Conventional Solvent Extraction

Solvent extraction is the term used for liquid-liquid extraction as well as leaching since a solvent is used to preferentially separate one or more constituents from either a liquid or a solid mixture.

3.1. Leaching

Leaching is a widely used separation process for the following:

1) Extraction of edible oils from seeds, beans, nuts, rice bran, wheat germ, coconut and other sources.

2) Extraction of essential oils from flowers, leaves and seeds.

3) Extraction of oleoresins from spices.

4) Extraction of sugar from sugarbeet and sugarcane.

5) Extraction of coffee and tea.

6) Extraction of fish meal.

7) Extraction of active ingredients from leaves, pods, seeds, flowers and barks e.g., extraction of tocopherols, etc.

Owing to its widespread use in diverse industries, leaching is also known as lixiviation, percolation, infusion, elutriation, decantation, and settling. The two main operations in leaching are: (a) contact of liquid solvent with solid for transfer of solute from the solid to the solvent and (b) separation of the extract, from the residual solid. Other auxiliary operations are preparation of the solids for extraction and recovery of solute from the solvent by distillation or evaporation.

3.1.1. Preparation of solid material

The mechanism of leaching may involve simple physical solution or solution due to chemical reaction. The rate controlling step in leaching may be the diffusion of solvent into the mass to be leached, diffusion of the solute into the solvent or diffusion of the extract solution out of inert material of solid. Whatever the mechanism, the leaching process is favoured by the reduction in size of the solid.

The preparation of the material for extraction is to make the solute more accessible to the solvent by size reduction of solid. This gives increased surface area per unit volume of solids to be leached and reduced distance to be traversed within the solid by the solvent and the extract. The preparation of the material for extraction involves crushing, grinding, flaking or cutting into pieces or cosettes. Grinding to a very fine size may cause packing of solids during extraction such that free flow of solvent during extraction is impeded. In case of material with cellular structure, the cell rupture due to grinding may lead to extraction of undesirable components.

3.1.2. Selection of solvent

The selection of solvent for extraction of food material is made on the basis of (1) solvent capacity, (2) selectivity, (3) chemical inertness, (4) thermophysical properties of the solvent such as density, viscosity, boiling point and latent heat of vaporization, (5) flammability, (6) toxicity, (7) cost, and (8) availability. Special requirements for food products are concentration and toxicity of the residual solvent. Growing awareness of carcinogenic tendencies of certain solvents have restricted their use. Similarly, concentration of the residual solvent from the permitted category has also been fixed by the regulatory authorities. Desolventizing has to be carefully monitored to achieve the permissible residual solvent level.

Normally n-hexane, short chain alcohols (methanol, ethanol), ketone (acetone), esters (ethyl acetate, n-butyl acetate), chlorinated hydrocarbons (methylene dichloride, ethylene dichloride) and liquid carbon dioxide have been used as solvents for leaching. In view of the very low maximum permissible limit of solvents such as chlorinated hydrocarbons in food materials and regulatory restrictions, the use of chlorinated solvents is discouraged.

3.1.3. Selection of operating temperature

Temperature plays an important role in solid extraction. Higher temperatures give higher solubility of solute in solvents, permitting higher rates of extraction. However, higher temperatures may also mean high solvent losses, extraction of some undesirable constituents, and damage to some sensitive components in the plant material. A compromise is necessary in the selection of operating temperature.

3.1.4. Leaching equipment

Extraction equipment may be classified based on the mode of operation as (1) batch or (2) continuous. The equipments are divided into two principal classes with respect to the solids handled. They are: (1) fixed bed contact and (2) dispersed contact. In the former, the solids are kept in the form of a fixed bed while in the latter the solids are dispersed by moving them in a liquid solvent. The fixed bed devices may have the solvent contacting done by three methods: (a) percolaton, (b) full immersion, and (c) intermittent drainage. The dispersed contact is usually effected by suitable agitation and is used for materials which disintegrate during leaching. The extraction may be performed in one stage or multistage arrangement. Most multistage operations are performed in a countercurrent manner. Extraction equipment of various types are described in various handbooks and review articles (Coefield Jr., 1951; Brennan et al, 1976; Prabhudesai, 1988; Pratt and Stevens, 1992; Wakeman, 1993).

