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• Has uniform flavor quality

• Is free from enzymes and tannins

However, the oil lacks the nonvolatile principles that contribute to the taste characteristics of the spice.

Oleoresin Ginger

The essential oil of ginger derived by steam distillation represents only the aromatic, odorous constituents of the spice; it does not contain the nonvolatile pungent principles for which ginger is highly esteemed. The oleoresin, obtained by extraction of the spice with volatile solvents, contains the aroma as well as the taste principles of ginger in highly concentrated form.

The oleoresin represents the wholesome flavor of the spice—a cumulative of the sensation of smell and taste. It consists of the volatile essential oil and the nonvolatile resinous fraction comprising taste components, fixatives, antioxidants, pigments and fixed oils naturally present in the spice. The oleoresin is, therefore, designated as "true essence" of the spice and can replace spice powders in food products without altering the flavor profile.

Manufacturing Process

Removal of a soluble fraction from a permeable solid phase with which it is associated by selective dissolution by a liquid solvent is technically known as leaching (Perry and Chilton, 1973). Although this term originally referred to the percolation of liquid through a fixed bed, it later became generic to all solid-liquid extractions (Treybal, 1968). In spice extraction, the process is commonly referred to as solvent extraction. The presence of a solid phase distinguishes this process from liquid-liquid extraction.

The oleoresin of ginger is obtained from the spice by solvent extraction. The process essentially comprises the following three steps: (1) contacting the spice powder and solvent in the extractor to effect the transfer of the functional components from the spice to the solvent, (2) separation of the resulting solution from the powder, and (3) distillation of the extract to recover the product. Extractors may be batch type or continuous.

Figure 11.5 gives the schematic representation of the operation in a batch extractor. Repeated washing with the solvent would be necessary to exhaust the spice. The solute concentration in the extract progressively diminishes with each wash.

Operation in a single extractor is rarely encountered in industrial practice. In industrial extraction, a number of batch contact units are arranged in a row called the extraction battery. Solids remain stationary in each extractor and are subjected to a multiple number of contacts with the extracts of progressively diminishing concentrations from the previous one. The final contact of the nearly exhausted solids is with fresh solvent while the concentrated solution leaves in contact with the fresh solids in another extractor. The concentrated solution is distilled while subsequent washings are directed to the next freshly loaded extractor. The extractors are discharged and reloaded one at a time. The operation using a battery of extractors can be represented as in Figure 11.6. This system enjoys the flexibility of batch extraction, but at the same time, the efficiency approaches that of continuous countercurrent operation once an equilibrium is established. The number of extractors in a battery varies from three to six.

Extractor

Fresh sovent

Fresh sovent

Spent Solids Extract

Figure 11.5 Solid-liquid operation in an extractor.

Fresh solvent

Figure 11.5 Solid-liquid operation in an extractor.

Fresh solvent

Ext 1

Ext 2

Ext 1

Ext 2

Nearly exhausted

Freshly loaded

Figure 11.6 Extraction in a battery of extractors.

For the extraction of ginger, the spice is disintegrated to a predetermined particle size and loaded in the extractors. The raw material should be cleaned free from dirt, foreign matter, filth, insect, rodent, and microbial contamination. It should be dried to an optimum moisture level since excessive moisture affects the percolation rate and product quality. Extractors are stainless steel cylindrical vessels with provisions for charging the feed from the top and removing the spent spice from the bottom. A perforated plate supported on a grid at the bottom of the extractor holds the charge. Extractor capacity ranges from 200 to 2000 kg based on the scale of operation. Solvent, admitted from the top, is sprayed onto the charge. As the solvent percolates down the charge, it dissolves the active principles from the spice. The concentrated solution obtained is filtered and the solvent distilled off. Repeated washings would be necessary to exhaust the spice. Lean solutions are directed to the next freshly loaded extractor. Once the spice is fully exhausted, the solvent adhering to the spent spice is recovered by injecting live steam. The bulk of the solvent from the extract is distilled out in an evaporator. The distilled solvent is recycled. Removal of the final traces of solvent is carried out in a desolventizer under controlled conditions of temperature and pressure to safeguard the delicate flavor principles. The level of residual solvent in the product is brought down well below the limits prescribed by regulatory bodies. The schematic layout of a simple batch extraction plant is shown in Figure 11.7. Figure 11.8 shows a commercial plant.

F = Filter

Figure 11.7 Schematic layout of a solvent extraction plant.

F = Filter

Figure 11.7 Schematic layout of a solvent extraction plant.

Figure 11.8 A spice extraction plant (batch extraction).

Continuous countercurrent extraction has also been recently introduced for spices. The extractor for continuous operation essentially consists of a moving belt that carries the feed through the extraction zone. Lean solutions of the extract of progressively diminishing concentration are sprayed onto the feed as it moves toward the discharge end. The exhausted spice is stripped off the residual solvent and discharged. The concentrated solution is distilled to remove the solvent. Continuous extraction is economical only if a minimum steady feed rate is guaranteed. Figure 11.9 shows a commercial continuous extraction plant.

