How to Process Medicinal and Aromatic Plants

Medicinal and aromatic plants (MAPs) go through a range of processing steps before they reach us as herbal teas, essential oils, tinctures, or therapeutic formulations. The raw plant material, no matter how potent, is highly perishable and often contains compounds that are difficult to use directly. Processing is what transforms these living plants into stable, usable products – preserving their most valuable compounds while extending their shelf life. Whether the goal is to dry a batch of tulsi leaves or to extract lavender essential oil, the method chosen has a direct impact on the quality and potency of the final product.

Why processing matters for medicinal and aromatic plants 🔗

Fresh medicinal and aromatic plants contain high levels of moisture – leaves typically hold 60-90% moisture, roots and rhizomes 70-85%, and seeds around 5-10%. At these moisture levels, the plant material is vulnerable to microbial contamination, enzymatic degradation, and rapid loss of active compounds. Processing brings the moisture content down to safe levels (usually around 10%) and captures the beneficial compounds in a stable form.

The right processing method depends on two things: the type of plant and the intended end use. Medicinal plants – valued for their therapeutic compounds like alkaloids, glycosides, and flavonoids – are often processed through drying, boiling, or solvent-based extraction. Aromatic plants – prized for their volatile essential oils – are typically processed through distillation or expression techniques. According to the United Nations Industrial Development Organization (UNIDO), the general extraction techniques for medicinal plants include maceration, infusion, percolation, decoction, and hot continuous extraction, while aromatic plants rely on hydrodistillation methods such as water distillation and steam distillation.

Drying: the foundation of medicinal plant processing 🔗

Drying is the most widely used and most fundamental method of preserving medicinal plants. It reduces the water content of the plant material to a level where microbial growth is inhibited, enzyme activity slows down, and the product becomes stable enough for long-term storage and transport.

However, drying is not a one-size-fits-all process. The temperature, airflow, and duration of drying all affect the quality of the final product. Research on medicinal plant drying has shown that temperatures between 30°C and 50°C are generally recommended to protect sensitive active ingredients. For instance, studies on sage (Salvia officinalis) found that the optimal drying temperature was 50°C – higher temperatures caused visible discolouration and loss of active compounds.

Common drying methods 🔗

Sun drying is the oldest and simplest method, still widely practised in tropical and subtropical regions. Fresh herbs are spread on ventilated racks and exposed directly to sunlight. While cost-effective, sun drying has significant drawbacks. It can cause substantial degradation of colour and aroma – for example, studies have found that sun-dried roman chamomile loses more of its key volatile compounds compared to samples dried with controlled hot air at 40°C.

Shade drying is a step up from sun drying. Plants are placed in a well-ventilated area away from direct sunlight, which helps preserve colour and volatile compounds better than sun drying. This method is especially important for leafy herbs and flowers where light exposure can break down sensitive phytochemicals.

Oven or hot-air drying uses controlled temperatures and airflow in a closed chamber. This method gives processors much more control over the final product quality and reduces drying time significantly. It is the preferred method for commercial-scale operations where consistency matters.

Solar dehydration is an improved version of sun drying that uses indirect solar power. The herbs are not exposed directly to sunlight but rather to solar-heated air. These dryers use the natural convection of rising hot air, making them energy-efficient and suitable for rural farming areas where electricity may be limited.

Freeze drying (lyophilisation) removes water by first freezing the material and then reducing the surrounding pressure to allow the frozen water to sublimate directly from solid to gas. This preserves heat-sensitive compounds far better than any thermal method but is expensive, making it suitable mainly for high-value products.

Extraction methods for medicinal plants 🔗

While drying preserves the whole plant material, many medicinal applications require the isolation of specific active compounds. This is where extraction comes in – the process of separating medicinally active constituents from plant tissues using selective solvents.