Some of the commonly used extractors described in the above-mentioned references are as follows :

B oilman extractor, Blaw-Knox extractor (Moving bed perforated basket type percolation systems) Rotocel extractor (Multicompartment countercurrent percolation extractor) Kennedy extractor (Multichamber unit with impellers) De Smet extractor, Lurgi extractor (Endless belt percolation extractors) Bonotto extractor (Vertical plate continuous dispersed solid extractor) Hildebrandt extractor, DDS extractor (Total immersion screw conveyor extractor) Sherwin-Williams extractor, Allis Chalmers extractor (Multistage mixer-settler disperse contact systems) Dorr extractor (Multistage decantation system) Diffusion battery (Multibatch countercurrent extraction system)

3.1.5. Applications

3.1.5.1. Extraction of edible oils

The major constituents of oilseed are oil, proteins, carbohydrates, crude fibre, moisture and inorganic matter. Some minor constituents are pigments, vitamins, antioxidants, and enzymes. The meat of the seed is predominantly composed of lipids and proteins, while the protective coating is largely composed of crude fibre and carbohydrates. Proteins and carbohydrates are insoluble in oil. Oil is a complex mixture of glycerides. Some esters exist as waxes which need to be recovered from the oil. Similarly, the undesirable complex lipids should not be present in the oil.

Preliminary operation before extraction consists of crushing the seed and dehulling. The meat is separated from the hulls. This is important as the oil content of hulls is very low (<1%). If hulls are not separated, they will absorb some oil as well as increase the material to be handled. The oil is distributed in the entire meat of the oilseed and is contained inside tough-walled cells. Size reduction helps in rupturing some of these cells to release the oil and also reduces the distance to which the solvent must diffuse to reach the oil.

The crushed, milled or flaked meat is charged to the extractors. Solvent, normally n-hexane, is fed to the extractors at about 50°C. Depending upon whether the operation is batch or continuous, the required quantity of solvent is contacted with the meal, using one or more contacts to reduce the oil content from the initial value of 15-19% to about 0.5% in the extracted mass.

After extraction the wet flakes contain about 40% solvent and 0.5% oil. These are carried to a desolventizer unit which may be steam jacketed paddle conveyor followed by an enclosed rotary drum dryer. The evaporator-dryer system removes the mixture of solvent and water which is condensed and the oil is separated and recycled.

The miscella, the oil-laden solvent, is fed to multiple effect evaporators and then a falling film evaporator. The evaporated solvent is condensed and recycled to extractors.

3.1.5.2. Extraction of "tea and coffee solubles

Extraction of dried, blended tea leaves with hot water is accomplished in 3 to 5 stages in a diffusion battery. Temperature for extraction is raised from 70°C in the initial stage to 90°C in the final stage using intermediate heat exchangers. The final solution usually contains 2.5-5% solids. For preparing instant tea, the solution is concentrated to about 50% solids in vacuum evaporators. The volatile aroma stripped off during this stage is condensed and blended back. The solution is dried by spray, freeze or vacuum belt dryers. In place of a diffusion battery, other extractors like Rotocel or continuous countercurrent multistage extraction system with split feed may be used.

In the extraction of coffee solubles, ground and roasted coffee beans are extracted with hot water under pressure. The diffusion battery containing 5 to 10 percolators may be used. The percolator may operate in a batch or semicontinuous mode with countecurrent flow of hot water. The extraction temperature is raised from 100°C in the initial stage to 180°C in the final stage. Intermediate heat exchangers are used for this purpose. The final solution contains 25-30% solids. This is evaporated and dried to get instant coffee.

