Recently, natural food additives consumers posed a question: why earlier well-known sub critical CO2 spices and herbs extracts are trying to be replaced by supercritical? What is their difference?
The difference is in the following:
- In much higher temperature and pressure during the supercritical extraction, this destroys extremely valuable unstable volatile elements;
- In obtaining extracts with various chemical composition from the same plant;
- In properties of the obtained extract and its applicability to an end-product, where the extract is used;
- In economic feasibility, when choosing the type of a used extract.
Let's consider the existing extraction ways.
Isolation of aromatic components from a cellulose matrix is usually conducted with the help of the water vapor distillation method or with the help of extraction methods (fig. 1).
Water vapor distillation of volatile components is conducted at the temperature of about 100 °С and allows to extract substances from vegetative raw materials at rather low boiling temperatures (150 - 250 °С). To the advantages of the steam distillation method could be added such benefits as its feasibility in field conditions and low cost of essential oils. However, at high distillation temperatures volatile substances undergo danaturation and some valuable components get modified, aromas get distorted. Besides, some essential oils require fining due to the formation of undesirable decomposition products. Storage periods of such oils are limited, already in 6-9 months periods, the polymerization process begins and smell gets even more deformed. Vacuum distillation is a more progressive method in comparison with the usual atmospheric ones, however it is used, mainly, for crude oils fining.
It is also widely known the extraction of valuable components from vegetative raw materials with the help of organic solvents: acetone, gasoline, hexane, dichlorethane, diethyl ether, isopropanol, petroleum-ether, ethyl chloride, ethanol, and et. Extraction oils, in most cases, have a higher quality than steam distillation oils, as the structure of extracts includes natural fixing agents and flavoring substances, which are usually absent in essential oils. The structure of extracts significantly depends on the used solvent/3/. For example, when using hydrophilic solvents (acetone, glycerin, ethanol) there are extracts obtained, which are soluble in the water and contain significant amounts of painting and other undesirable substances. Waterproof solvents (hexane, dichlorethane, tetrachloride carbon, etc.) better extract fatty substances, however they do not isolate carbohydrates, pitches, gum. World practice shows, that extract oils have their consumers and are widely used in the food, perfumery - cosmetic industry, household chemical goods and light industry. However, the majority of used solvents are flammable and explosive, some of them are toxic. High-temperature solvent distillation from miscella (as well as in case of steam distillation) results in undesirable qualitative structure changes of extracts.
Better quality extracts are obtained due to the use of liquefied gases as solvents.
The method of valuable substances extraction from raw materials with the help of the liquid food carbon dioxide, CO2 in the sub critical condition (critical point Р=7,4 МPa, t=31,10 °С) got practical application. Its main advantage, that CO2 - participates in the metabolism of all living substances on the planet and exists in a human body. Certainly the following conditions are attractive, when choosing the liquefied CO2, as an extracting agent: ecological safety and cleanliness, selectivity in relation to a number of biologically active substances, absence of residual solvents in the product, fire-and explosion safety, chemical inertness in the absence of a catalyst, high molecular diffusion coefficient, low viscosity and superficial tension. To the important advantages, except for the listed above, availability and low cost of carbon dioxide, as well as the multi-variant approach and durability of equipment operation could be attributed.
A parametrical line of extraction equipment has been developed, technological modes of valuable components extraction from more than 100 kinds of vegetative raw materials are mastered and constantly improved, and various ways of technological processes intensification are approved and used in manufacture.
On fig. 2 there is the scheme of one of the installations, successfully used at this enterprise. A distinctive feature of such installation is the possibility of solvent supply in the shop from a stationary large-tonnage tank; the advanced evaporator design, allowing conducting fast solvent distillation from miscella in a thin layer. In order to reduce the extraction process duration, ultrasonic fields of various intensity are used. Also the removal of air remains from an extractor with the loaded raw material is provided, which enables to conduct the components extraction process in strictly adjustable conditions. The condenser-cooling scheme has been modified and the technological defects of a draft tube (vapor tube) have been eliminated.

Raw materials - mainly spice-aromatic

Figure 1. Processing methods of aromatic vegetative raw material.

Figure 2. The scheme of the trial installation, mounted at the extraction factory
1 - extract collector, 2 - evaporator. 3 - miscella dispersion device, 4 - vapor pipe, 5 - heat exchanger 6 - condenser, 7 - liquid carbon dioxide collector, 8 - extractor loading hatch, 9 - vacuum pump, 10 - extractor, 11 - drainage device, 12 - reserve capacity for CO2, 13 - filter.

