Supercritical Fluid Extraction

Supercritical fluid extraction (SFE) is a unit operation that exploits the high dissolving power of fluids at temperatures and pressures above their critical values. In view of the recent advances in process and equipment design, SFE will have a significant impact on the food industry.

Advantages of SFE processes for the food industry:

1)Low temperature separations: It produces extracts from natural products where the heat-sensitive components remain undamaged.

2) Solvent residue: The extracted solute can be made almost solvent-free i.e., the residual solvent content can be reduced to almost zero ppm.

3) Solubility variation: The solubility variation of active constituents can be achieved by simply increasing or lowering extraction temperature and pressure.

4) Fractionated extraction and fractionated separation: The different constituents of natural products may be fractionally extracted and the extracted solutes may be fractionally separated.

5) Cheap solvents: Solvents can be completely recycled and also gases like carbon dioxide which are cheap may be used. This gives lower cost of solvents compared to conventional liquid-liquid extraction.

6) Low energy process: The SFE is a less energy intensive process requiring low utilities consumption.

7) Fast diffusion: The diffusivity of supercritical fluids is higher than that of liquids and viscosity is lower than that of liquids. This gives higher mass transfer rates.

8) Easy solvent recovery: Due to the high volatility of solvents they can be easily recovered.

In addition, there are no environmental risks and no fire hazards, when carbon dioxide is used as the solvent.

4.1. Industrial applications of supercritical fluid extraction in food industry

Commercial processes are available and used in the production of decaffeinated coffee and in preparing hops extract for beer brewers. These processes use carbon dioxide in the supercritical and in the liquid state.

In beer brewing industry there is a preference to carry out fermentation with hops extract, rather than using whole hops. This permits the preservation of raw material over a longer brewing season and provides a better quality raw material. Two variants of hops extraction process are used. One variant uses liquid carbon dioxide. This is offered for licence by the Brewers Research Foundation, London. Commercial plants using this process are at Pauls and Whites Brewery in U.K. and Carlton Ltd. in Australia. The other variant uses supercritical carbon dioxide. This is offered for licence by SGK of Mullheim, Germany. A plant using this technology is in operation at SKW AG, Trostberg, Germany. Pfizer in U.S.A. also uses this technology to separate the bittering agents from hops.

In addition, commercial activity or interest has been reported at over thirty companies including some end users of the technology, equipment suppliers and engineering companies. The activity is generally in the areas of extraction of spices, essential oils, oils and fats, cholesterol, flavours, pharmaceutical materials, and in supercritical fluid chromatography. All these process applications illustrate the wide range of applications of extraction with supercritical fluids in process industries. Excellent compilation for the use of supercritical fluid extraction are published from time to time. These give the status of the technology viz., laboratory scale, pilot plant scale or commercial scale. Brunner and Peter (1982), Randall (1982), Paulitis et al (1983) have presented some of the earlier exhaustive reviews. Koerner (1993) has given a listing of the pilot plants and commercial plants for supercritical fluid extraction. Table 1 highlights the use of supercritical fluid extraction in the processing of natural fats and oils (based on the information from various sources). Table 2 presents the operating conditions for the extraction of caffeine and coffee oil from coffee, humulones from hops, nicotine from tobacco and oleoresins from spices. As seen from the tables wide variation of conditions have been used for the extraction and separation.

4.2. Description of the process

A supercritical fluid extraction system consists of four basic components: an extractor, a separator, a utility section and a temperature/pressure control system. Additionally, other equipments such as system for the use of entrainer, valves, back pressure regulators, flow meters etc. are needed for the proper operation of the system. Schematic diagram of a supercritical fluid extraction unit is presented in Figure 1. The gaseous carbon dioxide is condensed and stored in the condenser, and brought to the required extraction pressure by a diaphragm metering pump. It is then heated to the required extraction temperature in a heat exchanger.

Table 1

Processing of natural fats and oils by supercritical fluid extraction

Mixture Extracting Entrainer Operating conditions of agent -

Extraction Separation

Mixture Extracting Entrainer Operating conditions of agent -

Extraction Separation

Lanoline from wool grease

Propane, propylene

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