Decaffeination of tea and coffee may be done by using'liquid-liquid extraction before drying.

3.1.5.3. Extraction of other materials

Spices such as black pepper, capsicum, celery and cumin and materials such as hops may be extracted by using solvents such as acetone, n-hexane, methylene dichloride, ethylene dichloride, ethanol or liquid carbon dioxide. Various patents, review articles and technical publications are available in literature. 10-15% yield of oleoresins is obtained from various spices. Flavours and aromas from fruits and fragrances from flowers are obtained by solid extraction followed by liquid-liquid extraction. It is necessary that the natural aroma, flavour, and fragrance be preserved by use of non-reactive solvents.

3.2. Liquid extraction

Liquid-liquid extraction is used in food processing to either selectively recover valuable components of a natural product or to remove undesirable components from it. Extraction of flavours and essences from crude extracts of citrus oils and separation of monoglycerides and lecithins from edible oils are examples of recovery of valuable components. Removal of fatty acids from edible oils, partitioning of the crude oils to obtain higher unsaturation are examples of the latter category. Decaffeination using liquidliquid extraction is an example of value addition to the coffee and tea as well as recovery of valuable caffeine.

3.2.1. Selection of solvent

The selection of solvent has been discussed in section 3.1.2 where the criteria are enumerated. In the case of liquid-liquid extraction, interfacial tension plays an important role. A low value of interfacial tension is desirable for getting a good dispersion with high interfacial area for extraction. However, a very low value will cause emulsion formation, which will create problems in phase separation. In the extraction of natural products some extracted material may also cause undesirable emulsification.

The solvent should have high solute capacity and selectivity. It should be inert to avoid formation of harmful artefacts. Robbins (1980) has given a table of solute-solvent interactions to be used for selection of solvents. Distribution coefficients may be predicted using UNIFAC or ASOG group contribution methods.

The toxicity is specially important in view of the extraction of food materials. The residual solvent concentration has to be within the specified limit. This reduces the number of acceptable solvents for extraction.

3.2.2. Liquid-liquid extraction equipment

The commercial extractors are classified in many ways. One classification is based on the manner in which the phases are contacted. The two classes of equipments are: (1) contact by gravity and (2) contact by centrifugal force. In the class of equipments where the countercurrent flow is produced by gravity, they are further classified by the type of agitation, viz., (a) unagitated columns, (b) pulsed columns, (c) mechanically agitated columns, and (d) miscellaneous devices. In unagitated columns, discrete stage contact is provided in sieve plate columns and differential contact is obtained in spray and packed columns. Similarly in pulsed columns, discrete contact is obtained in pulsed mixer-settlers and pulsed sieve plate columns and differential contact is obtained by mechanical or pneumatic pulsing. The mechanically agitated columns may be rotary devices or reciprocating devices. Mixer-settlers give discrete stage contact in this subcategory, while columns such as rotating disc contactor, Kuhni column and Schiebel column give differential contact. The reciprocating devices are typically reciprocating plate columns such as Karr extraction column. The miscellaneous equipment for extraction are static mixers, ultrasonic extractors, and pumping extractors. The centrifugal extractors also have discrete stage contact as in Luwesta and Robatel extractors and differential stage contact as in Podbielniak and Delaval extractors. Treybal (1963), Reissinger and Schröter (1978, 1984), Lo et al (1983, 1993), Robbins (1984), Walas (1988), and Pratt and Stevens (1992) discuss the constructional features, design, performance and selection of the liquid-liquid extraction equipment.

3.2.3. Applications

3.2.3.1. Extraction of edible oils

The hexane-extracted oil (described in section 3.1.5.1) is a mixture of triglycerides and fatty acids with small quantities of mono and diglycerides. The refining process for removal of free fatty acids is accomplished either by neutralization with aqueous alkali or by steam stripping. The neutralization process may be considered as extraction with reaction. Some suggested alternative liquid-liquid extraction processes are based on the use of solvents such as methanol, ethanol or acetone. The solvent may be an aqueous solution containing 85-95% organic solvent. Use of a mixture of hexane and aqueous ethanol for deacidification has been studied (Thomopoulos, 1971). Rotating disc contactors have been suggested for extractive deacidification. While in the case of alkali neutralization, mixer-settler units are used.

