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Extraction and its types

Jun 29, 2020
Categories:  Scientific publication
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Author:  Vita Forest

Extraction (from Latin extraho - I’m extracting) - is a method of extracting a substance from a solution or dry mixture with the help of the proper solvent (extractant).

All the existing methods of extraction can be classified - according to the way the process goes - into static and dynamic. During the static method the extractant is poured over the raw material every now and then and left to stay for a while for fermentation. Dynamic methods, on the contrary, involve a constant change of the extractant or both raw material and extractant being used.

Another way of classifying extraction methods is according to the direction of the flow - so there we find the continuous-flow extraction (the solvent and the raw material are flowing in the same current) and the countercurrent extraction (the solvent and the raw material are moving towards each other) .

In order to prepare a high-quality extract various extraction techniques are used:

  1. Maceration and re-maceration
  2. Percolation and re-percolation
  3. Countercurrent extraction
  4. Various techniques that include grinding inside of the extract, turbo extraction, use of ultrasound waves (ultrasonic extraction), impulse magnetic field, electro-impulse action/impact etc.

Maceration (or fermentation from Latin maceratio - soaking)

Ground raw material is put into the maceration container together with the required amount of solvent. Then one lets it ferment at the temperature of 15 to 20 °C constantly stirring the mixture. When not otherwise specified the standard fermentation time amounts to 7 days. Nowadays the exact time of fermentation is figured out individually for each type of raw material using extraction kinetics. After the fermentation the extract is poured out and the rest is pressed. The used raw material (schrott) is then washed out with solvent, pressed again and added to the previously poured out extract. Afterwards this mixture is left to ferment and more solvent is added to reach the necessary quantity.

The obvious advantages of maceration are the simplicity of the process and no need for complex equipment. However there are also disadvantages to be found such as that the substances do not get fully extracted, the process is too lengthy and includes multiple stages (double pressing and washing out the schrott) and there is excessive amount of dietary fibers left in the final product (pectins, proteins, high-molecular-mass compounds etc.)

Re-maceration (fractional maceration)

During this method the general amount of solvent is divided into 3 to 4 parts and add to the raw material gradually for fermentation. Every other time the ready extract is poured out. The time of the fermentation depends on the character of the raw material. Such extraction process allows to wear out the raw material faster to the full extent because of the significant difference in concentration between the raw material and the solvent.

Percolation (from Latin percolatio - straining through…) 

This process involves straining of the solvent through the plant raw material in order to take out the substances that can be solved in the extractant. Containers of various construction called percolators are used for the process.

The method includes three stages: soaking of the raw material, fermentation and percolation itself.

Soaking is carried out outside the percolator. Most often, maceration tanks or other containers are used, from which it is convenient to unload soaked raw materials. The proportion of extractant to raw material can vary from 50 to 100%. The raw material is left to stay for 4-5 hours in the closed container. During this time, the extractant penetrates between the particles of plant material and into the cells, the raw material swells, increasing in volume. In this case, the active substances dissolve inside the cell. This stage can be combined with insisting. However, if the raw material is capable of strongly swelling, the soaking stage must be carried out in a separate container, since due to a large increase in the volume of material in the percolator, it can be strongly compressed and do not miss the extractant at all.

Infusion is the second stage of the percolation process. After the raw plant material is put into the percolator the extractant is added and let to ferment for 24 hours (for the raw material that is easier subject to extraction and 48 hours for the drier material)

The percolation itself is a continuous straining of the solvent through the raw material and the collecting of the percolant. During this process the draining of the percolant is accompanied with the constant inflow of the solvent 

Re-percolation

Re-percolation meaning repeated percolation allows to use the solving capacity of the extractant to the full extent and obtain concentrated extract completely depleting the raw material. The process takes place in several (from 3 to 10) interacting percolators at once connected to each other. On this whole percolator chain the countercurrent of the raw material and the extractant is observed. The finished product is drained from the “head” percolator, in which there is always fresh raw materials, and the fresh extractant is fed to the “tail” percolator, in which the most depleted raw materials. Extracts from the “tail” percolator process raw materials in the previous percolator and so on throughout the battery — the subsequent raw materials are extracted with extracts obtained from previous percolators.