Further, we analyzed the extraction ability of carbon dioxide in sub- and supercritical conditions, using our own researches' results, the published data, and the data gathered from the Internet. In tab. 1 there are shown the extraction results of the basic organic compounds from vegetative raw material as a result of various extraction methods.
Essentially there is a question: whether it is necessary to extract completely all components from vegetative raw materials, or prefer a selective way and extract only those groups of substances, which are able to improve qualitative characteristics of a ready product. A unique extraction agent - carbon dioxide in the sub- critical conditions, meets these requirements.
It is particularly interesting, that at room temperature (+18 … +22 °С) liquid CO2 during the first extraction stage isolates volatile compounds, and in the process of their accumulation miscella obtains properties of a less selective solvent and its usage, as an extraction agent, allows to enrich the extract with other valuable components in the constant, natural conditions.
The best confirmation of the sub-critical СО2-exctraction development prospects can be the 12- year successful working experience of an extraction factory. Application of СО2-extracts in foodstuff is already not simply a part of technology, however the ideology of those, who aspire to conduct a good business in the manufacture of a quality and healthy product.
Now the company is able to provide the meat, fish and packing industries of the country with high-quality СО2-extracts.
The method of supercritical extraction (desorption) of valuable components from vegetative raw material started to be mastered rather recently, but not always for those kinds of raw material, where such method is justified.
The main advantage of the supercritical extraction is an opportunity to conduct a graduated, preparative extraction of certain valuable components from raw material. As for the disadvantages, such factors as the complexity of equipment, working under the pressure of up to 1000 atmospheres (100 МPа), necessity to maintain rather high processing temperatures: from 40 up to 90 °С.

The substance name Solvent and extraction method
Sub- critical CO2 up to 7 МPа Sub-critical CO2with a pulsator Sub- critical CO2 with an ultrasonic generator Sub- critical CO2 with the use of the CO-extraction effect. Sub- critical CO2 in a mix with hydro carbonic gases Supercritical CO2 up to 60 МPа Supercritical CO2 up to 100 МPа
Aldehydes, cetone            
Complex ethers            
Flavone aglycones            
Organic acids            
Phenolic compounds            
Mineral substances            
Amino acids              
Fibers, peptides              

It is not necessary to completely deny the advantages of the sub- and supercritical extraction methods. Each technology occupies its own consumer niche. For example, the supercritical extraction is successfully used abroad for caffeine extraction from coffee, nicotine from tobacco, oils from soybeans, etc. However, there is no point in applying supercritical extraction, for example, with regards to:
- Clove buds or laurel leaves (except for bitter fractions extractions, which are bad for a human being)
- Seeds of coriander, anise, fennel, caraway, anise tree, nutmeg and other aromatic raw materials with the high contents of fat oils (up to 20-40 %) for exception of less valuable fat oil extractions, reducing an extract storage period and decreasing the concentration of target aromatic and flavoring substances.
- Specific aromatic plants, which are valuable only because of essential oils presence, and that the deep extraction products don't have to be present in their extracts structure.
Supercritical extracts have lower cost in comparison to sub-critical CO2 extracts, however the contents of aromatic and flavoring substances in these extracts is lower. It is possible to conceder the processing of amaranth seeds, as an example. If the output of CO2 extracts is 4 %, when using the sub-critical extraction method of isolating biologically active substances from amaranth seeds with liquid CO2, and the contents of the most valuable main substance squalene in such an extract is 23 % (out of the sum of extraction substances), the output of the supercritical extract can be as high as 8 %, due to the fat oil, available in the seeds, however the squalene contents in this extract will be 12 %. In this case, the supercritical extracts should have the price twice below the amaranth CO2 extract price, which, probably, would be unprofitable to supercritical extracts manufacturers.
Scientific forecasts have shown that the extraction of valuable components by the sub-and supercritical carbon dioxide is going to remain the most perspective technological process for at least 10 years. These technologies are economically justified in case of valuable raw materials processing with the high contents of biologically active substances. Reduction in quality of dry spices also improves the prospects of using sub-critical CO2 extracts, as quality of such extracts is always stable. Creation of original recuperation and regeneration systems of solvents will make these technologies economic and ecologically safe. We would like to invite all interested firms to cooperate with us.

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