Monoglycerides present in the mixed glycerides have wide use as emulsifiers. Use of dual solvent system of ethanol and a straight chain hydrocarbon has been studied to get a fraction rich in monoglycerides (Arida et al, 1979).

Fractionation of triglycerides based on the degree of unsaturation of the fatty acid chains in the molecule was reported by Parsons (1976) using aqueous n-methyl pyrrolidone and dimethyl formamide as solvents. Use of propane at high pressure has been reported by Moore (1950). A special case of fish oil extraction using aqueous acetone to get enriched eicosopentanoic acid has been reported (Jap. Pat., 1984). Use of supercritical fluid extraction for the same purpose is reported by Krukonis (1988).

Separation of lecithin from crude soyabean oil using aqueous ethanol as solvent is reported by Liebing (1972). The extraction using supercritical carbon dioxide-propane (Peter et al, 1987) and supercritical carbon dioxide-acetone (Stahl and Quirin, 1985) for deoiling the lecithin have also been reported. Separation of alpha tocopherol from amaranth seed oil using conventional and supercritical carbon dioxide extraction have also been carried out (Nautiyal, 1995).

3.2.3.2. Extraction of caffeine

Caffeine content of the coffee beans is 1-2% (dry basis). The decaffeinated instant coffee contains 0.2-0.3% (dry basis) caffeine. The recovered caffeine finds ready market in the soft drinks industry. Hamm (1992) has given an excellent review of the different solvents for decaffeination, solubility of caffeine in different solvents, distribution coefficients of caffeine for water-solvent-caffeine systems at different temperatures (solvent: chloroform and dichloromethane) and solubility of caffeine in water and aqueous coffee extract (non-caffeine solubles). Solvents and extraction conditions for caffeine extraction from coffee beans are also summarized.

The liquid-liquid extraction of caffeine from the aqueous solution of coffee solubles is accomplished by using a chlorinated solvent such as dichloromethane. The first liquidliquid extraction stage is maintained at high temperature (40-80°C). The extraction column may be a rotating disc contactor (RDC) or reciprocating plate contactor (Karr). The caffeine-rich dichloromethane stream is again contacted with water to back-extract the caffeine to the aqueous phase in a secondary extractor. The temperature is maintained at 20-25°C with a very high phase ratio of aqueous to organic phases. The dichloromethane stripped off is recycled back to the primary extractor and aqueous stream is used to obtain caffeine. The extractor is again an RDC or Karr unit. The conditions of operation vary depending upon whether coffee solubles are obtained from green beans or from roasted coffee beans.

3.2.3.3. Extraction of other materials

Citrus oils such as lemon and orange oil contain a very large amount (>90%) of hydrocarbons and very small amount of flavour-imparting citrals, the monoterpene aldehydes. These aldehydes, neral and geranial need to be separated from the terpene and sesquiterpene hydrocarbons. Ethanol separates the citrals from the insoluble hydrocarbons. A solvent pair consisting of aqueous methanol with n-pentane may be used to separate and concentrate the citrals into the alcohol layer using a rotating disc contactor. Supercritical carbon dioxide with water as entrainer may also be used to separate the extract phase predominantly containing the citrals.

Extraction of aromas of fruit juices has been accomplished by using liquid carbon dioxide. The alpha-acids (humulones) are separated from the hop extracts by using either aqueous methanol with a hydrocarbon solvent or using aqueous potassium carbonate with petroleum spirit. Alpha-acids obtained in the polar solvent layer is stripped with n-butanol and isomerized to increase the flavour (Can. Pat., 1961; Brit. Pat., 1972).

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