Countercurrent extraction

The essence of the method is the stepwise advancement of the pure extractant from more depleted raw materials to less depleted. The most depleted plant material is extracted with a pure extractant, and concentrated extraction is collected from the extractor with freshly loaded raw materials. The countercurrent principle of supplying raw materials and extractant, the continuous movement of not only the liquid, but also the solid phase contributes to the achievement of a high difference in concentrations, convective diffusion of the extracted substances in the extractant layer and the creation of an effective extraction surface, and this greatly intensifies the process.

This type of extraction is carried out in various ways: in the battery of extractors, when the raw material is stationary, and only the extractant moves; in continuous extractors, where the raw materials and extractant move towards each other. The countercurrent continuous extraction method is used for large-scale production associated with the processing of large volumes of LRS.

Turbo-extraction

Turbo-extraction is based on a very intense stirring of raw material by high speed by turbine agitators or by agitators with multiple propeller blades. The high speed of the stirring creates proper conditions for the uneven pressure upon the flow of the mixture which causes such effects as cavitation and pulsation and as a result increases the speed of the inner diffusion. The extraction time is reduced to 10 minutes. During the intensive stirring the raw material is getting ground so the extraction process is completed through the washing out of the extractant substances. The extraction is saturated, but it contains many small particles of plant material, which greatly complicates further purification. Other disadvantages of this method include the increase in temperature during the operation of the mixers, which can affect the safety of biologically active substances.

Ultrasonic extraction

Ultrasonic oscillations are effectively used to accelerate and intensify the extraction process. The ultrasonic waves create regular pressure, cavitation and sonic wind. An ultrasound source is placed in the medium to be treated or attached to the body of the maceration tank in a place filled with extractant and raw materials. The greatest effect is manifested when the cell of the extracted material is well saturated with an ultrasonic conductive extractant. The impregnation of the raw material as well as the solving of the cell content is accelerated, increasing the speed at which 

Molecular diffusion inside the cells of the raw material and in the diffusion layer turns into convective diffusion which intensifies the masses exchange. An extract can be obtained within a few minutes, however, due to the destruction of cells, it will contain a lot of ballast substances and suspended particles of material, which complicates further cleaning.

Extraction with the use of impulse magnetic field

This method is based on the effect/impact of magnetic field on the raw plant material. The impulse magnetic extractor contains a membrane that is sending an impulse movement to the extractant. This occurs at the frequency at which the magnetic field changes. As a result of the oscillatory movement of the membrane a flat impulse of the regularly alternating pressure appears which triggers off the actual extraction through creating cavitation in the extractant.

The advantages of the method are the possibility of conducting the process with a small ratio of raw materials-extractant (1: 4), the absence of moving metal parts of the equipment, the reduction of microbial contamination of the processed raw materials and the reduction of energy consumption.

Extraction through electro-impulse impact

The use of electro-impulse discharges allows to accelerate the extraction from the raw material with cell structure. Electric discharges create conditions for the very high speed flow of inner cell diffusion while the molecular transmission of the substances is substituted with the convective transmission. Inside the extractor with the processed raw material, electrodes are placed, to which a high or ultra-high frequency pulse current is supplied. Under the impact of the high-voltage impulse discharge waves are created in the extract mixture that cause impulse pressure and potent cavitation processes. As a result, intensive mixing of the treated mixture occurs, convective diffusion increases, and cellular structures of plant material are partially destroyed. From the destroyed cells is the leaching of biologically active substances.

New methods of extraction for the plant raw material are continuously being designed and introduced into the industry every year. To obtain a high-quality extract producers opt for the best suitable method meeting their needs such as production facilities as well as character and type of raw material.