Maceration 🔗

In maceration, the plant material is soaked in a solvent (usually water, alcohol, or a mixture) at room temperature for an extended period, typically several days. The solvent slowly penetrates the plant cells and dissolves the active compounds. This is one of the simplest extraction methods and is commonly used in traditional medicine systems. It works well for soft, delicate plant parts.

Decoction 🔗

Decoction involves boiling plant material in water. It is particularly effective for tough plant parts like roots, bark, and woody stems where active compounds are difficult to extract at lower temperatures. Traditional Chinese medicine and Ayurveda rely heavily on this technique. The plant material is typically simmered for 20-60 minutes, depending on its hardness, and the resulting liquid concentrate is used therapeutically.

Percolation 🔗

Percolation passes a solvent through a bed of ground plant material. Fresh solvent continuously flows through, maintaining a concentration gradient that drives extraction more efficiently than simple soaking. This method is widely used in the pharmaceutical industry for producing tinctures and fluid extracts.

Soxhlet (hot continuous) extraction 🔗

Soxhlet extraction uses a specialised apparatus where a solvent is continuously heated, vaporised, condensed, and recycled through the plant material. This provides a very efficient extraction because fresh solvent repeatedly contacts the material. It is commonly used in laboratory and industrial settings for obtaining concentrated extracts from medicinal plants.

Steam distillation: the primary method for essential oil extraction 🔗

When it comes to aromatic plants, steam distillation is the dominant processing method. Approximately 93% of essential oils worldwide are extracted through steam distillation, with the remaining 7% obtained through other methods.

The process works as follows: steam is generated (usually in a separate boiler) and directed through plant material packed in a stainless steel vessel called a still. As the steam passes through, it ruptures the tiny glands in the plant tissue that hold the essential oil. The heat causes the volatile aromatic compounds to evaporate and get carried along with the steam. This vapour mixture then travels into a condenser, where it cools and returns to liquid form. Because essential oils are not soluble in water, the condensed liquid naturally separates into two layers – the essential oil floats on top, and the water (called hydrosol or hydrolat) settles below.

Temperature and pressure control are critical during steam distillation. Temperatures typically fall between 60°C and 100°C (140-212°F), with pressures around 15-20 PSI. Too much heat or pressure can degrade the essential oils or introduce unwanted non-aromatic compounds into the product. The duration also varies – some plants take just a few hours, while others may require several days of continuous distillation.

Different plants have different distillation requirements. Delicate flowers like rose petals need low-temperature, short-duration distillation. Tough, woody materials like sandalwood or vetiver roots may need prolonged distillation with higher steam pressure. Even harvest timing matters – jasmine flowers, for example, must be harvested at dawn when their aromatic compound concentration peaks.

Hydrodistillation 🔗

Hydrodistillation is a variation where the plant material is directly immersed in water rather than being contacted only by steam. The water and plant material are heated together, and the resulting steam carries the volatile compounds to the condenser. This method is often used for delicate flowers that might be damaged by direct, pressurised steam. It is one of the oldest distillation methods still in use today.

Solvent extraction 🔗

Some plant materials – particularly delicate flowers like jasmine, tuberose, and certain varieties of rose – do not yield their aromatic compounds well through steam distillation. For these, solvent extraction is the preferred method.

In this process, a food-grade solvent such as hexane or ethanol is passed through the plant material. The solvent dissolves the essential oils along with other plant substances like waxes and pigments. The resulting mixture is filtered and then the solvent is removed (usually through vacuum distillation), leaving behind a waxy substance called a concrete. When the concrete is further treated with alcohol, the waxes separate, and the remaining product is known as an absolute – a highly concentrated aromatic extract. Absolutes are widely used in the perfume industry because solvent extraction captures a broader range of aromatic compounds than distillation.

However, a small amount of solvent residue can remain in the final product, which is a consideration for products intended for therapeutic or food-grade use.

