Methods of herbal extraction
In the traditional extraction method, water is commonly used as a solvent. For scientific research, effective extraction methods are needed and organic solvents are generally selected according to the plant's medicinal constituents. Boiling is a sample of method using heat and suitable for heat stable compounds. For heat labile substances, maceration or other methods without heat are preferable. Advanced technology like supercritical carbon dioxide extraction is more effective but also more expensive.
Maceration
The crude drug should be soaked with a suitable solvent in a closed container for a few days without heat. Thus, the dried plant tissues would be softened to allow the solvent to fully penetrate and dissolve out the chemical compounds. Afterwards, the extractive is decanted and the residue is squeezed to remove all the extractives. Soaking the residue, again and repeating until it is exhausted, all extractives are combined then filtered. A stirrer may be inserted inside the container to stimulate the extracting capacity. The disadvantage is that multiple portions of solvent are used.
Percolation
Percolation is modified from maceration with the use of the equipment called a percolator. The soaked crude plant powder is packed steadily into the percolator, and more solvent is added and allowed to pass through slowly. The chemical constituents are extracted and collected in the receiver. New solvent will be added on top and the process is repeated until exhausted. The disadvantage is that this method still wastes many portions of the solvent.
Continuous Extraction
Soxhlet apparatus is a continuous extractor using heat. In this process, the equipment can be divided into 3 parts; boiling flask containing solvent on a heating mantle, extraction chamber containing a thimble packed with powdered drug, distillation arm and siphon sidearm and the upper part, a condenser with cold water running inside. The solvent used must have a low boiling point such as ethanol. The solvent is heated in a boiling flask and vaporized through the distillation arm, condensed in the condenser, then dripped over the thimble in the extraction chamber. When the solvent in the extraction chamber reaches the proper level, the siphon exit allows the extractive from the thimble to run down to the boiling flask where the extract becomes concentrated. This process repeats continuously until the extraction is exhausted. This method is preferable for heat-stabled constituents. The advantages are the fewer amounts of solvent used, more concentrated extract obtained, and more time saving.
Ultrasonic extraction
This is an extraction method without heat and safe for the environment. The plant material is placed in the container with solvent and immerse in a high-speed ultrasonic bath with frequency range from 20 kHz to 2,000 kHz. The ultrasonic vibration will result in cavitation bubbles in a solvent material. When these bubbles collapse near the cell walls, cause those cells walls to break and release their contents into the solvent. This extraction technology can reduce the processing time and solvent used.
Supercritical Carbon Dioxide extraction
Supercritical Carbon Dioxide Extraction is a method using liquid carbon dioxide which is modified to a supercritical temperature and pressure, resulting in effective performance in extracting nonpolar and volatile substances such as essential oils. This extraction method is the most effective and fast, without heat or toxic chemicals. The disadvantage is that this process is expensive.
Concentration and drying process
Normally, large portions of solvent are required to obtain exhausting extract. At a laboratory research and development scale, up to industrial scale, dried extract is needed because it is more practical for the quality control and formulation processes. Many techniques have been developed to remove the solvents from the extractive. Some are simple and cheap, but modern techniques are more effective, complicated and expensive.
Free Evaporation in Water Bath
Free evaporation is the simple process, especially for aqueous or alcoholic extracts of heat tolerant compounds. The extractive is poured over a dish and placed in a water bath. The disadvantages are the loss of solvent and other heat labile or volatile compounds, and if extracted with organic solvents such as chloroform, the vapor is toxic to the environment. Most of the concentrated products are viscous with dark colour.
Rotary Evaporator
A laboratory rotary evaporator is used for continuous distillation under reducing pressure. This reducing pressure allows the solvent to evaporate from the extract at a lower temperature, thus reducing the risk of decomposition of the active compounds. The solvent vapour is condensed in the condenser and can be recovered for usage. At an industrial scale, this equipment is augmented and made of stainless steel instead of glass.
Spray dryer
Spray drying process is suitable for heat-stable extract. To produce atomizing solution, the concentrated extract is subjected to a spray drying chamber with high pressure pump at a controlled feed rate, the temperature of during chamber is about 150 degree Celsius with low pressure for a few minute, allowing the solvent to evaporate leaving dried solids of essentially the same size and shape as the atomized droplets. Finally, the dried powder is separated from the gas stream and collected. Aqueous and hydro-alcoholic extractive are commonly used. The final product is in dried powder which can be easily formulated into tablets or capsules.
Freeze Dryer
Freeze Drying is a dehydration process typically used to preserve heat-labile and high cost substances. Freeze-drying works by freezing the material to at least from -20 °C up to -50 °C and then reducing the surrounding pressure to allow the frozen water in the plant extract to sublimate directly from the solid phase to the gas phase, leaving only a freeze-dried substance.