowers result in very slow diffusion. Pre-treatment is mandatory here: The material must be coarsely crushed in a mortar or ground with a grain mill to a particle size of approx. 2–4 mm. A maceration period of 6–8 weeks is realistic; shorter times yield very weak extracts. Alternatively, an alcohol extract (tincture) is more suitable here, as it accesses root contents much more efficiently.
Critical Parameters
Water activity (aw)
The most important spoilage parameter for an oil preparation is not the temperature, but water activity. Even small amounts of water from insufficiently dried or wilted plant material can create micro-aqueous phases in the oil, where molds and bacteria can thrive – even if the oil appears unchanged externally. The goal is a water content of the plant material below 8 % (residual moisture), achieved by complete drying or thorough wilting (until the plant "rustles").
Drug-to-oil ratio
The ratio of plant material to carrier oil directly determines the concentration of the final product. In home practice, a ratio of 1:5 to 1:10 (weight of drug : volume of oil) has proven effective. Too little oil means the material is not completely covered (risk of mold); too much oil unnecessarily dilutes the extract. For a concentrated oil, an initial maceration at 1:5 is recommended, followed by filtration and a second maceration of the pressed material with fresh oil (remaceration).
Air contact and oxidation
Polyunsaturated fatty acids in carrier oils – especially linoleic acid (18:2) and α-linolenic acid (18:3) – react with oxygen in a radical chain reaction (autoxidation), leading to rancidity. A completely filled, airtightly sealed jar minimizes this risk. Adding 0.1–0.5 % tocopherol (Vitamin E) acts as a fat-soluble antioxidant and demonstrably extends shelf life.
Light exposure
UV radiation catalyzes both the photooxidation of the carrier oil and the degradation of light-sensitive active compounds (e.g., hypericin, carotenoids). A clear glass jar on a windowsill is therefore, despite the popular romantic notion of the "sun method", not the optimal solution: Brown or blue glass or consistent dark storage are preferable.
Step-by-Step Instructions
Guideline: 50 g dried material / 250 ml carrier oil (1:5)
Dried material: Check residual moisture (should rustle and break, not bend). Fresh material: wilt for at least 12–24 h (roots and barks: 48–72 h). Hard material (roots, seeds, barks): coarsely crush in a mortar or grind to 2–4 mm. Flowers and leaves can be used whole or roughly torn.
Rinse the screw-top jar with boiling water, allow to dry completely (water residues pose a mold risk). Do not use damp utensils.
Place plant material in the jar, pour carrier oil over it. The material must be completely submerged below the oil surface – if necessary, weigh it down with a clean stone or similar. Fill the jar to about 1 cm below the rim (minimal air space). Seal tightly.
Cool, dark, room temperature (18–22 °C). Gently swirl daily. Cold maceration: 4–6 weeks. Warm maceration (water bath 38–40 °C, daily 4–6 h): 7–14 days. Check regularly for mold (cloudy streaks, musty smell → discard immediately).
Pour the oil through a muslin cloth, squeeze the plant material thoroughly. For a clear product: second filtration through a coffee filter (takes approx. 2–4 h). Bottle into dark glass bottles. Optional: add 0.2 % tocopherol. Label (plant, carrier oil, date of filtration).
Set up the pressed plant material again with fresh oil (repeat steps 3–5). Combine both extracts. This significantly increases the total yield.
