Perspective On The Future

Liquid-liquid extraction process as a separation process is less energy intensive compared to other processes. By selecting a suitable solvent having high selectivity and distribution coefficient for the solute, the energy requirement in the separation step of the extraction process may be significantly reduced. This is of paramount importance in view of the ever increasing energy cost.

The complex nature of molecules in food materials and their thermolabile characteristics restrict the use of solvents and temperature of operation. In addition, presence of surface active constituents reduces the interfacial tension between the contacting phases. The reduced density difference between the phases also leads to entrainment. In view of the above, the extractors have to be designed on the basis of experimental data for specific systems. Multistage extractors with reduced backmixing will be of interest. The mixed solvents and specially designed solvents will find more use in future.

Supercritical fluid extraction is an excellent separation process for food materials. Mild conditions of extraction permit the separation of components in an unaltered state with no residual solvent. The extraction process is amenable to multistage separation; this permits fractionation of the various components of the food material. Carbon dioxide, the most commonly used supercritical fluid is nonpolar in nature. The solubilities of food materials can be increased by several orders of magnitude by using a cosolvent (entrainer). The phenomenon of crossover in the solubility with changes in temperature and pressure has promise of utilization for separation of some food materials (Chimowitz and Pennisi, 1986). One of the biggest disadvantages of supercritical fluid extraction is the high capital cost of the equipment. This restricts its use to the extraction of high-value low-volume materials. Future technology should permit reduced cost of equipment and lower pressure operations.

ATPS have the characteristics of high biocompatibility and low interfacial tension. Polymer-polymer systems exhibit better selectivity as compared to polymer-salt systems but high viscosity and higher cost make their use economically unviable. ATPS can be used for bulk separations, becoming complementary to other purification methods such as electrophoresis or chromatography. Polymer-salt systems have lower viscosities, demix rapidly and are also cheaper. Therfore large scale operations will be based on salt based ATPS. Flavouring substances, dipeptides and nucleotides from acid hydrolysis in food industry can be easily handled with ATPS. The use of ATPS in isolation/purification of such compounds will be of interest in future.

The presence of bio-specific ligands can improve the partitioning of various bio-products. Since the affinity ligands are expensive, other ligands such as dyes, mimicking the biospecificity, need to be investigated in detail. The loss of such ligands from bound state may contaminate the bioproduct. Biocompatible affinity ligands thus need to be developed in future.

In the coming years, the enzymatic reactions, either substrate or product inhibited, will be increasingly investigated in ATPS which are more suitable than organic solvent based extractive conversions.

The reverse micelles can act as compatible hosts for proteins and amino acids. Their transparency lends them to be used for biomimetic phososynthetic systems. Enzymes in reverse micelles, after extraction, can be directly used for reaction in organic phase, e.g., lipase, alpha-chymotripsin and dehydrogenase. The use of mixed surfactant reverse micelles in extraction of proteins shows promising results over other techniques such as those based on pH and ionic strength.

Emulsion liquid membrane systems are similar to reverse micelle systems except that the encapsulated drops are much larger in size. The ELM extraction has been used for separation of carboxylic acids and penicillin-G. The problem of emulsion swelling and demulsification are major hurdles in its commercialization. It has good scope because of its fast extraction rates and use of conventional extraction columns.

The important issues in the development of separation techniques are selectivity, simplicity of systems and equipment, speed of separation, ease of scale-up and possibility of continuous operation. Some of the present and future work in extraction of food materials will look into these issues. Alternative separation techniques using adsorption systems, perfluorocarbon affinity separations will compete with the separation processes presented in this article.

Processes where simultaneous bioreaction and product separation is achieved by an integrated process will be of paramount interest in the future.

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