When choosing which method of extraction to employ on various medicinal plants, there are several factors to consider. Temperature, materials used, time and pressure can be all factors affecting the final yield. Depending on which kind of molecules are being extracted, different techniques or solvents can be used to maximize their extraction from natural sources.
In each instance, certain techniques may prove more fruitful than others. By analyzing how each extraction method works, it becomes easier for manufacturers to compare which process will ultimately produce the best results.
Maceration
Maceration is the simplest and cheapest form of extraction. Plant material, typically rough material like bark or leaves, is placed within a container which is then filled with some kind of solvent. Water or ethanol are common extraction solvents, but it is also possible to use oils or some other non-polar solvents.
The solution is then allowed to sit for an extended amount of time. Often, the minimum is for three days, but it is possible to extend this timeline to increase the saturation.
During the absorption period, the solution is regularly stirred to allow increased extraction of plant components into the solvent. At the end of this soaking period, the plant material can be separated by filtration, leaving behind the extract that can be then purified from the extractant. While this is a cheap and safe form of extraction, the primary downside is the time it takes to employ this method and the large amount of solvent used.
Infusion
Sharing a lot of similarities with maceration, infusion seeks to improve on some of its flaws.
Prior to the extraction process, the plant material to be used in infusion is typically ground to a fine powder to increase the surface area to volume ratio. This can help to speed up the extraction time by exposing more of the plant to the solvent. The other aspect that speeds up the extraction is to apply heat to the solution. This step is not mandatory, but it is a commonly used practice with infusion. These operations can shorten the extraction period when compared to maceration, however they also add additional costs: energy is required to grind the material, either mechanically or manually, and the heating of the solution will increase expenses and materials needed. Additionally, as heat is being applied, it is important to consider any safety risk with the solvent being used. Balancing the right amount of heat will prevent any potential explosion or loss of valuable thermolabile compounds for the final yield.
Digestion
For plant materials that are readily soluble, digestion may prove to be an effective extraction method. First the solvent is added to a clean container, followed by the plant material, before finally applying a low level of heat. This heat may be in the form of a water bath, an oven, or, in some occasions, with microwaves. The temperatures generally remain low, approximately 122 degrees Fahrenheit (50 degree Celsius), and are applied consistently through the extraction process. This method produces similar results as infusion, but instead of being like tea it is more like baking. Additionally, with lower temperatures, the risks of heat associate are limited as well. Digestion is an effective extraction method used mainly on tougher plant parts containing poorly soluble substances.
Decoction
Falling into a similar category of maceration, decoction is a water based extraction method that relies on heat to shorten the extraction time. Other solvents like aqueous ethanol or glycerol may be used instead of water. First, the plant material is ground or shredded into a fine powder before being mixed with water in a clean container. The ratio of water to plant materials varies, but it is typically in the range of 4:1 to 16:1. Heat is then consistently applied for 15 minutes while the solution is continually stirred. The stirring and the heat both help to break down the plant material to speed up the extraction. Times applied may vary, but the extraction can often be finished relatively quickly. The method is only used for compounds that are water-soluble and heat-resistant.
Percolation
Percolation marks the beginning of more complex extraction methods due to the equipment it requires. A percolator is a cone-shaped piece of glass with a large opening at the top and a much smaller opening at the bottom and a filter in the middle. The crude plant material, typically in powdered form, is combined with a solvent and allowed to soak for a period of around 4 hours. Afterward, the solution is put in the percolator, where it remains for a period of 24 hours straining with the soaked solvent through the filter. This gravitational pressure allows for up to 75% of the plant material to be extracted without the use of heat. Because of this, percolation has some similarities to maceration, but it is more efficient in both timeliness and extraction rates due to the equipment it uses. Often, once the percolation has finished, the solvent is also put through an additional filtration or decantation procedure to remove any unwanted plant materials.
Soxhlet Extraction
Soxhlet extraction is essentially a continuous heat based extraction. This method requires a complex equipment set-up: a round bottom flask is put at the bottom, with an extraction chamber, siphon tube, and a condenser placed above it. The solvent is placed into the flask at the bottom, and the plant material, typically finely ground, is added to the extraction chamber above it. As the solvent is heated, it evaporates up to the condenser, where it reforms into a liquid and passes through the plant material in the extraction chamber. From there, the solvent returns to the flask below, where it can be reheated and the process repeats. The advantage to this system is that it allows for the complete extraction of all plant material. As well, because the system is a closed loop, it only requires a relatively minimal amount of solvent. This allows for maximum yields with minimal wasted materials. There are two considerations prior to employing this method. The first is that it can be dangerous if it is used with any materials that are sensitive to heat. Second, the plant material must be soluble in whatever solvent is being employed, otherwise it will only destroy the compounds with heat without proper extraction.
Microwave Assisted Extraction
Microwave assisted extraction has the potential to significantly decrease both the extraction time and the amount of required solvent. This method works by converting electromagnetic energy into heat thanks to microwaves, allowing for a very rapid heating of the plant material. This is most effective when using a polar solvent, as the microwaves can cause a dipolar rotation to penetrate the plant material promoting an efficient extraction. With this technique it is possible to enhance the extraction efficiency and improve yields: the heat produced permits to lower the extraction temperatures and speed up the overall process. Nevertheless thermolabile compounds could be affected and the extraction could proceed with low selectivity.
Ultrasonic Assisted Extraction
Ultrasonic assisted extraction utilizes sound (typically around 20 KHz) to break down plant cells, allowing for maximum solvent penetration and mass transfer. This means that increased yields can be obtained without relying on any additional heat or extended extraction time. A possible disadvantage of this technique is that prolonged exposure to ultrasonic waves could induce changes in the extracted molecule leading to possible artifacts and side products formation.