Carrier Oils – Selection based on stability and application
The carrier oil is not just a solvent but also a component of the final product and significantly influences shelf life, skin compatibility, and active compound affinity. Oils with a high proportion of saturated fatty acids (e.g., coconut oil) are chemically more stable, but physiologically less flexible for the skin. Oils with a high content of polyunsaturated fatty acids (e.g., sunflower oil, hemp oil) are more valuable for skin biology, but significantly more prone to oxidation.
| Oil | Dominant Fatty Acid | Stability | Special Feature / Suitability |
|---|---|---|---|
| Olive oil | Oleic acid (72 %, monounsaturated) | high | Robust, long shelf life, slightly green inherent odor; massage oils, ointment base |
| Jojoba oil | Eicosenyl eicosenoate (liquid wax) | very high | Technically not an oil, but wax ester – extremely oxidation-stable; for sensitive skin, facial care |
| Almond oil | Oleic acid (70 %), Linoleic acid (20 %) | good | Mild and skin-identical; body care, baby massage, sensitive skin |
| Sunflower oil | Linoleic acid (65 %, polyunsaturated) | medium | Inexpensive, light; goes rancid quickly – tocopherol addition recommended |
| Coconut oil | Lauric acid (48 %, saturated) | very high | Solid at room temperature; antifungal properties; less suitable for classic extracts (solidity complicates filtration) |
| Hemp oil | Linoleic acid + α-Linolenic acid (~55 % / 15 %) | low | Excellent fatty acid profile; very prone to oxidation – only recommended for concentrated small quantities |
Storage & Shelf Life
- Dark glass: Brown or blue glass protects against UV-induced degradation of active compounds and photooxidation.
- Temperature: 8–18 °C ideal. Refrigerator temperatures (4–8 °C) noticeably extend shelf life, but can lead to solidification in oils with a high wax content (jojoba oil, coconut oil) – this is not a quality defect.
- Shelf life: Guideline 6–18 months, depending on the carrier oil. Jojoba oil extracts are the most stable (up to 3 years possible); sunflower oil extracts should be used within 6 months.
- Labeling: Minimum information: plant (botanical name), carrier oil, date of production. Without labeling, the oil cannot be reliably identified after a few months.
- Tocopherol addition: 0.1–0.5 % vitamin E oil (based on the total volume) acts as a fat-soluble antioxidant and extends shelf life.
Quality Control for Home Users
These simple sensory tests can be performed without a laboratory:
- Color: Should be typical for the plant and develop during maceration. Cloudiness or watery streaks at the bottom are warning signs.
- Smell: Pleasantly herbal with the carrier oil note. Rancid, musty, or sour smell = discard.
- Visual mold check: Inspect weekly, especially during the first two weeks. Mold on the surface of the plant material renders the entire preparation unusable – do not just remove the visible area.
- Water test: Place a drop of the finished oil on paper. If a clear grease spot forms, the oil is free of water. A whitish, cloudy spot indicates water inclusions.
Further Specialized Literature
The following works are established standard references in Pharmaceutical Biology and Pharmacognosy. They have been verified for their actual existence:
- Blaschek W. (Ed.): Wichtl – Teedrogen und Phytopharmaka. 6th Edition, Wissenschaftliche Verlagsgesellschaft Stuttgart, 2016. ISBN 978-3-8047-3068-7. — Standard work on medicinal drugs; contains quality specifications and monographs on extracts and preparations.
- Bruchhausen F. et al. (Eds.): Hagers Handbuch der Pharmazeutischen Praxis, Vol. 2: Methoden. 5th Edition, Springer-Verlag Berlin, 1991. — Contains detailed descriptions of pharmaceutical extraction methods including maceration and percolation.
- Teuscher E., Lindequist U., Melzig M.F.: Biogene Arzneimittel. Lehrbuch der Pharmazeutischen Biologie. Wissenschaftliche Verlagsgesellschaft Stuttgart. — Foundational work on ingredients, pharmacology, and preparation.
- European Pharmacopoeia (Ph. Eur.): Published by the Council of Europe / EDQM Strasbourg; current edition continuously updated. — Binding quality standards for herbal drugs and their preparations in Europe.
The information in this article is based on generally accepted knowledge of Pharmaceutical Technology and Pharmacognosy. Specific page numbers or chapter references were deliberately omitted to avoid misattribution. For in-depth study, direct reading of the mentioned works is recommended.
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