Supercritical CO₂ extraction: the modern alternative 🔗

Supercritical carbon dioxide (CO₂) extraction has emerged as a cutting-edge processing technique for both medicinal and aromatic plants. CO₂ is used as a supercritical solvent because it is odourless, non-toxic, non-flammable, and recyclable, making it an environmentally friendly choice.

The process works by compressing CO₂ above its critical point (31.1°C and 73.8 bar), where it takes on properties of both a liquid and a gas. In this supercritical state, CO₂ becomes an excellent solvent that can penetrate plant material and dissolve targeted compounds. Once extraction is complete, the pressure is released, the CO₂ returns to its gaseous state and evaporates completely, leaving behind a pure extract with zero solvent residue.

Supercritical CO₂ extraction operates at lower temperatures compared to steam distillation (typically 35-60°C versus 100°C+), which means heat-sensitive bioactive compounds are preserved much better. The selectivity of the method can also be fine-tuned by adjusting temperature, pressure, and flow rate. For extracting polar compounds, small amounts of co-solvents like ethanol or water can be added.

The main drawback is cost – the equipment is expensive, and the high-pressure systems require significant investment. For this reason, supercritical CO₂ extraction is typically reserved for high-value products in the pharmaceutical, nutraceutical, and premium cosmetics industries.

Cold pressing 🔗

Cold pressing is a mechanical extraction method used primarily for citrus oils. The process involves pricking or scraping the fruit peel and then pressing it to release the essential oil, which is rinsed away with water and then separated. Cold-pressed oils retain their natural flavours, colours, and vitamins since no heat is involved. However, this method only works well for a limited range of plants (mainly citrus fruits) and is not efficient for large-scale extraction of most MAPs.

Post-processing: storage and quality control 🔗

Processing does not end with extraction or drying. Proper storage is essential to maintain the quality of the processed product. Essential oils must be stored in dark glass bottles to prevent light degradation. Dried herbs should be kept in airtight containers that prevent moisture absorption. Both types of products should be stored in cool, dark conditions with minimal exposure to oxygen.

For commercial products, standardisation is a major challenge. Unlike synthetic chemicals, plant extracts can vary significantly depending on growing conditions, harvest timing, geographic location, and processing parameters. Reliable producers invest in testing protocols, supplier management, and blending techniques to maintain batch-to-batch consistency.

Emerging technologies in MAP processing 🔗

The field of MAP processing is evolving rapidly. Several newer techniques are gaining ground in commercial operations:

Microwave-assisted extraction (MAE) uses microwave energy to heat the solvent and plant material simultaneously, dramatically reducing extraction time while improving yield. Ultrasound-assisted extraction (UAE) uses sound waves to disrupt cell walls, making it easier for solvents to reach and dissolve active compounds. Enzyme-assisted extraction uses specific enzymes to break down cell walls before or during extraction, improving the accessibility of target compounds.

These methods share common advantages: shorter processing times, lower energy consumption, and better preservation of heat-sensitive compounds. As these technologies become more affordable, their adoption in MAP processing is expected to grow significantly.

What do you think? Given that different processing methods can significantly alter the potency and quality of the final product, how important is it for consumers to know how their herbal products were processed? And with emerging green technologies like supercritical CO₂ extraction becoming more accessible, could these eventually replace traditional methods like steam distillation for most applications?

References

1. https://edepot.wur.nl/137179
2. https://www.unido.org/sites/default/files/2009-10/Extraction_technologies_for_medicinal_and_aromatic_plants_0.pdf
3. https://www.researchgate.net/publication/266214502_Drying_of_Medicinal_Plants
4. https://www.tandfonline.com/doi/full/10.1080/10408398.2020.1765309
5. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/steam-distillation
6. https://www.doterra.com/US/en/blog/science-research-news-steam-distillation
7. https://www.purodem.com/the-process-of-essential-oil-distillation/
8. https://www.customprocessingservices.com/blog/top-4-ways-to-extract-essential-oils-from-plants
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC6270407/
10. https://pmc.ncbi.nlm.nih.gov/articles/PMC11359946/