As discussed earlier, oleoresin is a composite of the volatile and the nonvolatile flavor principles of the spice. These two fractions can be retrieved from the spice separately using a two-stage extraction process or together through a single-stage process.

Figure 11.9 A spice extraction plant (continuous extraction).

Two-Stage Extraction: Here the clean dry spice is ground and first steam distilled to obtain the volatile oil. This oil represents the aroma of the spice. The deoiled spice is then subjected to solvent extraction to recover the nonvolatile taste principles. The aroma and taste fractions are proportionately blended to give the oleoresin of the spice. Figure 11.10 gives the flow diagram for a typical two-stage ginger extraction. Two-stage extraction provides flexibility in product formulation. Since the aroma and pungency fractions are isolated individually, their relative percentages in the endproduct can be varied to meet exact customer specifications.

Single Stage Extraction: The process can also be carried out in a single stage bypassing the steam distillation step (Figure 11.11). However, this method restricts the flexibility in finetuning the oleoresin to the customers' specifications.

The oleoresin of ginger is a dark brown viscous liquid and usually contains 25 to 35 percent volatile oil. The yield of oleoresin from dried ginger is normally 4 to 6 percent, but varies with the variety and the extraction medium.

Figure 11.10 Steps in two-stage extraction.
Figure 11.11 Steps in single-stage extraction. Factors Affecting the Extraction Efficiency

The major factors affecting the efficiency of solvent extraction are:

The Particle Size: To achieve adequate solute-solvent contact in solvent extraction, the raw material should be comminuted. Solute is usually surrounded by a matrix of insoluble matter. The solvent must, therefore, diffuse into the mass and the resulting solution must diffuse out. Grinding of the raw material will greatly accelerate the leaching action since more of the solute is exposed to the solvent. Each plant material has a characteristic shape, size, texture, and hardness. The choice of size-reduction equipment depends on these factors. Even though size reduction enhances the transfer of solute to the solvent, very fine powder tends to restrict the percolation of the solvent through the charge due to decrease in the porosity of the bed. This will adversely affect the extraction rate. It is, therefore, necessary to select an optimum particle size for extraction.

Extraction Medium: The selection of solvent primarily focuses on an optimum quantity of extractives of the desired quality and not necessarily maximum yield. A good extraction solvent should:

• Be able to dissolve the active principles selectively and minimise the extraction of undesirable constituents.

• Be chemically pure, since residues of impurities can impart objectionable off-flavor to the product.

• Be reasonably low boiling to facilitate distillation. However, too low a boiling point can lead to excessive loss of solvent during processing.

• Be chemically inert, i.e., should not react with the constituents of the product.

• Have low specific and latent heats.

• Be nontoxic and should not pose health hazards.

• Be nonflammable and nonexplosive.

• Be readily available and reasonably priced.

• Be acceptable under the food laws of the country where the product is to be used.

Spice extraction involves the use of organic solvents. Regulatory bodies have specified permissible limits of the residues of various solvents in the oleoresin. Limits laid down by the Code of Federal Regulations of the FDA (CFR,1995) for common extraction solvents for spices are listed in Table 11.22.

Acetone, methanol, isopropanol, methylene chloride, ethyl acetate, and ethyl alcohol are popular extraction solvents for ginger. Ethylene dichloride is an efficient extractant; however, its use is restricted due to alleged carcinogenicity.

Temperature of Extraction: Generally, an increase in temperature improves extraction efficiency. This, in turn, helps to reduce the solvent quantity and the process time. Extractors can be provided with steam jackets to heat the contents or hot solvent may be sprayed onto the charge. However, high temperatures may lead to the extraction of excessive amounts of undesirable compounds from the spice, which can affect the quality of the product.

Extraction Using Supercritical Fluids

Conventional spice extraction involves the use of organic solvents, which can leave their residues in the final product. Moreover, at the distillation temperature, some deterioration or chemical modification of the labile components is also likely.

Supercritical fluid extraction (SFE) is a novel isolation method that can overcome the above issues. An oleoresin, free from chemical alterations brought about by heat and water and without solvent residues and other artifacts, can be obtained by this method. Carbon dioxide (CO2) is the popular medium for the supercritical extraction of spices.

Principle of Supercritical Fluid Extraction

Supercritical fluid extraction involves the use of a compound above its critical temperature and pressure as extraction medium.

Figure 11.12 gives the phase diagram for a substance. The equilibrium curves for the three states—solid, liquid, and gas—meet at TP, the triple point of the substance. At

Table 11.22 Limits for residual solvents in spice oleoresins

Solvent

Limit, ppm max

Acetone

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